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

Real vs. Ideal Compression(Example with Reciprocating Compressor)

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 ηv

(book page 82-86, Fig 4.6)

Define how isentropic is our

compression

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

Example ProblemExplain the principle of operation of vapor compression based dehumidifier and show

how it affects the indoor environment.

If space conditions are T=25ºC, RH=70% 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%.

Coil Extended Surfaces Compact Heat Exchangers

• Fins added to refrigerant tubes

• Important parameters for heat exchange?

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

What about compact heat exchangers?

• Analysis is very complex

• Assume flat circular-plate fin

Overall Heat Transfer

• Q = U0A0ΔTm

Heat Exchangers

• Parallel flow

• Counterflow

• Crossflow

Ref: Incropera & Dewitt (2002)

Heat Exchanger Analysis

ocih

icoh

ocihicohm

tt

tt

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ln

Counterflow

b

a

bam

t

t

ttt

ln

Parallel