Objectives

1
Objectives

• Compare real and ideal compression process
• Learn about expansion valves (Ch. 4)
• Compare residential and commercial systems
• Introduce heat exchangers (ch.11)
• Next two weeks

2
Real vs. Ideal Compression(Example with
Reciprocating Compressor)
3
Reciprocating Compressor

• Piston compressing volume
• PVn constant C
• For all stages, if we assume no heat transfer
• Can measure n, but dependent on many factors
• Often use isentropic n in absence of better
  values
• R-12 n 1.07
• R-22 n 1.12
• R-717 n 1.29

n and volumetric efficiency ?v (book page 82-86,
Fig 4.6) Define how isentropic is our
compression
4
Expansion Valves

• Throttles the refrigerant from condenser
  temperature to evaporator temperature
• Connected to evaporator superheat
• Increased compressor power consumption
• Decreased pumping capacity
• Increased discharge temperature
• Can do it with a fixed orifice (pressure reducing
  device), but does not guarantee evaporator
  pressure

5
Thermostatic Expansion Valve (TXV)

• Variable refrigerant flow to maintain desired
  superheat

6
AEV

• Maintains constant evaporator pressure by
  increasing flow as load decreases

7
Summary

• Expansion valves make a big difference in
  refrigeration system performance
• Trade-offs
• Cost, refrigerant amount
• Complexity/moving parts

8
Refrigerants
9
What are desirable properties of refrigerants?

• Pressure and boiling point
• Critical temperature
• Latent heat of vaporization
• Heat transfer properties
• Viscosity
• Stability

10
In Addition.

• Toxicity
• Flammability
• Ozone-depletion
• Greenhouse potential
• Cost
• Leak detection
• Oil solubility
• Water solubility

11
Refrigerants

• What does R-12 mean?
• ASHRAE classifications
• From right to left ?
• fluorine atoms
• hydrogen atoms 1
• C atoms 1 (omit if zero)
• CC double bonds (omit if zero)
• B at end means bromine instead of chlorine
• a or b at end means different isomer

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Refrigerant Conventions

• Mixtures show mass fractions
• Zeotropic mixtures
• Change composition/saturation temperature as they
  change phase at a constant pressure
• Azeotropic mixtures
• Behaves as a monolithic substance
• Composition stays same as phase changes

14
Inorganic Refrigerants

• Ammonia (R717)
• Boiling point?
• Critical temp 271 F
• Freezing temp -108 F
• Latent heat of vaporization?
• Small compressors
• Excellent heat transfer capabilities
• Not particularly flammable
• But

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Carbon Dioxide (R744)

• Cheap, non-toxic, non-flammable
• Critical temp?
• Huge operating pressures

16
Water (R718)

• Two main disadvantages?
• ASHRAE Handbook of Fundamentals Ch. 20

17
Water in refrigerant

• Water Halocarbon Refrigerant (strong) acids
  or bases
• Corrosion
• Solubility
• Free water freezes on expansion valves
• Use a dryer (desiccant)
• Keep the system dry during installation/maintenanc
  e

18
Oil

• Miscible refrigerants
• High enough velocity to limit deposition
• Especially in evaporator
• Immiscible refrigerants
• Use a separator to keep oil contained in
  compressor
• Intermediate

19
The Moral of the Story

• No ideal refrigerants
• Always compromising on one or more criteria

20
Example Problem

• Explain the principle of operation of vapor
  compression based dehumidifier and show how it
  affects the indoor environment.
• If space conditions are T25ºC, RH70 and flow
  rate through humidifier is 360 m3/h, calculate T
  and RH in the dehumidifier discharge jet and
  amount of energy that this dehumidifier uses.
• Assume that
• Temperature of R22 in the evaporator is 2ºC,
• Average surface temperature of cooling coil is
  10ºC above temperature of evaporation,
• Temperature of air leaving evaporator is 15ºC,
• Temperature of condensation is 10ºC above
  temperature of air that leaves condenser,
• We have isentropic compression and compressor
  motor efficiency 80,
• Air pressure drop in evaporator is 80 Pa and in
  condenser is 50Pa,
• Fan motor efficiency of 50.

21
Coil Extended Surfaces Compact Heat Exchangers

• Fins added to refrigerant tubes
• Important parameters for heat exchange?

22
Some HX (Heat Exchanger) truths

• All of the energy that leaves/enters the
  refrigerant enters/leaves the heat transfer
  medium
• If a HX surface is not below the dew point of the
  air, you will not get any dehumidification
• Water takes time to drain off of the coil
• Heat exchanger effectivness varies greatly

23
What about compact heat exchangers?

• Analysis is very complex
• Assume flat circular-plate fin

24
Overall Heat Transfer

• Q U0A0?Tm

25
Heat Exchangers

• Parallel flow
• Counterflow
• Crossflow

Ref Incropera Dewitt (2002)
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Heat Exchanger Analysis
Counterflow
Parallel