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Rooftop Unit Economizer – Operation & Control
Perform the economizer functions of a packaged rooftop unit using digital control
Steve Calabrese is a Project Engineer with a large controls contractor serving the Chicagoland area, and author of the book Practical Controls: A Guide To Mechanical Systems. You can visit his website at www.pcs-engineering.com.
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July - RTU Operation Via Conventional & Digital Controls
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One of the most clever features of a typical, standard rooftop unit, and perhaps also one of the most misunderstood, is the economizer section. The economizer section of a rooftop unit, or for any air handler for that matter, is designed to utilize outside air as a source of “free cooling” if outside air conditions permit. The economizer (minimally) consists of the following components:
Outside and return air dampers
Damper actuator(s) – only one if the outside and return air dampers are mechanically linked
Outside air enthalpy (temperature & humidity) sensor or dry bulb (temperature only) sensor
Discharge air temperature sensor
Economizer controller – performs all facets of economizer system operation
What follows is a Sequence of Operation for the economizer section of a typical commercial rooftop unit, along with a brief analysis of each part:
<<The outside air enthalpy or dry bulb sensor monitors outside air conditions. If outside air conditions are suitable for “free cooling” as determined by the sensor and the economizer controller “enable” setting, then economizer operation is allowed.>>
Dry bulb sensing accounts for temperature only, whereas enthalpy sensing takes humidity into account as well. For the sake of further explanation, let’s assume dry bulb sensing with an economizer enable setting of 65 degrees, and an outside air temperature of 60 degrees.
<<Upon a call for “first stage cooling” (Y1 from the space-mounted thermostat), economizer operation is initiated. Outside and return air dampers begin to modulate (from minimum position) in order to maintain a suitable discharge air temperature (typically around 55 degrees). If necessary, the outside air damper will modulate fully open in an effort to achieve this temperature, though depending upon the outside air temperature (and the economizer controller “enable” setting) may not be able to. In other words, if the economizer controller is set to allow economizer operation at a “higher” outside air enthalpy or dry bulb temperature, then the temperature of the outside air may be higher than what is trying to be maintained at the discharge, and therefore the outside air damper will modulate fully open without succeeding to achieve this discharge air temperature setpoint. Depending upon the space cooling requirements, this may or may not be sufficient to satisfy the call for “first stage cooling”.>>
On a 60 degree day, the economizer would be hard-pressed to achieve that “suitable discharge air temperature” setpoint of 55 degrees. The reality is that the outside damper modulates fully open (with the return air damper going fully closed), and the resulting discharge air temperature is 60 degrees, the temperature of the outside air.
<<If economizer operation alone is not sufficient to satisfy the requirement for cooling, resulting in a call for “second stage cooling” (Y2 from the space-mounted thermostat), then “first stage DX cooling” is engaged (compressor number 1). The DX coil becomes active, and the outside and return air dampers continue to modulate to maintain a suitable discharge air temperature as sensed by the discharge air sensor, which is located downstream of the coil. With the DX coil active, the resulting action of the economizer dampers is that they modulate back down toward minimum position in order to continue to maintain the discharge air temperature setpoint (as sensed downstream of the DX coil).>>
The economizer controller routes the call for second stage cooling to the first compressor. The economizer is truly the first stage of cooling in this scenario. Compressor number 1 is energized upon a call for second stage cooling, and the DX coil imparts (more or less) a 10 degree temperature drop to the air flowing across it. The economizer dampers once again begin to modulate. The outside air damper closes down and the return air damper opens, to provide the proper portions of outside and return air as required to maintain a discharge air temperature of 55 degrees (after the coil).
<<As the requirement for cooling is satisfied, then DX cooling and economizer operation are disengaged in the reverse order that they were engaged.>>
For rooftop units equipped with two compressors, the terminals Y1 and Y2 correspond to compressors number 1 and 2 respectively, when economizer operation is not allowed. When economizer operation is allowed, then Y1 corresponds to the economizer, and Y2 corresponds to compressor number 1. Compressor number 2 is not allowed to function during economizer operation.
For rooftop units with only one compressor, a two-stage thermostat may still be utilized. It is sometimes mistakenly assumed that since such a unit has only one compressor, then a single stage thermostat is all that is needed. In reality, the manufacturer may very well have made provision for this scheme of control, thereby supplying the Y2 connection at the terminal strip.
Unless equipped with low ambient controls (which allow for safe operation of compressors during colder outdoor air temperatures), the first stage of DX cooling of the rooftop unit, depending upon the climate, may need to be equipped with an optional outside air temperature “lockout” controller. This controller prohibits compressor operation if the outside air temperature is below the setpoint of the device (typically 50 degrees). This means that, if outside air conditions are suitable for free cooling, and the outside air temperature is below 50 degrees, then economizer operation alone is permitted (DX cooling is disabled). This is acceptable, because if the outside air temperature is below 50 degrees, then the economizer will be able to maintain discharge air temperature setpoint, and therefore economizer operation should be sufficient to satisfy the cooling requirements of the space.
If a rooftop unit does not have an economizer, then (depending upon the climate) it should be equipped with low ambient controls, which would allow the compressor(s) to operate at low outside air temperatures. Otherwise, without an economizer section, and without low ambient controls, the rooftop unit has no means of providing cooling when the outside air temperature is below what the compressors can “safely” operate at (generally around 50 degrees or so). In this case, the thermostat’s mode switch should be set so as not to allow calls for cooling.
Of course in this digital age of commercial building control, it’s all about controlling package rooftop equipment not from a conventional thermostat, but from a digital controller networked into a Building Automation System. The thermostat is replaced by a space temperature sensor, which may or may not have setpoint adjustability. The space sensor serves as an input to the digital controller. The outputs of the controller interface to the rooftop unit’s terminal strip, similar as to how the thermostat would. The controller processes space temperature and setpoint information into calls for cooling as required. Typically the economizer controls are left intact, thus allowing the rooftop unit’s economizer controller to perform economizer enable/disable, and to perform economizer control. However it’s becoming common practice to “decommission” the on-board economizer controller, and take direct control of the outside and return air dampers using an analog output on the digital controller. Add a discharge air temperature sensor as an input to the controller for control purposes, and base the decision to economize on temperature/humidity information as reported from the Building Automation System’s main outside air temperature and humidity sensors. Ah, the best of both worlds: packaged rooftop equipment and direct digital control!
|Tip of the Month: It also needs to be understood as to how the economizer controller routes cooling calls, when economizer operation is allowed, and when economizer operation is prohibited.|
Tip of the Month: When relegating economizer control from a rooftop unit’s on-board economizer controller to an external (digital) controller, it’s not as simple as disconnecting the outside and return air damper actuator from the economizer controller, and connecting it to the digital controller. Certainly that’s part of what needs to be done. But it also needs to be understood as to how the economizer controller routes cooling calls, when economizer operation is allowed, and when economizer operation is prohibited. The goal here is to be able to take direct control of the compressors, however that can’t be done by simply wiring those outputs of the digital controller assigned as cooling stages directly to the rooftop unit’s Y1 and Y2 terminals. The good news is that many manufacturers have recognized this to be a trend among building automation contractors, and have become sensitive to this issue, some even going so far as to have generated some instruction on how to take complete control of the unit’s basic functions. The tip here is to exercise caution in this scenario, and by all means request that the rooftop unit manufacturer provide information on how to transfer economizer control from internally to externally, if such information is indeed available.
About the Author
Steve Calabrese earned his BSEE degree in 1990 from the University of Illinois at Chicago (UIC). He has since spent much of his professional career working for a mechanical contracting company, in various roles including mechanical systems design, control systems design, project management, and department management. Currently employed by a large Chicagoland controls company, Steve couples his broad mechanical knowledge and experience with a strong background in the area of electricity and electronics. His control systems expertise includes electrical and electronic stand-alone controls, as well as microprocessor-based direct digital controls (DDC) and networked Building Automation Systems (BAS). You can visit his website at www.pcs-engineering.com.
In 2003 Steve’s book, Practical Controls: A Guide To Mechanical Systems, was published. Geared toward the HVAC professional, the book details practical methods of controls and defines the role of HVAC controls in an easy-to-understand format. Steve brings his mechanical and controls contracting experience to this writing, and offers practical approaches to control systems issues.
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