Measurement and Verification: Of Roof Top Units With Economizers
| Authors: | Brad Acker, Kevin Van Den Wymelenberg |
| Published: | February 20, 2009 |
| Links: | Pre-print pdf |
| Background: | An HVAC economizer system is designed to save cooling energy by using outdoor air (OA) instead of return air in order to avoid compressor operation, hence, cooling in economizer mode is often referred to as ‘free cooling’. The economizer system operates when there is a call for cooling in the space and OA conditions are favorable to provide cooling to the space. Figure 1 below shows a schematic view of the different air steams and dampers in a typical economizer system, the blue line represents the cooling coil. In full economizer mode the OA damper are open to the maximum position and the mixed air damper is closed. Some control systems can also throttle the mixed air damper to provide the desired supply air temperature. When outdoor air dampers (OAD) are set to the minimum position, they remain slightly open to meet interior fresh air requirements and to help control envelope infiltration by maintaining building pressurization.
Economizers can be controlled either locally at individual roof top units (RTU) or from a central energy management system (EMS). The physical components that make up economizer systems can be factory installed or field installed as an aftermarket energy efficiency measure. Control strategies and setpoints are configurable and adjustable based on building owner/operator preferences. Best practices based on climate zone may also be employed. Several control sequences exist in order to allow the OAD to open and provide free cooling to the space. Two common control sequences include 1) comparing outdoor air properties to a fixed value selected on the controller and 2) to compare return airstream properties to the outdoor airstream conditions. When return air properties are compared to OA properties it is called ‘differential control’. With either of these sequences, two different air properties are commonly monitored, dry bulb air temperature and air enthalpy. Dry bulb air temperature is a measure taken without concern for the water vapor content of the air. Air enthalpy is a measure of the energy content of the air and is dependent both on dry bulb air temperature and water vapor content of the air. This study is concerned with the energy saving aspects of economizers in general and does not contrast different control types or set points. |