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

Example 1

• R-22 condensing temp of 30 °C and evaporating temp of 0°C

• Determinea) qcarnot wcarnot

b) COPcarnot

c) ηR

Example 2

• R-22 condensing temp of 60 °C and evaporating temp of 0°C

• Determine the ηR

Example 3

• Two stage compressor

• R-22 condensing temp of 60 °C

• Evaporating temp of compressor A 30°C and for compressor B 0°C.

• Determine the ηR

Assume that condenser temperature for compressor B is the same like evaporation temperature of compressor A

Systems: residential

Indoor Air

Outdoor Air

System componentsLarge building system

Plate heat

exchanger

Water form

cooling tower

Water to

cooling tower

6oC 11oC

Water to

building Water from

building

25oC 35oC

Chiller

Compressors

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

Reciprocating compressor

Reciprocating Compressor

• Figures 4.4, 4.6

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

Rotary Compressors

• Higher efficiency, lower noise and vibration

• Cylinder rotating eccentrically in side housing

Rotary Compressor

Scroll Compressors

• One scroll is fixed

• The other scroll “wobbles” inside compressing refrigerant

• Often requires heat transfer from refrigerant to cool scrolls

Scroll compressor

»http://www.youtube.com/watch?v=f_6xolDoqs0

Scroll Compressors

• Constant displacement

• Higher efficiency, but harder to manufacture

• Close tolerance between scrolls

• Ugly to analyze – see text for details

Screw compressor

»http://www.youtube.com/watch?v=xO7IhhzImMU&feature=related

Screw Compressors

• Rotating meshed screws

• One or two screws

Summary

• Many compressors available• ASHRAE Handbook is good source of more

detailed information• Very large industry

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

Thermostatic Expansion Valve (TXV)

• Variable refrigerant flow to maintain desired superheat

AEV

• Maintains constant evaporator pressure by increasing flow as load decreases

Summary

• Expansion valves make a big difference in refrigeration system performance

• Trade-offs• Cost, refrigerant amount• Complexity/moving parts

Refrigerants

What are desirable properties of refrigerants?

• Pressure and boiling point

• Critical temperature

• Latent heat of vaporization

• Heat transfer properties

• Viscosity

• Stability

In Addition….

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

Refrigerants

• What does R-12 mean?• ASHRAE classifications• From right to left ←

• # fluorine atoms

• # hydrogen atoms +1

• # C atoms – 1 (omit if zero)

• # C=C double bonds (omit if zero)

• B at end means bromine instead of chlorine• a or b at end means different isomer

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

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…

Carbon Dioxide (R744)

• Cheap, non-toxic, non-flammable

• Critical temp?

• Huge operating pressures

Water (R718)

• Two main disadvantages?

• ASHRAE Handbook of Fundamentals Ch. 20

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/maintenance

Oil

• Miscible refrigerants

• High enough velocity to limit deposition• Especially in evaporator

• Immiscible refrigerants • Use a separator to keep oil contained in

compressor

• Intermediate

The Moral of the Story

• No ideal refrigerants

• Always compromising on one or more criteria

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.