Lecture Objectives: Finish cooling / heat pump example Review Psychrometrics Swimming pool example

Heat pump example A recreation center Heat Pump Ice Rink Swimming pool Condenser Evaporator 85-89oF 30oF Consumes heat energy Rejects heat energy Electric power for a compressor

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 R = gas constant P = pressure V = volume T = absolute temperature W = 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 heat h = enthalpy T = absolute temperature t = temperature W = humidity ratio Subscripts: w is water vapor, a is dry air, g is saturated water vapor

Adiabatic mixing Governing equation External heat

Sensible heating

Dehumidification by Cooling

Real Dehumidification Process

Transport of saturated air tsurface < tdp Condensation Mold in a duct

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: hm Tpool Tair pool W s W ≈ hm Ws Tpool Wair pool Depends on the air speed and even more on the condition of the water surface (people splashing in the water) Further analysis can be conducted in the psychrometric chart

Process in AHUs Example AHUs for swimming pools MENERGA ThermoCond 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 air stream Evaporator Compressor of the heat pump Condenser in the swimming pool

Recirculation of air with dehumidification catalog