Handbook of air conditioning and refrigeration / Shan K

21.12 CHAPTER TWENTY-ONE
Consider a single-zone VAV system that serves a building located in Phoenix, Arizona. Assume
that the outdoor temperature is 85°F (29.4°C), the humidity ratio is 0.006 lb/lb (kg/kg),
and the conditioned air leaves the coil at a temperature of 55°F (12.8°C), with a relative humidity
of 95 percent. The recirculating air enters the AHU or PU at a temperature of 78°F (25.5°C) and
a humidity ratio of 0.0094 lb/lb (kg/kg). If a differential enthalpy economizer control is used,
the state point of outdoor air o h falls in region III, and if a differential dry-bulb economizer control
is used, the state point of outdoor air o T falls in region I. The properties of moist air are as
follows:
A differential dry-bulb economizer control is required to cool the mixture of outdoor and recirculating
air down to an enthalpy difference of 6.2 Btu/lb (14.4 kJ/kg) instead of 7.2 Btu/lb (16.7
kJ/kg) using a differential enthalpy economizer control.
Therefore, consider the following:
1. For locations having hot and humid climates where many status points of outdoor air fall in the
dotted area A in Fig. 21.3a, a differential enthalpy economizer control is recommended.
2. For most locations having moderate outdoor humidity ratios, according to Spitler et al. (1987),
the energy savings offered by using a differential enthalpy economizer control instead of a temperature-based
economizer are small. The two types of economizer control may differ by only
about 5 to 10 percent. A fixed dry-bulb economizer control is recommended.
3. For locations having dry climates, a fixed dry-bulb of economizer control with a temperature setting
of 75 to 78°F (23.9 to 25.1°C) sometimes even saves more cooling energy than a differential
enthalpy economizer control. A fixed dry-bulb economizer control is strongly recommended.
4. For locations with humid climates, where the space requires a lower relative humidity, a fixed
dry-bulb economizer control using a set point T db a few degrees Farenheit lower than space temperature
sometimes is beneficial.
5. A survey including the simulation and comparison of various air economizer control alternatives
is often beneficial.
Water Economizer and Control
Differential enthalpy Differential dry bulb
Outdoor Off-coil Recirculating Outdoor Mixture Off-coil
Temperature, °F 85 55 78 85 79.5 55
Relative humidity, % 24 65 45 24 40 90
Humidity ratio, lb/lb 0.006 0.006 0.0094 0.006 0.0086 0.0086
Enthalpy, Btu/lb 27.0 19.8 28.6 22.4
Enthalpy difference, Btu/lb 7.2 6.2
Amount of outdoor air, % 100 20
Consider a typical water economizer that uses a precooling coil in either an AHU or PU, as shown
in Fig. 21.4b, and has the following operating characteristics:
● The cooling tower approach is assumed to be 5°F (2.8°C).
● The mixing temperature of outdoor air and recirculating air T m, is 78°F (25.5°C).
● The minimum temperature difference between T m and the condenser water temperature T con,w is
4°F (2.2°C).
● When the outdoor wet-bulb temperature T o��40°F (4.4°C), the temperature of the condenser
water used to replace the entire refrigeration through a heat exchanger is less than 45°F (7.2°C).