Q 108 QAT102a

where q = hourly ventilation heating load, Btu/hr

Q = volumetric flow rate of air, cfm

AT = temperature increase, °F

The economizer cycle is most appropriate for thermally massive buildings which have high internal loads and require cooling in interior zones year round. It is ineffective in thermally light buildings and buildings whose heating and cooling loads are dominated by thermal transmission through the envelope. Economizer cycles will provide the greatest benefit in climates having more than 2000 heating degree days per year, since warmer climates will have few days cold enough to permit the use of outside air for cooling.

In theory 100 percent outside air would be utilized for cooling whenever the outside air temperature is below the return air temperature, and this is done in dry climates. But if the outside air has a higher relative humidity than return air and contains significant latent heat, the economizer would not be activated at this temperature.

The simplest type of economizer utilizes a dry-bulb temperature control which activates the economizer at a predetermined outside dry-bulb temperature, usually the supply air temperature, or about 55°F (13°C). Above 55°F (13°C), minimum outdoor air is supplied for ventilation. Below 55°F (13°C), the quantity of outdoor air is gradually reduced from 100 percent and blended with return air to make 55°F (13°C) supply air.

Eq. 10.2a predicts that the energy required to cool room (return) air 20°F (13°C) from 75°F (24°C) with a supply air temperature of 55°F (13°C) is 1.08 x Q x 20 = 21.6 Q. The energy saved [E, Btu/hr] from the use of a simple economizer which blends outside air with return to provide a supply air temperature, at sea level, is therefore:

Because it is possible to utilize outdoor air at temperatures above 55°F (13°C) to save cooling energy, a modified dry-bulb temperature control can be used. This is identical to the simpler dry-bulb temperature control except that when the outside temperature is between 55°F (13°C) and a preselected higher temperature based on the typical humidity, 100 percent outdoor air is used, but cooled to 55°F for supply air.

The third and most efficient type of control for an economizer cycle is enthalpy control, which can instantaneously determine and compare the amount of energy required to cool 100 percent outdoor air with that required to cool the normal blend of return air. It then selects the source which requires the least energy for cooling.

Air-handling units which lack adequate provision for 100 percent outside air can utilize an alternative "wet-side" economizer. This energy-saving technique is a potential retrofit for existing older buildings. Several variations on the wet-side economizer exist, all of which allow the chiller to be shut down. The simplest waterside economizer is a coil in the air-handling unit through which cooling tower water can be circulated to provide free cooling. An alternative arrangement for large chilled water systems interconnects the chilled water circuit directly with the cooling tower, to allow the chiller to be bypassed. To reduce contamination of the chilled water, the cooling tower water should only be coupled to the chilled water circuit through a heat exchanger (Figure 10.13).

The energy saved [E, kWh] due to the use of a wet-side economizer can be determined from a knowledge of the hours [hr] of chiller operation during when the outside temperature is below 40°F (4°C), the capacity of chiller in tons and the kW/ton rating of the chiller:

A word of caution is advised. A wet-side economizer uses condenser water at 40-45°F (4-7°C). But the chiller typically operates with condenser water much higher than this. In some cases, the lowest condenser water temperature at which the chiller will operate is 65°F (18°C). This means that when the chiller is turned off there would be no useful coolant until the cooling tower cooled to 45°F (7°C). This "cool down" period is typically no more than 30 minutes. For additional discussion of the wet-side economizer, see §10.6.6.

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