MENG 4003: Sustainable Energy System Design

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MENG 4003 Sustainable Energy System Design
Lecture 3 Duct Design Methods
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Background

• Duct design involves supply and distribution of
  conditioned air (usually a mixture of return and
  outside air)
• Basic calculations assume air is incompressible
  fluid
• The equation of continuity and Bernoullis
  equation are used
• The calculation procedure is similar to analysis
  of flow in pipes
• Friction and minor losses must be evaluated

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(12.7m/s)
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Example 1

• Table 6
• 5.86 ft2 ? 843.84 inches2
• ? De 32.8
• ? Class 4
• ? side length 14x 76

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Adiabatic Flow

• In thermodynamics, an adiabatic process is a
  process in which no heat is transferred to or
  from working fluid. The term "adiabatic"
  literally means an absence of heat transfer for
  example, an adiabatic boundary is a boundary that
  is impermeable to heat transfer and the system is
  said to be adiabatically (or thermally)
  insulated. An insulated wall approximates an
  adiabatic boundary. Another example is the
  adiabatic flame temperature, which is the
  temperature that would be achieved by a flame in
  the absence of heat loss to the surroundings. An
  adiabatic process which is also reversible is
  called an isentropic process.

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Total Head

• In fluid dynamics, head is the difference in
  elevation between two points in a column of
  fluid, and the resulting pressure of the fluid at
  the lower point. It is possible to express head
  in either units of height (e.g. meters) or in
  units of pressure such as pascals (the SI unit).
  Head refers to the constant right hand side in
  the incompressible steady version of Bernoulli's
  equation.
• This is best understood by considering a
  waterwheel the head is the vertical distance
  from the top of the waterwheel to the free
  surface of the millpond.
• More generally, when considering a flow, one says
  that head is lost if energy is dissipated,
  usually through turbulence equations such as the
  Prony equation and the Darcy-Weisbach equation
  have been used to calculate the head loss due to
  friction. In the context of steam trains, one
  talks of a good head of steam, referring to the
  pressure in the boiler.
• The static head of a pump is the maximum height
  (pressure) it can deliver. The capacity of the
  pump can be read from its Q-H curve (flow vs.
  height).
• Head is used to describe the energy in
  incompressible fluids. Head has units of distance
  and equals the fluid's energy per unit weight.
  Head is useful in specifying centrifugal pumps
  because their pumping characteristics tend to be
  independent of the fluid's density.
• There are four types of head used to calculate
  the total head in and out of a pump
• Static head is due to gravitational force on a
  column of fluid.
• Velocity head is due to the motion of a fluid.
• resistance head is due to the frictional forces
  against a fluid's motion.
• Pressure head is due to other mechanical forces
  acting on a fluid

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• 1700 fpm 8.64m/s

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• Duct Research Destroys Design Myths Video Seeing
  is believing! Features the history of ductwork
  and shows which type of duct system branch
  configuration will perform best and will
  demonstrate that engineers' uses of the loss
  coefficient tables are often incorrect. The video
  includes fittings from the HVAC Duct Construction
  Standards - Metal and Flexible manual (2nd Ed.,
  1995). A fan blowing bubbles through the ductwork
  system enables the viewer to see the actual air
  flow turbulence present in duct systems mains,
  branches and elbows. VHS. 32 mins., 1996. Order
  1676, List Price 122

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Lecture

• Todays lecture will only focus on Cooling Load
  Calculations.
• Heating Load calculations are similar refer to
  references for further information
• Exam question on this topic will be based on
  cooling load calculations only
• All data tables for this topic are available on
  the web the appropriate tables will be supplied
  during the exam
• Calculations for this topic will be for Northern
  Hemisphere, Data tables for southern Hemisphere
  are available in ASHRAE

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Definitions

• A diffuser in thermodynamics is the mechanical
  device that is designed to control the
  characteristics of a fluid at the entrance to a
  thermodynamic open system. When the diffuser
  compresses, this causes the velocity to decrease.
  In other words, when the diffuser is put to work,
  the velocity of the fluid in the system
  decreases. Diffusers usually work with nozzles in
  controlling fluid flow. Flow through nozzles and
  diffusers may or may not be assumed to be
  adiabatic. Frictional effects may sometimes be
  important, but usually they are neglected.
  However, the external work transfer is always
  assumed to be zero. It is also assumed that
  changes in thermal energy are significantly
  greater than changes in potential energy and
  therefore the latter can usually be neglected for
  the purpose of analysis.
• Retrieved from "http//en.wikipedia.org/wiki/Diffu
  ser_28thermodynamics29"

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Definitions

• In thermodynamics, an adiabatic process is a
  process in which no heat is transferred to or
  from working fluid. The term "adiabatic"
  literally means an absence of heat transfer for
  example, an adiabatic boundary is a boundary that
  is impermeable to heat transfer and the system is
  said to be adiabatically (or thermally)
  insulated. An insulated wall approximates an
  adiabatic boundary. Another example is the
  adiabatic flame temperature, which is the
  temperature that would be achieved by a flame in
  the absence of heat loss to the surroundings. An
  adiabatic process which is also reversible is
  called an isentropic process.

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Definitions

• Static pressure is a term used in ventilation
  engineering, airspeed indication, fluid statics,
  hydraulics and flow measurement. It is defined as
  the potential pressure exerted in all directions
  by a fluid or gas at rest. For a fluid or gas in
  motion, static pressure is measured in a
  direction at right angles to the direction of
  flow. Thus static pressure shows a tendency to
  burst or collapse a ventilation duct, a water dam
  or balloon. Static pressure in a ventilation
  system is usually expressed in inches of water
  gauge, which can be either negative or positive.
  Other units that are used to express static
  pressure are pascals and millimeters of mercury
  (mmHg). A vacuum is a special case of static
  pressure. Fluid pressure is another case of
  static pressure.

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Definitions

• Discharge pressure (also called high side
  pressure or head pressure) is the pressure
  generated on the output side of a gas compressor
  in a refrigeration or air conditioning system.
  The discharge pressure is affected by several
  factors size and speed of the condenser fan,
  condition and cleanliness of the condenser coil
  and the size of the discharge line. An extremely
  high discharge pressure coupled with an extremely
  low suction pressure is an indicator of a
  refrigerant restriction.

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Definitions
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12.10
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