Objectives Learn about refrigerants, compressors, and expansion valves (Ch. 4) Introduce heat exchangers (ch.11)

Objectives

• Learn about refrigerants, compressors, and expansion valves (Ch. 4)

• Introduce heat exchangers (ch.11)

Reciprocating Compressor

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

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

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

Air-liquid Tube heat exchanger

Plate heat exchanger

Heat exchangers

Air-air

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

Heat Exchanger Effectiveness (ε)

C=mcp

exchangeheatposible Maximum

exchangedHeat

Location B Location A

THout

TCin

TCout

THin

Mass flow rate Specific capacity of fluid

Example:What is the saving with the residential heat recovery system?

Furnace

72ºF

32ºF 72ºF

Outdoor Air

For ε=0.5 and if mass flow rate for outdoor and exhausted air are the same

50% of heating energy for ventilation is recovered!

For ε=1 → free ventilation! (or maybe not)

52ºF Exhaust

Gas

Combustion

products

Fresh Air

Air-Liquid Heat Exchangers

• Fins added to refrigerant tubes

• Important parameters for heat exchange?

Coil Extended Surfaces Compact Heat Exchangers

What about compact heat exchangers?

• Geometry is very complex

• Assume flat circular-plate fin

Overall Heat Transfer

Q = U0A0Δtm

Overall Heat

Transfer Coefficient

Mean temperature

difference

Heat Exchangers

• Parallel flow

• Counterflow

• Crossflow

Ref: Incropera & Dewitt (2002)

Heat Exchanger Analysis - Δtm

Heat Exchanger Analysis - Δtm

ocih

icoh

ocihicohm

tt

tt

ttttt

,,

,,

,,,,

ln

Counterflow

A

B

ABm

tt

ttt

ln

For parallel flow is the same

or

A

B

ABm

tt

ttt

ln