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Title: Modern Refrigeration and


1
Modern Refrigeration and Air Conditioning
Althouse Turnquist Bracciano
PowerPoint Presentation by Associated
Technical Authors
PublisherThe Goodheart-Willcox Company,
Inc.Tinley Park, Illinois
2
Chapter 13
Commercial Systems
3
Learning Objectives
  • Explain the differences between the mechanism of
    commercial refrigeration systems and domestic
    systems.
  • Compare the differences between various
    commercial mechanisms.
  • Describe how each mechanism (condenser,
    evaporator, and compressor) operates.
  • Discuss the theory and operation of control
    devices.
  • Follow approved safety procedures.

4
Modules
  • Commercial Systems
  • Commercial SystemsControls

5
Chapter 13
COMMERCIAL SYSTEMS MODLULE
6
Construction ofRefrigeration Components
13.1
  • Varies from domestic systems in the following
    ways
  • Number of evaporators connected to a single
    condenser.
  • Electrical voltage.
  • Compressor design and size.
  • Condenser unit designs and size.
  • Motor controls (both temperature and pressure).
  • Refrigerant controls (liquid and vapor).

7
Construction ofRefrigeration Components
13.1
  • Varies from domestic systems in the following
    ways
  • Piping.
  • Evaporator designs.
  • Defrosting systems.
  • Variety of refrigerants used.

8
Refrigeration Components
13.2
  • Hermetic compressors used on small commercial
    systems such as beverage dispensers, ice cube
    makers, and ice cream machines.
  • Semihermetic compressors are used on larger
    commercial applications such as multiple
    evaporator storage rooms, large fresh-food cases,
    and multiple compressor units.
  • Commercial systems are either packaged or split
    systems.

9
Refrigeration Componentscontinued
13.2
  • Packaged systems are designed, built, and shipped
    by manufacturer and include all components needed
    for a complete system.
  • Split systems are site engineered. Components
    are assembled on site. These systems are often
    custom designed for specific applications.

10
Packaged Commercial System Components
13.3
  • High-pressure side includes
  • Compressor, usually hermetic.
  • Condenser, usually air-cooled.
  • Liquid receiver, when TXV or AXV are used.
  • High-pressure safety motor control.
  • Liquid line with drier and sight glass.
  • NoteThe refrigerant control is the division
    point between the low side and the high side of
    the system. It consists of an AEV or capillary
    tube.

11
Packaged Commercial System Componentscontinued
13.3
  • Low-pressure side includes
  • Evaporator.
  • Low-pressure or temperature motor control.
  • Suction linesome with filter-driers and surge
    tanks.

12
Packaged System withMultiple Evaporators
13.3
  • High-pressure side includes
  • Compressor, often with an oil separator.
  • Condenser water- or air-cooled.
  • Liquid receiver.
  • High-pressure motor control.
  • Liquid lines with a drier and a sight glass.
  • Water valve, used with a water-cooled unit.

13
Packages System withMultiple Evaporators
13.3
14
Packages System withMultiple Evaporatorscontinue
d
13.3
  • NoteThe refrigerant control is the division
    point between the high-pressure side and the
    low-pressure side.
  • Low-pressure side includes
  • Refrigerant controls (two or more)usually
    thermostatic expansion valves.
  • Evaporators (two or more)may be natural
    convection, forced convection, or submerged.
  • Motor controlusually operated by pressure.
  • Suction lines with drier and suction pressure
    regulator.

15
Packages System withMultiple Evaporatorscontinue
d
13.3
  • Low-pressure side includes (continued)
  • Two-temperature valves for multiple temperature
    installation.
  • Surge tanks for reducing rapid pressure changes.
  • Check valves for multiple temperature
    installations.

16
Subcooling
13.3
  • Used on low-temperature units such as display
    cases, freezers, etc.
  • The process reduces the refrigerant temperature
    in the liquid line below the saturated
    temperature. The lower the temperature in the
    liquid line, the greater the systems heat
    removal capacity, resulting in a more efficient
    system.
  • Accomplished by refrigerating the liquid line on
    a low-temperature system. A high-temperature
    system is used since it removes Btus three times
    more efficiently than low-temperature
    refrigeration systems. Together the two systems
    increase overall efficiency of the refrigeration
    process.

17
Subcooling
13.3
18
Subcoolingcontinued
13.3
External drive unit Condensers are mounted on
steel base. Motor is mounted outside the
compressor. Motor drives the compressor either
directly or with one or more belts.
19
Subcoolingcontinued
13.3
Hermetic unit Motor is connected directly to
compressor. Crankcase and system pressures are
equalized on start-up, preventing oil from
leaving the compressor during start-up.
20
Subcoolingcontinued
13.3
  • Compressors may be named after their cylinder
    arrangementvertical single, horizontal single,
    vertical two cylinder, V-type four cylinder, etc.
  • Shown is a serviceable six-cylinder W-type
    compressor.

21
Commercial Hermetic Units
13.3.1
  • Units with bolted assembly are referred to as
    field serviceable or accessible. Both have
    service valves. Some units are sealed in a welded
    casing. These units may be connected to any type
    of evaporator.
  • Advantage of hermetics in commercial field is the
    elimination of crankshaft seals and belts.
    Moisture and dirt must be kept out of system
    during servicing.

22
Commercial Hermetic Unitscontinued
13.3.1
Outdoor hermetically sealed air-cooled condenser
has a fan condenser, shroud, and service valve.
23
Commercial Hermetic Unitscontinued
13.3.1
Inside of four cylinder welded hermetic motor
compressor used for air conditioning, heat pump,
and commercial condensers.ACrankshaft.BConnect
ing rod.CPiston.DMotor windings.EElectrical
terminals.FSuction and discharge openings.
24
Commercial Hermetic Unitscontinued
13.3.1
Smaller units have single-phase motors. Units
over 5 hp generally have three-phase motors.
Rotor
Stator
25
Commercial Hermetic Unitscontinued
13.3.1
  • Condensers may be installed in different rooms or
    outside the building.
  • Manufacturer-assembled condensing unit can be
    matched with an evaporator assembly and
    precharged refrigeration lines, to meet a wide
    range of cooling needs.

26
Commercial Hermetic Unitscontinued
13.3.1
  • For large installations, two-motor compressors
    may be used.
  • Tandem assembly motor compressor Connects two
    motor compressors together at the motor end.
    These units can be run separately for low load or
    together for full load.

27
Commercial Hermetic Unitscontinued
13.3.1
  • For large installations, two-motor compressors
    may be used.
  • Parallel assembly motor compressors Connects two
    or more units in parallel by piping. The units
    also require a compressor oil piping system to
    ensure all compressors have the correct oil
    amount in each crankcase while operating.

28
Outdoor Air-CooledCondensing Units
13.3.2
  • Save space when air conditioning commercial
    buildings and homes.
  • Save cost of plumbing for water circuits.
  • Useful when chemicals in water make water-cooling
    impractical.
  • May be mounted on the roof, outside wall, or at
    ground level.

29
Outdoor Air-CooledCondensing Unitscontinued
13.3.2
  • Four major provisions to using outdoor air-cooled
    condensing units
  • Must be a head pressure control if unit is
    exposed to outdoor weather below the operating
    cabinet temperature.
  • Method of preventing short cycling must be
    designed into the system.
  • Means provided to prevent dilution of the
    compressor oil by liquid refrigerant.
  • Completed condenser must be constructed and
    installed so it is virtually weatherproof.

30
Outdoor Air-CooledCondensing Unitscontinued
13.3.2
  • Low ambient temperatures will cause low head
    pressures, which may stop flow of refrigerant. To
    maintain pressure
  • Partially fill condenser with liquid refrigerant.
  • Stop or slow condenser fans.
  • Partially or completely close ambient air
    louvers.
  • Heat the condenser.

31
Dual-Compressor Model
13.3.2
  • Contains two completely separate refrigeration
    systems.
  • Range from 6 tons to 35 tons and provide a
    partial standby system.

32
Dual-Compressor Modelcontinued
13.3.2
  • Basic operation similar to two-stage compressor.
    Each compressor is activated individually from a
    two-stage space thermostat.
  • Provides two-stage heating and two-stage cooling
    with automatic changeover.

33
Operation ofDual-Compressor Model
13.3.2
  • System No. 1 turns on first stage of the
    thermostat.
  • If load is light, system No. 1 carries the load.
    The compressor cycle is on and off at the call of
    the thermostat.
  • If system No. 1 is not adequate to handle the
    load, the room thermostat automatically turns on
    the second compressor.

34
Operation ofDual-Compressor Modelcontinued
13.3.2
  • It will signal on and off to carry the rest of
    the load, while compressor No. 1 runs constantly,
    removing moisture from the air.
  • When the load drops and the facility temperature
    is lowered, compressor No. 2 shuts down and
    compressor No. 1 cycles to carry the reduced load.

35
Maintaining Condensing Pressures by Design Change
13.3.2
  • When using outdoor units, it is important to
    maintain full operating capacity at the
    thermostatic expansion valve during cold weather.
  • Capacity depends on pressure difference across
    the valve. If condensing pressure is reduced,
    valve capacity will drop and not enough liquid
    refrigerant will flow. The fixture temperatures
    may rise too high. The unit will short cycle.

36
Maintaining Condensing Pressures by Design
Changecontinued
13.3.2
A design change can alleviate this problem. The
unit is made to nearly fill the condenser tubes
with liquid. Just enough condensing surface is
left to maintain the pressure.
37
Maintaining Condensing Pressures by Design
Changecontinued
13.3.2
  • Installation specifications must be carefully
    checked. The receiver must hold enough liquid
    refrigerant to flood most of the condenser in the
    winter. It must also safely hold the refrigerant
    during the warm season.
  • The check valve and limiter valve ensure good
    condensing pressure during cold weather.
  • Another method of maintaining pressures is to
    install a pressure-sensitive device connected to
    the condenser tubing. This head pressure device
    will move the rod out as pressures increase,
    opening the louvers.

38
Maintaining Condensing Pressures by Design
Changecontinued
13.3.2
  • The adjustable louvers will close as head
    pressure decreases. A system may also use a
    fan-cycling pressure control to sense condenser
    pressure.
  • Controls lower the fan speed when the head
    pressure drops. Electrically controlled modulated
    fan speeds are used for this purpose.

39
Maintaining Condensing Pressures by Design
Changecontinued
13.3.2
  • The system operates with a thermistor on the
    condenser and a special fan motor.
  • Electric heating elements are often placed in or
    around the receiver in an effort to keep receiver
    temperature warmer than cabinet temperature. If
    the receiver became too cold, it would act like a
    condenser.

40
Maintaining Condensing Pressures by Design
Changecontinued
13.3.2
Systems may use a bypass from the compressor to
the receiver. The bypass feeds hot refrigerant
vapor to the receiver to keep it warm. The bypass
has a check valve mounted in it to ensure one-way
refrigerant flow.
41
Maintaining Condensing Pressures by Design
Changecontinued
13.3.2
  • Compressor is kept warm by electric heating
    elements that surround it. They are
    thermostatically operated to energize the heating
    element at about 50ºF (10ºC). The heater usually
    has a 100W to 200W capacity.
  • Windy conditions can prevent damper and fan
    operation. The unit must be installed in a
    position to avoid high-velocity cold winds. It
    should be as weatherproof as possible with walls
    built around all four sides.
  • Head pressure control valves that are thermostat
    operated are often used. A check valve in the
    condenser outlet prevents the flow of refrigerant
    to the cold receiver.

42
Maintaining Condensing Pressures by Design
Changecontinued
13.3.2
  • A system with a pressure-control valve will open
    as the receiver pressure falls. Hot gas is
    allowed to bypass into the receiver (at about 20
    psi pressure difference), raising the receiver
    pressure and increasing the flow of liquid
    refrigerant to the evaporators.

43
Maintaining Condensing Pressures by Design
Changecontinued
13.3.2
44
Maintaining Condensing Pressures by Design
Changecontinued
13.3.2
  • The valve has two openings, B and C. As one
    closes, the other opens. Valves must be sized to
    capacity of system.
  • System is charged with twice as much refrigerants
    as without the condenser flooding feature. A
    receiver that can store all extra refrigerant
    during the summer is needed. For service
    purposes, the receiver should be twice the normal
    size.
  • The compressor may collect small amounts of
    liquid refrigerant during the off cycle. A trap
    may be needed in the compressor discharge line.

45
The Compressor
13.3.3
  • Commercial compressors are either external drive
    or hermetic.
  • There are several types of commercial hermetic
    compressors. A bolted hermetic compressor uses
    temperature limit controls and an oil pressure
    sensing safety control.

46
The Compressorcontinued
13.3.3
Large units may have winter hydraulic or electric
unloading devices to control the number of
cylinders pumping. The higher the load, the more
cylinders used to pump the vapor.
47
The Compressorcontinued
13.3.3
  • A welded hermetic motor compressornot field
    serviceableis built in sizes from 1/6 hp to 20
    hp. Design varies with size and manufacturer.
    Some are spring mounted internally others use
    outside mounting springs. Smaller units have one
    cylinder. Larger units have two or more
    cylinders. Small units may be either two or
    four-pole (single phase). Three-phase motors are
    used in larger units.

48
The Compressorcontinued
13.3.3
  • Each compressor has a minimum and maximum
  • Revolutions per minute (rpm) for efficiency.
  • Compression ratio (a maximum pressure difference
    between low side and high side).
  • Discharge temperature.
  • Volume of gas it can pump.
  • Prior to using a compressor, check the
    manufacturers operating specifications.

49
Cascade Systems
13.3.3
  • Used in many low-temperature systems.
  • First stage compressor may be reciprocating unit,
    but rotary units are also used. Rotary compressor
    pressure limit is about 45 psi across the
    compressor. Works well with a compression ratio
    of 41. Also works well with a discharge
    temperature of about 200ºF (93ºC).
  • Rotary has a high volumetric efficiency. A check
    valve is usually placed in the discharge to
    prevent backup of refrigerant during off cycle.
    Check valve should also be placed in the oil
    lines.

50
Cascade Systemscontinued
13.3.3
Compressors may have from one to twelve
cylinders. There are numerous cylinder
arrangements.
51
Cascade Systemscontinued
13.3.3
  • Internal unloaders are usually operated by oil
    pressure. A spring holds the intake valve open
    until oil pressure builds up, causing all intake
    valves to operate. This also reduces pumping
    capacity during low-load conditions. Solenoid
    valves are mounted in the oil lines to unloaders.
    When the solenoid closes, the oil pressure drops
    in the unloader. The intake valves are kept open.

52
Cascade Systemscontinued
13.3.3
  • Low-side pressure switches operate the solenoids.
    A timer bypass pressure switch operates the
    system at full capacity for a minute each hour or
    two. External unloaders use a bypass to the
    evaporator inlet ensuring suction vapor is cool.
    (De-superheating.)

53
Air-Cooled Condenser
13.3.4
  • Common in large commercial systems. May be cooled
    by a big fan built onto the motor or into the
    compressor flywheel on external drive units.

54
Air-Cooled Condensercontinued
13.3.4
Placing a metal shroud around the air-cooled
condenser may increase fan efficiency. More than
one fan may be used. Air is drawn and forced
through the condensers.
55
Air-Cooled Condenser
13.3.4
The condensers have fins and frequently use a
double or triple row of tubes. A variety of fin
arrangements and constructions may be used.
56
Outdoor Air-Cooled Condensers
13.3.5
  • Motor compressor and liquid receiver may be
    located indoors and the air-cooled condenser
    located outdoors.
  • The compressor discharge line carries the hot
    high-pressure vapor to the outdoor air-cooled
    condenser.
  • The condensed liquid is piped back into the
    building.

57
Water-Cooled Condenser
13.3.6
  • Large commercial refrigerating units often use
    water-cooled condensers. They are built in three
    styles
  • Shell and tube.
  • Shell and coil.
  • Tube-within-a-tube.

58
Shell and Tube Condenser
13.3.6
  • Cylinders usually made of steel with copper tubes
    inside. Water circulates through the tubes
    condensing hot vapors in the cylinder into a
    liquid.
  • The bottom part of the shell serves as a liquid
    receiver.

59
Shell and Tube Condenser
13.3.6
60
Shell and Tube Condenser
13.3.6
Advantages include compact, needs no fans, and
combines condenser and receiver in one. When
manifold ends are removed, water tubes can easily
be cleaned of deposits.
61
Shell and Coil Condenser
13.3.6
  • Very similar to shell and tube water-cooled
    condenser.
  • Has a coil of water tubing inside the shell
    rather than a straight tube.
  • Often used in smaller commercial units.

62
Tube-within-a-Tube Condenser
13.3.6
  • Popular because it is easy to construct.
  • Water passing through the inside tube cools the
    refrigerant in the outer tube. The outer tubing
    is cooled by air in the room. Double cooling
    improves efficiency.

63
Tube-within-a-Tube Condensercontinued
13.3.6
This type of condenser may be constructed in a
cylindrical, spiral, or rectangular style.
64
Tube-within-a-Tube Condensercontinued
13.3.6
The inner tube shows a six-lead and an eight-lead
grooved inner tube that is designed to increase
heat transfer.
65
Tube-within-a-Tube Condensercontinued
13.3.6
A double-walled inner tube with a grooved design
achieves venting of refrigerant vapor in the
event of a leak.
66
Tube-within-a-Tube Condensercontinued
13.3.6
  • The tube-within-a-tube design has water entering
    the condenser at the refrigerant outlet. The
    water leaves the condenser at the point where the
    hot vapor from the compressor enters. This is
    called a counterflow design. The warmest water is
    adjacent to the warmest refrigerant and the
    coolest refrigerant is next to the coolest water.

67
Cooling Towers
13.3.7
  • Water-cooling towers save on water consumption.
    The towers serve the same purpose as the spray
    towers in large systems.
  • There are a variety of cooling tower designs.
  • Cooling towers generate excessive noise due to
    their large fans and water sprays. Therefore,
    they should be located away from areas such as
    offices, restaurants, and residences.
  • Cooling towers are made of corrosion-resistant
    materials, including steel, copper, stainless
    steel, plastic, and treated wood.

68
Cooling Towers
13.3.7
69
Cooling Towerscontinued
13.3.7
  • The more water surface in contact with the air
    flowing through the cooling tower, the more
    efficient the cooling action.
  • The following factors impact performance of a
    cooling tower
  • Design conditions.
  • Humidity requirements.
  • Tower heat load.
  • Design wet bulb temperature.
  • Water quality.

70
Cross-Flow Cooling Tower
13.3.7
  • A cross-flow cooling tower takes the water from
    the heat source through an inlet on the side of
    the unit to the hot water distribution basins on
    each side.
  • Gravity flow nozzles distribute the water evenly
    over the wet deck surface. Air is drawn through
    the air inlet louvers and across the wet deck,
    causing some water to evaporate, removing heat
    from the remaining water.

71
Cross-Flow Cooling Towercontinued
13.3.7
  • Cooled water flows into the lower sump and
    returns to the heat source. Cooled water collects
    in the bottom of the enclosure and passes through
    a screen removing foreign material. The water is
    then re-circulated through the condenser.
  • A float-controlled valve in the lower water pan
    adds water as needed.
  • A drain continually bleeds some water out of the
    pan to keep water hardness to a minimum.
    Chemicals are added to retard rust, algae,
    fungus, and bacteria.

72
Cross-Flow Cooling Tower
13.3.7
73
Fills
13.3.7
  • A fill is a material that allows water to flow
    over materials in thin films, ensuring increased
    contact with the airflow and more efficient
    cooling action.
  • Fills are made of many materials metal fins,
    wood slats, plastic, etc.
  • The shapes of the surfaces vary with the most
    popular being cellular (honeycomb).
  • The distribution system (nozzles, troughs,
    V-notches) must be kept clean and must distribute
    water evenly to prevent scale buildup.

74
PreventingCooling Tower Freezing
13.3.7
  • Normally no danger of freezing while in
    operation. Electric heat in the form or immersion
    and convection heaters can keep water temperature
    up during shutdowns.
  • An electric heater may be installed in the pump
    circuit to prevent freezing.
  • Hot water or steam may be used to prevent
    reservoir freeze-up. Pipes may require insulation
    or electric heater tape.

75
Screens for Cooling Towers
13.3.7
  • Use only coarse screens on pump inlets. All
    suction lines must be below water level in the
    cooling tower or air may enter the suction line,
    causing drop in pump volume and damage.
  • Pump outlets require fine screens. The water pump
    should push water through the system to prevent
    low water pressures in the condenser tubes or
    pipes.
  • See manufacturers literature for details of
    tower sizes and capacities.

76
Cooling Towers andLegionnaires Disease
13.3.7
  • Legionella Pneumophila is a bacterium that causes
    Legionnaires disease.
  • First found in 1976 at a Legionnaires convention
    in Philadelphia.
  • During convention, over two hundred people became
    ill and thirty-four people died.
  • Symptoms include headache, high fever, and
    respiratory problems.
  • Disease is caused by contaminated cooling water
    from a cooling tower. Bacteria grow in stagnant
    cooling-tower water and are then transferred from
    the water sprays into the buildings air
    conditioning ductwork.

77
Cooling Towers andLegionnaires Diseasecontinued
13.3.7
  • Preventive measures include placing cooling
    towers downwind from buildings and ductwork, and
    periodic disinfecting of cooling towers.
  • When bacteria count is high, action must be taken
    to reduce levels.
  • Check state and local requirements for testing
    procedures.

78
Evaporative Condensers
13.3.8
  • The evaporative condenser system carries
    refrigerant into a condenser.
  • An enclosure much like a cooling tower, and is
    usually mounted outdoors.
  • Water is sprayed or drips over the condenser,
    cooling it.
  • Water cycle is in the condenser cabinet only.

79
Evaporative Condenserscontinued
13.3.8
  • Variety of systems are available including one
    that pumps water to a trough above the condenser.
  • The water drips over coils as air is forced
    through them. A thermostat can be used to control
    the water flow.
  • A fan blows air over the condenser whenever the
    condenser is operating.
  • The condenser is cooled by air alone until the
    condenser temperature reaches 80ºF (26.7ºC) or
    more. A thermostat then initiates water cooling.

80
Evaporative Condenserscontinued
13.3.8
  • One method subcools the refrigerant as it leaves
    the receiver.
  • The liquid line goes through the evaporative
    condenser.
  • Temperature of the refrigerant can be dropped
    10ºF (6ºC) by subcooling.
  • When the temperature reaches 45ºF (7.2ºC) or
    lower, the water is shut off.
  • The condenser can still carry the load as an
    air-cooled condenser.

81
Liquid Receiver
13.3.9
  • The liquid receiver is a welded steel tank with
    two service valves.
  • One service valve is mounted between the liquid
    receiver and the condenser. The other is located
    between the receiver and the liquid line.
  • Receivers should have safety devices, minimally a
    thermal release plug. Some receivers should have
    both thermal and pressure releases. A special
    line should be installed on relief valves to the
    refrigerant recovery system.

82
Liquid Receivercontinued
13.3.9
  • Receivers may be mounted vertically or
    horizontally. The horizontal style hangs
    underneath the compressor and motor frame.
  • Some receivers have a sight glass, magnet floats,
    or valves for determining level of liquid
    refrigerant. The receiver should be able to hold
    the entire refrigerant charge in the system.

83
Liquid Receiver
13.3.9
84
Commercial Evaporators
13.4
  • Divided into two main groups
  • Those used for cooling air in turn, the air
    cools the contents of the cabinet.
  • Those submerged in a liquid, such as brine or a
    beverage.
  • Evaporators for cooling are of two primary types
  • Natural convection.
  • Forced convection.

85
Commercial Evaporatorscontinued
13.4
  • Natural air convection evaporators
  • Air circulation depends on gravitational or
    thermal circulation.
  • Three classes of natural convection, air cooling
    evaporators
  • Frosting.
  • Defrosting.
  • Nonfrosting.
  • Conditions (temperature range, temperature
    difference between evaporator and cabinet) under
    which an evaporator must work determine its
    classification.

86
Frosting Evaporators
13.4.1
  • Frequently used in low temperature and frozen
    food fixtures.
  • Builds up frost continuously when operating.
  • Operates at 5ºF (15ºC) refrigerant temperature
    cut-in.
  • Machine must shut down from time to time to get
    rid of frost.
  • Frost on evaporator comes from moisture in air.
    Therefore, air in cabinet is dry.
  • As frost grows thicker, cooling efficiency of
    evaporator is reduced.

87
Defrosting Cycle Evaporators
13.4.2
  • While condenser is running, temperature of the
    evaporator is low, causing frost to accumulate on
    it.
  • When compressor shuts off, the coil warms above
    32ºF (0ºC). The frost melts.
  • While compressor is running, the evaporator will
    remain at 20ºF to 22ºF (6.6ºC to 5.6ºC).

88
Defrosting Cycle Evaporatorscontinued
13.4.2
  • This defrosting process is called air defrosting.
    It clears the evaporator surfaces of frost and
    provides efficient heat transfer.
  • It maintains a high relative humidity of 9095.
  • This sacrifices temperature differences between
    the evaporator refrigerant and the air in the
    cabinet.
  • Greater evaporator area is needed to make up for
    this loss.

89
Defrosting Cycle Evaporatorscontinued
13.4.2
  • Often present problems. Top of evaporator may
    defrost and moisture flows down the surface,
    freezing on the lower parts of the evaporator.
  • This ice accumulation will block air circulation
    around the evaporator and interfere with proper
    refrigeration.
  • The drier coil prevents blockage.

90
Nonfrosting Evaporators
13.4.3
  • Operate at temperatures not much below 32ºF
    (0ºC), so frost does not form on evaporator.
  • Evaporators operate at temperature of 33ºF
    (0.6ºC) to 34ºF (1.1ºC). Refrigerant temperature
    inside the evaporator will be around 20ºF
    (6.7ºC) to 22ºF (5.6ºC).
  • Since they do not frost over, little moisture
    from inside the cabinet is lost. Therefore, a
    relative humidity (RH) of 75 to 85 can be
    maintained in cabinet, keeping produce fresh.

91
Nonfrosting Evaporatorscontinued
13.4.3
Fin-Type
Plate-Type
Two types of evaporator construction are used.
92
Forced Circulation Evaporator
13.4.4
  • A compact arrangement of refrigerant-cooled tubes
    and fins. A fan driven by an electric motor blows
    air over them.
  • Evaporator and fan are usually enclosed in a
    metal housing.

93
Forced Circulation Evaporatorcontinued
13.4.4
  • Forced circulation evaporators tend to cause
    rapid dehydration of food. Drying can be
    minimized if the evaporator is large.
  • It should operate at a small temperature
    difference 10ºF to 12ºF (6ºC to 7ºC).
  • Air should be circulated slowly.

94
Forced Circulation Evaporatorcontinued
13.4.4
Fan motor can be any size and may run
continuously. Refrigerant temperature is usually
kept quite low. Rapid air circulation keeps
evaporator from frosting up. Considerable
sweating occurs so drainage must be provided.
95
Forced Circulation Evaporatorcontinued
13.4.4
  • Blower evaporators operated at low refrigerant
    temperatures need special defrosting care. Frost
    or ice may interfere drastically with heat
    transfer.
  • A microprocessor controls the operation of the
    expansion valve, defrost system, and fan.

96
Forced Circulation Evaporatorcontinued
13.4.4
A condensate pump may be used to remove
condensate. The pump is mounted on the drain and
is self-priming. It uses 10W and operates
continuously. It may also be used for pumping
slightly acidic condensate produced by
high-efficiency gas furnaces.
97
Forced Circulation Evaporatorcontinued
13.4.4
Many evaporators use a hot gas bypass system. It
maintains above-freezing temperatures when
cooling load is decreased. Hot gas is provided to
prevent evaporator frosting during low head
conditions.
98
Liquid-Cooling Evaporator
13.4.5
  • Used to refrigerate drinks, water, and beverages.
  • Three types of cooling evaporators are used
  • Bottled liquids.
  • Liquids under atmospheric pressure.
  • Liquids under pressure.

99
Immersed Evaporator (Brine)
13.4.5
  • Evaporator mounted inside the liquid being cooled
    is referred to as an immersed evaporator.
  • Usually a small, plain tube evaporator is used.
  • Immersion makes the evaporator more efficient.
    Liquids transfer heat to metals faster than air.
    An efficiency ratio of 501 to 1001 is common.
  • A submerged evaporator can remove 50 to 100 Btu
    per hour, per degree temperature difference, per
    square foot of evaporator surface.
  • Air-cooling evaporators can only remove 1 Btu
    under the same conditions.

100
Immersed Evaporator(Sweet Water)
13.4.5
  • Immerses the evaporator in ordinary tap water
    called a sweet water bath.
  • Allows the sweet water to freeze around the
    evaporator during the nonload period.
  • Light ice accumulation acts as a reserve of
    refrigeration.

101
Immersed Evaporator(Sweet Water)continued
13.4.5
  • Usually have a thermostatic expansion valve
    refrigerant control. Evaporator should reach
    bottom if the bath temperature is to be less than
    39.1ºF (3.9ºC).
  • Water, as it cools from 39.1ºF to 32ºF (3.9ºC to
    0ºC) expands and rises. Therefore, the coldest
    water is on top.

102
Pressure (Beverage) Evaporators
13.4.5
  • Liquid refrigerant is carried in a tube submerged
    in the beverage to be cooled. The beverage itself
    is under pressure. The temperature of the
    beverage must never be low enough to freeze to
    any extent.
  • An all-metal beverage evaporator takes advantage
    of the high heat conductivity of aluminum. Two
    separate copper tubes are coiled in a helix
    (spiral) design. They are then molded in a hollow
    cylinder of aluminum.
  • Refrigerant is evaporated in one coil. Heat will
    then flow from the liquid in the other coil.
    Coolers may have three or more separate tubes
    enclosed by aluminum when more than one liquid
    must be cooled.

103
Ice Cube Maker Evaporator
13.4.6
  • To create flaked ice, the following process
    occurs
  • Water is made to flow over an evaporator shaped
    like a cylinder. The evaporator surface is cold,
    9ºF (12.7ºC), so the water freezes rapidly.

104
Ice Cube Maker Evaporatorcontinued
13.4.6
  • To create flaked ice, the following process
    occurs (continued)
  • A heavy steel auger is driven by an electric
    motor. It cuts and scrapes the ice from the
    surface.

105
Ice Cube Maker Evaporatorcontinued
13.4.6
  • To create flaked ice, the following process
    occurs (continued)
  • Float level control of the water is provided. A
    shutoff mechanism is located in the storage bin
    halts ice making when bin is full.

106
Ice Cube Maker Evaporatorcontinued
13.4.6
Square or cylindrically shaped cubes may be
formed by using a tube-within-a-tube.
107
Ice Cube Maker Evaporatorcontinued
13.4.6
The water and ice circuit of an automatic ice
cube maker includes an inlet water strainer,
water valve, evaporators, and ice forming tubes.
108
Ice Cube Maker Evaporatorcontinued
13.4.6
  • Ice is harvested when a hot gas solenoid valve is
    energized allowing high-pressure vapor to
    enterthe evaporator.

109
Ice Cube Maker Evaporatorcontinued
13.4.6
  • Small commercial ice cube makers are often used
    in fast-food restaurants.
  • Basic components
  • Evaporator.
  • Automatic expansion valve.
  • Gearbox.
  • Water reservoir.

110
Ice Cube Maker Evaporatorcontinued
13.4.6
111
Ice Cube Maker Evaporatorcontinued
13.4.6
  • Process for small commercial ice cube maker
  • Water enters at water inlet and goes through
    water supply assembly.
  • Water enters the bottom of the evaporator and
    freezes on the evaporator cylinder.
  • The auger inside the freezer evaporator moves the
    ice upward into the storage bin.
  • When the bin is filled, the ice-making switch
    shuts off the flow of ice.

112
Ice Cube Maker Evaporatorcontinued
13.4.6
  • Caution It is very important to follow the
    plumbing code in piping away the drain water. A
    loss of water pressure in the buildings water
    supply could cause reverse siphoning. This may
    result in contamination of the drinking water in
    the building.

113
Freezing Dispenser Evaporators
13.4.8
The freezing evaporator is a cylinder in which a
motor-driven dasher churns the mix. The
refrigerant then travels to a premix storage
compartment.
114
Freezing Dispenser Evaporatorscontinued
13.4.8
A serviceable hermetic motor compressor, a
water-cooled condenser, and a freezer-mixing
blade drive motor, are components of a
freezer-dispenser.
115
Evaporator Defrosting
13.4.9
  • Many evaporators operate at temperatures below
    freezing. Low temperatures and small fin spacing
    make frequent defrosting necessary.
  • Defrosting is usually automatic.
  • Some evaporators defrost during each off part
    of the cycle.
  • Others use a timer control.

116
Evaporator Defrostingcontinued
13.4.9
  • Six defrosting methods
  • Hot refrigerant vapor system.
  • Nonfreezing solution system.
  • Water system.
  • Electric heater system.
  • Reverse cycle defrost system.
  • Warm air system.
  • These defrosting methods either heat the
    evaporator from the inside or outside to melt the
    frost. It is important to clean the evaporator,
    drain pans, and drain lines frequently.

117
Hot Gas Defrost System
13.4.9
  • Hot refrigerant vapor is pumped directly through
    the evaporator tubing. System has a refrigerant
    line running directly from the compressor
    discharge line to the evaporator.
  • Line may be connected between the thermostatic
    expansion valve or the capillary tube and the
    evaporator.
  • Line is opened and closed by a solenoid shutoff
    valve.

118
Hot Gas Defrost Systemcontinued
13.4.9
  • At a predetermined time, when the system is not
    in use, a timer closes the circuit.
  • Compressor starts, opens the solenoid valve, and
    stops the evaporator fan motors.
  • Hot compressed vapor rushes through the
    evaporator and warms it.
  • It then returns to the compressor along the
    suction line.

119
Hot Gas Defrost Systemcontinued
13.4.9
120
Hot Gas Defrost Systemcontinued
13.4.9
  • Defrost process usually takes 5 to 10 minutes.
  • Defrost water must be kept from freezing in the
    drain pan and tube.
  • A hot gas defrost line may be installed under the
    drain pan and pipe or a small electric heater can
    be installed here.

121
Evaporation of Condensate during Defrost Cycle
13.4.9
  • A defrost bypass places some hot gas into the
    suction line to vaporize any liquid refrigerant.
  • A TEV is mounted on a line between the liquid
    line and the suction line.
  • The sensing bulb is mounted on the suction line.
  • If gas returning to compressor becomes warm, TEV
    will open.
  • Liquid refrigerant mixes with hot bypass gas.
  • Mix is kept between 45ºF and 65ºF (7.2ºC and
    18.3ºC).
  • A solenoid valve in the bypass TEV inlet shuts
    off the TEV when the system is operating normally
    or on a full load.

122
Evaporation of Condensate during Defrost
Cyclecontinued
13.4.9
  • Heat is applied to vaporize the returning
    refrigerant.
  • A blower-evaporator may be installed in
    connection with suction line, allowing only vapor
    to return to the compressor.
  • The blower only works when unit is on defrost. An
    accumulator mounted in the suction line will trap
    liquid refrigerant and vaporize it prior to it
    reaching the compressor.

123
Evaporation of Condensate during Defrost
Cyclecontinued
13.4.9
124
Evaporation of Condensate during Defrost
Cyclecontinued
13.4.9
  • Sometimes hot gas is fed backwards into the
    evaporator.
  • Condensed refrigerant bypasses the TEV by means
    of a check valve, forcing the liquid into the
    receiver by way of the liquid line.

125
Evaporation of Condensate during Defrost
Cyclecontinued
13.4.9
126
Hot Gas Cooling Cycle
13.4.9
  • Refrigerant that condenses during the defrost
    cycle should be evaporated.
  • During the defrost cycle, hot gas from the
    compressor and receiver travel back to the
    evaporator by way of the suction line.
  • Condensed liquid refrigerant is traveling around
    TEVs by way of check valve and is returning to
    the receiver through the liquid line.
  • A heater in the receiver provides more hot gas
    for defrosting.

127
Hot Gas Bypass Valves
13.4.9
  • Pressure Operated
  • Connected to the suction line.
  • Valve is adjusted by low-side pressure of the
    vapor going to the compressor.
  • Opens wider as suction pressure drops and starts
    to close as low-side pressure rises.

128
Hot Gas Bypass Valvescontinued
13.4.9
  • Pilot Controlled Valve
  • Dependent on suction line pressure.

129
Multiple Systems withSeveral Evaporators
13.4.9
  • Evaporators should be defrosted one at a time.
  • Others are used to evaporate the liquid from the
    defrosting evaporator.
  • This ensures no liquid refrigerant will reach the
    low side of the motor compressor.

130
Multiple Systems withSeveral Evaporators
13.4.9
131
Multiple Systems withSeveral Evaporatorscontinue
d
13.4.9
  • Valve may be used in place of two suction line
    valves on each evaporator.
  • Evaporator gas at point A travels into valve
    during normal operation. Hot gas travels out of
    valve at A during defrosting.
  • When pilot solenoid valve at A is energized, it
    opens, allowing high-pressure gas from discharge
    connection to push down on the double valve.
  • This closes the top valve B, stopping flow from
    evaporator into suction line, opening bottom
    valve C. High-pressure hot gas flows from
    discharge connection up into the evaporator.

132
Multiple Systems withSeveral Evaporatorscontinue
d
13.4.9
133
Nonfreezing SolutionDefrost System
13.4.9
  • The hot fluid defrost system has a container in
    which brine is stored.
  • Refrigerant vapor from the compressor is pumped
    through heat storage container then into the
    condenser.
  • The brine may be electrically heated during the
    normal running part of refrigerating cycle.
  • When system shuts off, the defrost timer closes a
    solenoid valve. This is the beginning of the
    defrost cycle. The evaporator fan is usually shut
    off.
  • The brine solution is pumped through its own
    piping along the drain line, drain pan, and
    evaporator. It then returns to its container.

134
Nonfreezing SolutionDefrost System
13.4.9
135
Water Defrost Systems
13.4.9
  • Runs warm tap water over evaporator when system
    is off. Done manually or automatically.
  • Water may be sprayed over evaporator, or fed to a
    pan located over the evaporator. Holes in the pan
    feed the water evenly over the evaporator.
    During the operation, evaporator louvers are
    closed.
  • Water lines must be completely drained before
    unit is turned on, or water will freeze. Electric
    timer provides automatic operation.
  • Pump may be employed to recirculate brine.
    Eliminator plates are needed to prevent brine
    spray from passing into the refrigerated space.

136
Water Defrost Systems
13.4.9
137
Electric Heater Defrost System
13.4.9
  • Popular for defrosting low-temperature
    evaporators. Heating coils are installed in the
    evaporator, around it, or within the refrigerant
    passages.
  • May use resistance wire heating elements mounted
    underneath the evaporator, under the drain pan,
    and along the drainpipe.
  • A timer stops the refrigeration unit and closes
    the liquid line.
  • It then pumps the refrigerant out of the
    evaporator. Blowers and electric heaters are
    turned on.
  • Heaters melt the frost from the evaporator and
    water drains away.

138
Pump Down
13.4.9
  • A control system.
  • An extra relay is wired into the compressor
    circuit in parallel to the normal relay. It is
    connected to the thermostat circuit. The extra
    relay operates the start button on the normal
    starting relay. The compressor cannot restart
    until the thermostat points close.
  • The thermostat operates a solenoid in the liquid
    line. At same time, a low-pressure switch
    operates the compressor, to prevent flow of
    liquid refrigerant from the evaporator to the
    compressor.

139
Pump Downcontinued
13.4.9
  • When thermostat is satisfied, it opens and liquid
    line solenoid closes.
  • Compressor continues to run, removing refrigerant
    vapor from the evaporator and suction line.
  • When proper low-side pressure is reached,
    low-pressure switch opens and the compressor
    stops. Crankcase heaters may be needed to drive
    the refrigerant during the off cycle.

140
Optional ElectricDefrost Systems
13.4.9
  • An immersion electric heater heats a separate
    charge of refrigerant. The warm refrigerant
    circulates in the evaporator while the unit is
    turned off, defrosting the system.

141
Optional ElectricDefrost Systemscontinued
13.4.9
  • In a double tube evaporator system, refrigerant
    passes through the passageway between the tubes
    during normal refrigeration.
  • Electric heating elements are inserted in the
    center tube.
  • In the defrost operation, the system is stopped
    and the electric heating elements are turned on.
    The evaporator tubes cause defrosting from the
    inside.

142
Reverse Cycle Defrost System
13.4.9
  • Evaporators are defrosted by reversing the flow
    of refrigerant, causing the evaporator to become
    the condenser and the condenser an evaporator.
  • The evaporator, functioning as a condenser, melts
    the frost. A four-way valve handles the reversing.

143
Warm Air Defrosting
13.4.9
  • Cabinet air is used for defrosting if it is at
    the correct temperature.
  • Cycles must be frequent enough and long enough to
    defrost the evaporator completely.
  • Outside air may be used for defrosting, using a
    controlled duct system with blowers and a fan.

144
Heat Exchangers
13.4.10
  • Heat exchanger mounted in the suction and liquid
    line has three advantages
  • Subcools the liquid refrigerant and increases
    operating efficiency.
  • Reduces flash gas in the liquid line.
  • Reduces liquid refrigerant in the suction line.

145
Heat Exchangerscontinued
13.4.10
Heat is transferred from the warmer liquid in the
liquid line to the cool vapor coming from the
evaporator.
146
Heat Exchangerscontinued
13.4.10
  • Reduces flash vapor (flash gas). Liquid cooled by
    10ºF to 20ºF (5ºC to 11ºC) at prevailing head
    pressure absorbs more latent heat. This occurs as
    it changes to a vapor in the evaporator. This
    subcooled liquid reduces the chance of flash gas
    forming in the liquid line.
  • Prevents sweat backs or frost backs on the
    suction line. Low-temperature liquid refrigerant
    in the returning suction vapor will evaporate in
    the heat exchanger, as the refrigerant absorbs
    heat from the liquid line.

147
Questions
  • What is subcooling and where does it take place
    in a refrigeration system?

Subcooling is lowering the temperature of a
liquid below its saturation temperature. It takes
place at the end of the condenser and in the
liquid line.
  • What special provisions must be made to outdoor
    refrigeration condensing units?

They must have a head pressure control and a
crankcase heater.
  • Name three types of head pressure controls.

Airflow louvers, cycling of condenser fans, and a
head pressure control valve.
148
Questionscontinued
  • How does a head pressure control valve maintain
    high-side pressures?

It bypasses hot gas into the receiver and floods
the condenser.
  • Name three types of water-cooled condensers.

Tube-within-a-tube, shell and tube, and shell and
coil.
  • How does the counter-flow principle work for a
    tube-within-a-tube condenser?

The coldest (entering) water is in contact with
the coldest (leaving) refrigerant.
149
Questionscontinued
  • Name two factors that affect the operation of a
    cooling tower.

Wetbulb temperature and water quality.
  • What is the purpose of a float control valve in a
    cooling tower?

It is used to add makeup water to the system when
needed.
  • If cooling towers are not properly maintained,
    certain bacteria in the water can cause a fatal
    disease called _________________.

Legionnaires disease
150
Questionscontinued
  • Which type of condenser uses both airflow and
    water flow for heat transfer?

An evaporative condenser.
  • What is the purpose of a receiver?

It is a storage tank that holds the systems
refrigerant charge.
  • Name two types of direct expansion evaporators.

Natural and forced convection evaporators.
151
Questionscontinued
  • Name two types of defrost systems.

Electric and hot gas defrost.
  • On a hot gas defrost system, which component must
    be de-energized during the defrost mode?

The evaporator fans.
  • Which component must be added to the suction line
    on a hot gas defrost system?

An accumulator.
152
Questionscontinued
  • With an automatic pumpdown system, which
    component does the temperature control operate?

The liquid line solenoid.
  • On an automatic pumpdown system, which component
    does the low-pressure control operate?

The compressor.
  • Which type of defrost system uses a four-way
    solenoid valve?

A reverse cycle defrost system.
153
Questionscontinued
  • What is the purpose of a heat exchanger?

It is used to subcool the liquid refrigerant
before the expansion valve to increase system
efficiency.
154
Chapter 13
COMMERCIAL SYSTEMSCONTROLS MODULE
155
Refrigerant Controls
13.5
  • Refrigerant Controls
  • A single evaporator and condenser system
    typically use one of the following refrigerant
    controls
  • Thermostatic expansion valves.
  • Automatic expansion valves.
  • High-side floats.
  • Low-side floats.
  • Capillary tubes.
  • Systems with more than one evaporator usually use
    multiple thermostatic expansion valves or
    low-side floats.

156
Motor Controls
13.6
  • Commercial refrigeration uses two basic types
  • Thermostatic.
  • Pressure.
  • Large systems use magnetic starters operated by
    motor controls.

157
Motor Controlscontinued
13.6
  • In multiple evaporator commercial systems,
    pressure motor controls are often used because
  • Low-side pressure is an indication of the
    temperature in the evaporators.
  • One control works well regardless of the number
    of evaporators connected to it.
  • Pressure motor controls provide variety of range
    and differential adjustments.

158
Motor Controlscontinued
13.6
159
Motor Controlscontinued
13.6
There are recommended average pressure motor
control settings.
160
Pressure Motor Control
13.6.1
  • Usually mounted on the condenser.
  • Operated by low-side pressure.
  • Control should be placed about 10' to 15' (3m to
    4.6m) from the compressor.
  • Range settings vary with application. Cut-out
    pressure should be set about 10ºF (6ºC) below the
    desired evaporator outside surface temperature.
    Cut-in pressure should be about the same as the
    highest allowable evaporator temperature.

161
Pressure Motor Control
13.6.1
162
Pressure Motor Controlcontinued
13.6.1
  • Differential setting will vary, depending on the
    temperature accuracy required.
  • A wide pressure difference will allow some
    variation in cabinet temperature and lengthen the
    operating cycle interval of the condenser. (The
    compressor will not run as often.)
  • A differential set to close limits will maintain
    a more uniform cabinet temperature, and will
    shorten the cycling interval of the condenser.
    (The unit will run more often.)

163
Pressure Motor Controlcontinued
13.6.1
  • Pressure difference between cut-in and cut-out
    point varies with the refrigerant used.
  • Common pressure difference is 20 psi for R-12,22
    psi for R-22, 16 psi for R-500, and 25 psi for
    R-502.

164
Thermostatic Motor Control
13.6.2
  • Similar to pressure motor control in design, but
    sensing bulb and capillary tube are different.
  • Used in large single installations.
  • If used in multiple evaporator system, is used to
    sense warmest evaporator case.

165
Thermostatic Motor Controlcontinued
13.6.2
  • Sized so when the unit shuts off the compressor,
    there is sufficient cooling in the colder
    conditioned space. Allows each separate cabinet
    in a multiple installation to be controlled.
  • May have a very close differential, such as 1ºF
    (0.5ºC), in display cases, frost alarms, liquid
    chillers, and refrigerated trucks.
  • May have double-throw contacts and operate other
    devices (fans, defrost systems).

166
Safety Motor Controls
13.6.3
  • Commercial systems may use high-pressure safety
    cutout and oil pressure safety cutout.
  • High-pressure safety device
  • A bellows built into the control and connected to
    the high-pressure side of the system.
  • Often connected to cylinder head for easy
    disconnecting of the control from the system.

167
Safety Motor Controlscontinued
13.6.3
  • High-pressure safety device
  • A bellows built into the control and connected to
    the high-pressure side of the system.
  • Often connected to cylinder head for easy
    disconnecting of the control from the system.

168
Safety Motor Controlscontinued
13.6.3
  • High-pressure safety device
  • When head pressure becomes too high, bellows will
    expand. The bellows are attached to plunger that
    is pushed against the switch, shutting off the
    motor.
  • High-pressure safety device prevents buildup of
    dangerous pressures with the system.
  • Usually set to cut out at 20 above normal head
    pressure.
  • R-12 set at 150-160 psi, R-22 set at 260-270 psi,
    R-502 set at 280-290 psi, and R-500 set at
    190-200 psi.

169
Safety Motor Controlscontinued
13.6.3
  • Oil pressure safety cutout
  • Shuts off electrical power if oil pressure fails
    or drops below normal.
  • A differential control, using two bellows.
  • One bellows responds to crank pressure, the other
    to oil pressure.
  • Oil pressure must always be above the crank
    pressure for oil to flow.

170
Safety Motor Controlscontinued
13.6.3
  • Oil pressure safety cutout (continued)
  • Control opens circuit if pressure difference
    between two bellows drops below required oil
    pressure.
  • Used in large commercial systems. Control points
    may be in compressor motor circuit or through a
    bimetal strip.

171
Safety Motor Controlscontinued
13.6.3
  • Oil pressure safety cutout (continued)
  • Refrigerant level kept within safe limits by
    float switch. If level is too high, float switch
    closes an electrical circuit. If level is too
    low, actuates a circuit allowing refrigerant to
    flow into the system.

172
Motor Contactors
13.6.4
  • Allows commercial controls to handle larger
    motors (larger loads) using a magnetic starter
    (contactor).
  • An electromagnetic device whose magnetism is
    controlled by the electricity that flows through
    the motor control.
  • Magnetism attracts an armature. The armature
    moves, closing large contact points.

173
Motor Contactorscontinued
13.6.4
  • Contact points safely carry large current flow
    needed for large motors.
  • Mounted in an approved metal box with safety
    access door. May incorporate manual shutoff
    switch, fuses, and overload thermal safety
    breaker switch.

174
Motor Contactorscontinued
13.6.4
  • Mounted in an approved metal box with safety
    access door. May incorporate manual shutoff
    switch, fuses, and overload thermal safety
    breaker switch.
  • Safety switch operated by heating element in
    motor circuit black lead inside the contactor box
    or starter.

175
Motor Contactorscontinued
13.6.4
If motor demands too m
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