Heating and Air Conditioning I

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Heating and Air Conditioning I

• Principles of Heating, Ventilating and Air
  Conditioning
• R.H. Howell, H.J. Sauer, and W.J. Coad
• ASHRAE, 2005

basic textbook/reference material For ME 421 John
P. Renie Adjunct Professor Spring 2009
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Chapter 3 Basic HVAC Calculations

• Applying Thermodynamics to HVAC Processes
• Looking at a simplified (but complete)
  air-conditioning system
• Terminology qsensible, mwater, qL, hw, solar
  gains
• Look at first law of thermodynamics (energy) and
  conservation of mass
• Air is removed from the room, returned to the
  air-conditioning apparatus where it is
  reconditioned, and then supplied again to the
  room.
• Many cases, it is mixed with outside air required
  for ventilation
• Outdoor air (o) is mixed with return air (r) from
  the room and enters the apparatus at condition
  (m)
• Air flows through the conditioner and is supplied
  to the space (s).
• The air supplied to the space absorbs heat qs and
  moisture mw, and the cycle continues.

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Chapter 3 Basic HVAC Calculations

• Applying Thermodynamics to HVAC Processes

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Chapter 3 Basic HVAC Calculations

• Applying Thermodynamics to HVAC Processes

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Chapter 3 Basic HVAC Calculations

• Applying Thermodynamics to HVAC Processes

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Chapter 3 Basic HVAC Calculations

• Absorption of Space Heat and Moisture Gains
• AC usually reduces to determining the quantity of
  moist air that must supplied and the condition it
  must have to remove given amounts of energy and
  water from the space to be withdrawn at a
  specified condition.
• Sensible heat gain addition of energy only
  not wrt water

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Chapter 3 Basic HVAC Calculations

• Heating or Cooling of Air without moisture gain
  or loss straight line on psychrometric chart
  since humidity ratio is constant

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Chapter 3 Basic HVAC Calculations

• Cooling and Dehumidifying Air
• Moist air brought down below its dew point
  temperature some of the water will condense and
  leaves the air stream
• Assume condensed water is cooled to the final air
  temperature before draining from the system

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Chapter 3 Basic HVAC Calculations

• Cooling and Dehumidifying Air

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Chapter 3 Basic HVAC Calculations

• Cooling and Dehumidifying Air
• Moist air brought down below its dew point
  temperature some of the water will condense and
  leaves the air stream
• Assume condensed water is cooled to the final air
  temperature before draining from the system
• Cooling and dehumidifying process involves both
  sensible heat transfer and latent heat transfer
  where sensible heat transfer is associated with
  the decrease in dry-bulb temperature and the
  latent heat transfer is associated with the
  decrease in humidity ratio.

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Chapter 3 Basic HVAC Calculations

• Heating and Humidifying Air

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Chapter 3 Basic HVAC Calculations

• Adiabatic Mixing of Two Streams of Air

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Chapter 3 Basic HVAC Calculations

• Adiabatic Mixing of Moist Air with Injected Water

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Chapter 3 Basic HVAC Calculations

• Moving Air

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Chapter 3 Basic HVAC Calculations

• Approximate Equations Using Volume Flow Rates
• Since volumes of air change need to make
  calculations with mass of dry air instead of
  volume. But volumetric flow rates define
  selection of fans, ducts, coils, etc.
• Use volume while still considering mass by using
  volume rates based on standard air conditions
• Dry air at 20 oC and 101.325 kPa (68 oF and 14.7
  psia)
• Density is 1.204 kg/m3 (0.075 lb/ft3) dry air
• Specific volume is 0.83 m3/kg (13.3 ft3/lb) dry
  air
• Saturated air at 15 oC has about same density and
  volume
• Need to convert actual volumetric flow conditions
  to standard
• Say you need 1,000 cfm outside air rate at
  standard conditions
• Outside measured at 35 oC dry bulb and 23.8 oC
  wet bulb corresponding to a specific volume of
  14.3 ft3/lb.
• Then, the actual flow rate would be 1,000
  (14.3/13.3) 1,080 cfm
• 1,000/13.3 1,080/14.3 mass rate (lb/min) of
  moist air

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Chapter 3 Basic HVAC Calculations

• Sensible heat gain corresponding to the change of
  dry-bulb temperature for a given airflow (at
  standard conditions)

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Chapter 3 Basic HVAC Calculations

• Latent heat gain corresponding to the change of
  humidity ratio W for a given airflow (at standard
  conditions).
• The latent heat gain in Watts (Btu/h) as a result
  of a difference in humidity ratio DW between the
  incoming and leaving air flowing at standard
  conditions.

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Chapter 3 Basic HVAC Calculations

• Total heat gain corresponding to the change of
  dry-bulb temperature and humidity ratio W for a
  given airflow (at standard conditions).
• The total heat gain in Watts (Btu/h) as a result
  of a difference in enthalpy Dh between the
  incoming and leaving air flowing at standard
  conditions.

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Chapter 3 Basic HVAC Calculations

• Total heat gain corresponding to the change of
  dry-bulb temperature and humidity ratio W for a
  given airflow (at standard conditions).

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Chapter 3 Basic HVAC Calculations

• Single-Path Systems
• Simplest form of all-air HVAC system serving a
  single temperature control zone
• Responds to one set of space conditions, where
  conditions vary uniformly and the load is
  stable.
• Schematic of system return fan necessary under
  certain conditions of Dp.
• Need for reheat necessary to control humidity
  independent of the temperature requirements.
• Equations for single-path systems air supplied
  must be adequate to take care of each rooms peak
  load conditions. Peak loads may be governed by
  sensible or latent room cooling loads, heating
  loads, outdoor air requirements, air motion, and
  exhaust. let us look at each of these loads and
  what air volume is required to satisfy these
  demands.

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Chapter 3 Basic HVAC Calculations

• Single-Path Systems - schematic

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Chapter 3 Basic HVAC Calculations

• Single-Path Systems equations for supply air

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Chapter 3 Basic HVAC Calculations

• Single-Path Systems equations for supply air

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Chapter 3 Basic HVAC Calculations

• Single-Path Systems supply air for ventilation
• Supply air for ventilation needed when the
  amount of outside air is not adequate
• Supply air not adequate for the amount of exhaust
  makeup required no return air comes from the
  room and entire volume of make-up ventilation air
  becomes an outside air burden to system
• Desired air exchange rate not satisfied supply
  air is determined
• Desired air movement not satisfied, based on area
  index parameter, K.
• Each of the above conditions are used at
  different times Case 1 when outside air
  governs, Cases 3 and 4 when air movement governs,
  and Case 2 when exhaust governs.

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Chapter 3 Basic HVAC Calculations

• Single-Path Systems Example Problem 3-3

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Chapter 3 Basic HVAC Calculations

• Single-Path Systems Example Problem 3-3

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Chapter 3 Basic HVAC Calculations

• Single-Path Systems Example Problem 3-3

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Chapter 3 Basic HVAC Calculations

• Single-Path Systems Example Problem 3-3

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Chapter 3 Basic HVAC Calculations

• Single-Path Systems Cycle Diagram

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Chapter 3 Basic HVAC Calculations

• Single-Path Systems Cycle Diagram
• Each state point is identified both in summer and
  winter
• Change of Dt is result of sensible heat loss or
  gain, qS
• Change in DW is result of latent heat loss or
  gain, gL
• All return air is assumed to pass from the room
  through a hung-ceiling return air plenum
• Supply air CFMS at the fan discharge temperature
  tsf (summer mode) absorbs the transmitted supply
  duct heat qsd and supply air fan velocity
  pressure energy qsf,vp thereby raising the
  temperature to ts

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Chapter 3 Basic HVAC Calculations

• Single-Path Systems Cycle Diagram
• Room supply air absorbs room sensible and latent
  heat qSR and qLR along the room sensible heat
  factor (SHR) line s-R, reaching the desired room
  state, tR and WR.

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Chapter 3 Basic HVAC Calculations

• Single-Path Systems Cycle Diagram
• Room (internal) sensible loads which determine
  the CFMs consist of

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Chapter 3 Basic HVAC Calculations

• Single-Path Systems Cycle Diagram

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Chapter 3 Basic HVAC Calculations

• Single-Path Systems Psychrometric Representation

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Chapter 3 Basic HVAC Calculations

• Single-Path Systems Psychrometric Representation

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Chapter 3 Basic HVAC Calculations

• Single-Path System - Psychrometric Representation

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Chapter 3 Basic HVAC Calculations

• Single-Path System Sensible Heat Factor (Ratio)
  
• Sensible heat factor (ratio), SHF or SHR, is the
  ratio of sensible heat for a process to the total
  of sensible and latent heat for the process.
• The sensible and latent combined is referred to
  as the total heat
• On psychrometric chart, the protractor provides
  this ratio and may be used to establish the
  process line for changes in the conditions of the
  air across the room or the conditioner on the
  chart
• The supply air to a conditioned space must have
  the capabilty to offset both the rooms sensible
  and latent heat loads.
• Connecting the room and supply points with a
  straight line provides the sensible heat factor
  condition. The conditioner provides the
  simultaneous cooling and dehumidifying that
  occurs.
• Horizontal line would be SHF 0.0 (only
  sensible)
• Line with SHF 0.5 would be half sensible and
  half latent

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Chapter 3 Basic HVAC Calculations

• Single-Path System Final Example

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Chapter 3 Basic HVAC Calculations

• Single-Path System Final Example
• Sensible and latent loads given
• Room Conditions (75 oF and 55 RH) Supply at
  58 oF
• Outside Conditions 96 oF DB, 77 oF WB and 20
  of total flow

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Chapter 3 Basic HVAC Calculations

• Single-Path System Final Example

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Chapter 3 Basic HVAC Calculations

• Single-Path System Final Example

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Chapter 3 Basic HVAC Calculations

• Single-Path System Final Example

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Chapter 3 Basic HVAC Calculations

• Single-Path System Final Example

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Chapter 3 Basic HVAC Calculations

• Single-Path System Final Example