Objective

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Objective

• Discuss Expansion Valves and Refrigerants
• Heat Exchangers
• Learn about different types
• Define Heat Exchanger Effectiveness (e)

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AEV

• Maintains constant evaporator pressure by
  increasing flow as load decreases

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Thermostatic Expansion Valve (TXV)

• Variable refrigerant flow to maintain desired
  superheat

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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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Heat exchangers
Air-liquid
Tube heat exchanger
Plate heat exchanger
Air-air
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Some Heat Exchanger Facts

• All of the energy that leaves the hot fluid
  enters the cold fluid
• If a heat exchanger 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

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Example
What is the saving with the residential heat
recovery system?
Outdoor Air
32ºF
72ºF
72ºF
Combustion products
52ºF
Furnace
Exhaust
Fresh Air
Gas
For e0.5 and if mass flow rate for outdoor and
exhaust air are the same 50 of heating energy
for ventilation is recovered! For e1 ? free
ventilation! (or maybe not)
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Heat Exchanger Effectivness (e)
Cmcp
Mass flow rate
Specific capacity of fluid
THin
TCout
THout
TCin
Location B
Location A
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Air-Liquid Heat Exchangers
Coil Extended Surfaces Compact Heat Exchangers

• Fins added to refrigerant tubes
• Important parameters for heat exchange?

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What about compact heat exchangers?

• Geometry is very complex
• Assume flat circular-plate fin

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Overall Heat Transfer

• Q U0A0?tm
• Overall Heat
• Transfer Coefficient

Mean temperature difference
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Heat Exchangers

• Parallel flow
• Counterflow
• Crossflow

Ref Incropera Dewitt (2002)
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Heat Exchanger Analysis - ?tm
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Heat Exchanger Analysis - ?tm
Counterflow
For parallel flow is the same
or
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Counterflow Heat Exchangers
Important parameters
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What about crossflow heat exchangers?

• ?tm F?tm,cf

Correction factor
?t for counterflow
Derivation of F is in the book
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• Example
• Calculate ?tm for the residential heat recovery
  system if mcp,hot 0.8 mc p,cold
• th,i72 ºF, tc,i32 ºF
• For e 0.5 ? th,o52 ºF, th,i48 ºF ? R1.25,
  P0.4 ? F0.89
• ?tm,cf(20-16)/ln(20/16)1
  7.9 ºF, ?tm17.9 0.8915.9 ºF

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Overall Heat Transfer

• Q U0A0?tm

Need to find this
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Heat Transfer
From the pipe and fins we will find
t

• tP,o

tF,m
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Resistance model

• Q U0A0?tm
• Often neglect conduction through tube walls
• Often add fouling coefficients

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Heat exchanger performance (Book section 11.3)

• NTU absolute sizing ( of transfer units)
• e relative sizing (effectiveness)

Criteria
NTU
e P RP
cr
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Fin Efficiency

• Assume entire fin is at fin base temperature
• Maximum possible heat transfer
• Perfect fin
• Efficiency is ratio of actual heat transfer to
  perfect case
• Non-dimensional parameter

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Summary

• Calculate efficiency of extended surface
• Add thermal resistances in series
• If you know temperatures
• Calculate R and P to get F, e, NTU
• Might be iterative
• If you know e, NTU
• Calculate R,P and get F, temps