Lecture Objectives:

• Finish cooling / heat pump example

• Review Psychrometrics • Swimming pool example

Heat pump example

Heat Pump

Consumes heat energy Rejects heat energy

A recreation center

Ice Rink Swimming pool

Electric power for a compressor

EvaporatorCondenser

85-89oF 30oF

Ice Rinks Energy flow

Challenges for this problem

• Circulating fluid

• Adjustment of evaporation and condensation temperatures

• Adjustment of capacity for design condition

• Control of capacity for non design conditions• Need to study load profiles • Design sophisticated control • Provide backup system

Psychrometrics (Swimming pool example)

High humidity

High humidity and low temperature

Psychrometric Chart

• Need two quantities for a state point• Can get all other quantities from a state point

• Can do all calculations without a chart• Often require iteration• Many “digital” psychrometric charts available

• Can make your own

• Best source is ASHRAE fundamentals (Chapter 6)• Or electronic version at:

http://www.handsdownsoftware.com/

Connection between W and Pw(W humidity ratio – Pw water partial pressure)

• PV = mRT (Ideal Gas Law )

• P = Pw + Pa

w

a

a

w

TRVP

TRVP

a

w

R

R

P

P

m

mW

a

a

w

w

w

w

PP

PW

622.0

R = gas constantP = pressureV = volumeT = absolute temperatureW = humidity ratio

Subscripts: w is water vapor, a is dry air

Calculation of psychometric quantities

• For an ideal gas,• hda = ∫cpadT, hw = ∫cpwdT

• So, hda = cp,dat which assumes a reference state of 0 °F or 0 °C – Tables A4• Note different reference

• hw = cpwt + hg0

• h = cp,dat + W(cpwt + hg0)Or you can use:• h = cpt + W∙hg0, cp = cp,da + Wcpw

cp = specific heath = enthalpyT = absolute temperaturet = temperature W = humidity ratio

Subscripts: w is water vapor, a is dry air, g is saturated water vapor

Adiabatic mixing

• Governing equation hmQhmoutin

External heat

Sensible heating

tcmQ p

Dehumidification by Cooling

Real Dehumidification Process

Mold in a duct

Transport of saturated air

tsurface < tdp Condensation

Humidification

hw Specific enthalpy of water added to system

hg Specific enthalpy of saturated water vapor

Summary

• Describe psychrometric quantities

• Given any two psychrometric quantities, calculate any other quantity

• Use Tables A4 or psychrometric charts to look up psychrometric quantities

• Calculate psychrometric quantities at non-standard conditions

Swimming pool energy and mass balance

Evaporation

• Heat loss by evaporation:

pool W s W Tair

Tpool

hm pool WsTpool Wair

Further analysis can be conducted in the psychrometric chart

≈

Depends on the air speed and even more on the condition of the water surface (people splashing in the water)

Process in AHUs

• Example AHUs for swimming pools

MENERGAThermoCond 19

Swimming pool heating

Dehumidification using outside air in Winter and Summer

More advance control

MENERGA ThermoCond 29

Dehumidification using outside air in winter

Condenser in the swimming pool

Evaporator

Condenser in the air stream

Compressor of the heat pump

Recirculation of air with dehumidification

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