refrigerants compressors

Objectives

• Finish up discussion of cycles

• Differentiate refrigerants• Identify qualities of a good refrigerant

• Compare compressors

• Describe expansion valves

Administrative

• Oral presentations will take longer than one class period• I propose doing the presentations one evening. I will

provide pizza etc. and we can probably finish them in 2.5 – 3 hours

• What evenings don’t work for you?• Monday December 5th

• Tuesday December 6th

• Wednesday December 7th

• Thursday December 8th

• Should an evening replace one or two classes?

Missing classes

• October 18th (AAAR conference)

• November 1st (ISEA conference)

Multistage Compression Cycles

• Combine multiple cycles to improve efficiency• Prevents excessive compressor discharge

temperature• Allows low evaporating temperatures (cryogenics)

What are desirable properties of refrigerants?

• Pressure

• Critical temperature

• Latent heat of vaporization

• Heat transfer properties

• Viscosity

• Stability

Oh Yeah….

• 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 (less symmetric)

Refrigerant Conventions

• Mixtures show mass fractions• Zeotropic mixtures

• Change composition/saturation temperature as they change phase at a constant pressure

• 400 series (if commercialized)

• Azeotropic mixtures• Behaves as a monolithic substance• Composition stays same as phase changes• 500 series (if commercialized)

More Refrigerant Arcana

• Misc organic refrigerants – 600 series

• Inorganic refrigerants 700 + molecular weight

• Composition designation• (H) + (BCF) + C #

Inorganic Refrigerants

• Ammonia (R717)• Boiling point?• Critical temp = 271 °F• Freezing temp = -108 °F• Latent heat of vaporization?

• Small compressors and linesets

• Excellent heat transfer capabilities• Not particularly flammable

• But…

Carbon Dioxide (R744)

• Recent ASHRAE papers• Evaluation of carbon dioxide as R-22 substitute for residential air-conditioning

Brown, J. Steven (Department of Mechanical Engineering, Catholic University of America); Kim, Yongchan; Domanski, Piotr A. Source: ASHRAE Transactions, v 108 PART 2, 2002, p 954-963Abstract: This paper compares the performance of CO2 and R-22 in residential air-conditioning applications using semi-theoretical vapor compression and transcritical cycle models. The simulated R-22 system had a conventional component configuration, while the CO2 system also included a liquid-line/suction-line heat exchanger. The CO2 evaporator and gas cooler were microchannel heat exchangers originally designed for CO2. The R-22 heat exchangers employed the same microchannel heat exchangers as CO2 with the difference that we modified the refrigerant passages to obtain reasonable pressure drops. The study covers several heat exchanger sizes. The R-22 system had a significantly better coefficient of performance (COP) than the CO2 system when equivalent heat exchangers were used in the CO2 and R-22 systems, which indicates that the better transport properties and compressor isentropic efficiency of CO2 did not compensate for the thermodynamic disadvantage of the transcritical cycle in comfort cooling applications. An entropy generation analysis showed that the CO2 evaporator operated with fewer irreversibilities than did the R-22 evaporator. However, the CO2 gas cooler and expansion device generated more entropy than their R-22 counterparts and were mainly responsible for the low COP of the CO2 system. (33 refs.)

• Cheap, non-toxic, non-flammable• Critical temp?• Huge operating pressures• Often no phase change

Water (R718)

• Two main disadvantages

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 (11,12, 21,113)• High enough velocity to limit deposition• Especially in evaporator

• Immiscible refrigerants (717,744,13,14)• Use a separator to keep oil contained in

compressor

• Intermediate (22, 114)

The Moral of the Story

• No ideal refrigerants

• Always compromising on one or more criteria

• Should be able to look up properties and analyze good candidates for refrigeration cycles

Compressor

• Workhorse of the system

• Several types – all compress gas with varying degrees of efficiency• Far from isentropic (our assumption earlier)

• CD = mve for ideal conditions

• Wshaft = work done by shaft

• Welec = electric power requirements

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

Reciprocating Compressor Work

• Define compressor volumetric efficiency ηV=mv3/CD

bb

c

v

CD

P

PCCm

n

1

1

11

1n

n

b

cbb P

PvP

n

nw

Rotary Compressors

• Higher efficiency, lower noise and vibration

• Cylinder rotating eccentrically in side housing

• Positive displacement

Rotary Compressor

BA

BA

BAW

V

VCD

shaftcomp

s

comps

1

422

Scroll Compressors

• One scroll is fixed

• The other scroll “wobbles” inside compressing refrigerant

• Often requires heat transfer from refrigerant to cool scrolls

Scroll Compressors

• Constant displacement

• Higher efficiency, but harder to manufacture

• Close tolerance between scrolls

• Ugly to analyze – see text for details

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

• Where are we going• Expansion valves• Coils (evaporators and compressors)

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

• For Thursday, look at Example 4.4 and practice problem in References Section of Class Website