Air - side Economizer

In many buildings nowadays, even when the outdoor air temperature is moderate or very cool, many commercial buildings still need mechanical refrigeration to meet normal demands. The mechanical refrigeration load usually comes from the internal areas of commercial buildings. This is because the heat loss of the building envelope cannot offset the internal heat gains in the internal areas, such as heat dissipation from computers, lights, and people. The internal heat gain caused by solar radiation is also an important factor increasing the cooling load, especially when a large number of windows are arranged on the west and south sides of the building.

The air - side economizer can provide a perfect low - cost solution for this design requirement. The air - side economizer is a device with ducts and dampers equipped with an automatic control system.

Working together, they enable the cooling system to supply outdoor air, reducing or eliminating the need for mechanical refrigeration in suitable - temperature and cold weather, thus achieving a two - fold effect: (1) providing more outdoor air to save refrigeration energy; (2) improving indoor air quality. The efficiency of the economizer depends on the load characteristics of the building, the HVAC system, and the local climate characteristics.

Integrating this device into the HVAC system has a very obvious effect. ASHRAE Standard 90.1, one of the most widely used standards for building performance, includes air - side economizers in both prescriptive and performance - based approaches. In the Energy Cost Budget (ECB) method of ASHRAE Standard 90.1, air - side economizers are exempted only in climate zones 1a and 1b. This is because in these “very hot - humid” (CZ 1a) and “very hot - dry” (CZ 1b) climate zones, the relatively high outdoor air enthalpy throughout the year eliminates the advantages of using economizers. In the contiguous United States, some southern cities in Florida and Texas are exempt from using air - side economizers in their HVAC systems.

In all remaining areas, when the system cooling capacity exceeds 4.5 tons of refrigeration (≈16 KW), an air - side economizer is required.

Figure 1. Creating fixed dry - bulb temperature and differential enthalpy economizers in ApacheHVAC

As an upper - limit shut - off, the air - side economizer must be carefully set.

If not properly controlled, it may introduce excessive outdoor air, thereby increasing the cooling load. The California Energy Commission requires functional testing of the economizer control components to ensure reliability.

To avoid excessive outdoor air under adverse weather conditions, the economizer uses upper - limit shut - off control. ASHRAE Standard 90.1 cites four control strategies:

Note: ASHRAE Standard 90.

1 - 2013 eliminated the use of electronic enthalpy, dew - point temperature, and dry - bulb temperature control options from earlier versions.

In the ApacheHVAC application module of the IESVE software, it is very convenient to set up an air - side economizer with four upper - limit shut - off controls. Figure 2 shows the operating results of an economizer with “differential enthalpy and fixed dry - bulb temperature” (Option 4). The integrated economizer will intake excessive outdoor air (blue) when the following conditions are met:

When one or both of these conditions are not met, the economizer will continue to maintain the minimum amount of outdoor air required by the system (e.g., as per ASHRAE 62.1).

Figure 2. Hour - by - hour data in IESVE software describing the optimal economizer operation

All four economizer control strategies were simulated using a 15,000 - square - foot (≈1400 m²) office building model in Chicago (CZ 5a). The results show that compared with the system without an economizer, the “differential enthalpy and fixed dry - bulb temperature” (Option 4) control strategy is the best choice. It operates in economizer mode for 1900 hours, saving 18% of cooling energy.

In the HVAC prototype system of ApacheHVAC, economizers with different upper - limit shut - off controls have been pre - built on the air - side. Designers have the opportunity to analyze the operation of the economizer at hourly or quarter - hourly time steps. This provides more detailed results regarding the effectiveness of the economizer and optimizes the control strategy to maximize cooling energy savings.

Figure 3. Comparison of cooling energy savings and economizer operating time

Download the.asp file of the air - side economizer upper - limit shut - off strategy and place it in the software installation location (.../apps/Templates/HVAC/Library), and then you can open, view, and apply it in the software.

Figure 4. Four economizer control strategies