Objectives

1
Objectives

• Solve examples
• Learn about refrigerants, compressors, and
  expansion valves (Ch. 4)
• Compare residential and commercial systems
• Introduce heat exchangers (ch.11)
• Next two weeks

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Example 1

• R-22 condensing temp of 30 C and evaporating
  temp of 0C
• Determine
• qcarnot wcarnot
• COPcarnot
• ?R

3
Example 2

• R-22 condensing temp of 60 C and evaporating
  temp of 0C
• Determine the ?R

4
Example 3

• Two stage compressor
• R-22 condensing temp of 60 C
• Evaporating temp of compressor A 30C and for
  compressor B 0C.
• Determine the ?R
• Assume that condenser temperature for compressor
  B is the same like evaporation temperature of
  compressor A

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Systems residential
Outdoor Air
Indoor Air
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System componentsLarge building system
Water to cooling tower
Water form cooling tower
35oC
25oC
Plate heat exchanger
6oC
11oC
Water to building
Water from building
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Chiller
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Compressors
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Compressor

• Workhorse of the system
• Several types all compress gas with varying
  degrees of efficiency
• Far from isentropic (our assumption earlier)
• Wshaft work done by shaft
• Welec electric power requirements

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Reciprocating compressor
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Reciprocating Compressor

• Figures 4.4, 4.6

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Reciprocating

• 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

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Rotary Compressors

• Higher efficiency, lower noise and vibration
• Cylinder rotating eccentrically in side housing

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Rotary Compressor
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Scroll Compressors

• One scroll is fixed
• The other scroll wobbles inside compressing
  refrigerant
• Often requires heat transfer from refrigerant to
  cool scrolls

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Scroll compressor

• http//www.youtube.com/watch?vf_6xolDoqs0

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Scroll Compressors

• Constant displacement
• Higher efficiency, but harder to manufacture
• Close tolerance between scrolls
• Ugly to analyze see text for details

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Screw compressor

• http//www.youtube.com/watch?vxO7IhhzImMUfeature
  related

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Screw Compressors

• Rotating meshed screws
• One or two screws

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Summary

• Many compressors available
• ASHRAE Handbook is good source of more detailed
  information
• Very large industry

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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

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Thermostatic Expansion Valve (TXV)

• Variable refrigerant flow to maintain desired
  superheat

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AEV

• Maintains constant evaporator pressure by
  increasing flow as load decreases

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Summary

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

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Refrigerants
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What are desirable properties of refrigerants?

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

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In Addition.

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

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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

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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

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Water (R718)

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

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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

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Oil

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

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The Moral of the Story

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

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HW3

• Five problems
• 1) Book 3.1,
• 2) Book 3.5,
• 3) Solve 3. 5 for ammonia,
• 4) Same like 3.5 for R22 with no intercooler
• 5) Finish example problem 3 (two cycle and two
  compressors) for ammonia.
• Deadline 03/11 in class.