The best way to approach this is from the beginning, so here we go. Many commercial buildings require year round cooling due to the heat gain from lighting, computers, and people. During the summer it's a no-brain-er, you run mechanical refrigeration to cool the space. However, when the outdoor air is cold we another option, we can use the cool outside air to cool the space, is done by opening and closing dampers that are installed in the air handling equipment. One damper opens up to the outside, the other closes off the return air to the unit, making the unit draw in outside air. The method is the same whether it's a packaged roof top unit or an air handler.
Can something be too simple?
The problem lies in that the mechanics and function of an economizer are so simple no one puts any real effort into understanding how they operate. Notice I used the term "mechanics" and "function". Yes, the mechanics of an economizer is simple—a couple of dampers opening and closing—but that's not the misunderstanding. The sequence of operation and various control configurations or "Control Strategies" of the economizer is often not truly understood.
Free Cooling Strategies
There are four changeover strategies for free cooling, and the strategy depends on the conditioned space's use.
Before this gets too confusing, let's break out the different Free Cooling Strategies. There are four different changeover strategies that can be used to determine when the outdoor air can be used for free cooling. You can only use one of the four; they cannot be combined.
(1)The most common is outdoor dry bulb, this uses an outdoor temperature sensor to control change over. Very simply, if it's set at the typical 55 degrees, below that the unit uses outside air for cooling, above that, it uses mechanical cooling. The problem with this strategy is it does not look at humidity. Look at it this way, 53 degree air may be great to cool the space, but not at 80% humidity. Using this change over strategy would result in curled paper jamming printers and copiers, warping ceiling tiles, and a cold, clammy feeling for the tenants. Obviously, this would be better suited for areas that have low outdoor humidity.
(2)The next type of strategy is outdoor enthalpy or single point enthalpy, this works like the dry bulb style except it also takes into account outdoor humidity. Instead of a numeric temperature set point for change over it uses alpha settings of A-B-C-D that correspond to a curve on a enthalpy chart provided by the manufacturer of the economizer control (see insert). The reason is the change over temperature will change depending on the outdoor humidity, for example, at a setting of "C", 65 degrees at 50% outdoor humidity is usable, but 65 degrees at 70% is not usable. The change over value can be determined from the manufactures enthalpy change over chart.
(3) The third style is differential dry bulb, this configuration has a temperature sensor outside, and a temperature sensor in the return duct of the HVAC unit. Regardless of what the outdoor temperature is, if it's cooler than the temperature inside the building outside air will be used for cooling. Example, it may be 75 degrees outside, but if its 80 degrees in the building then the economizer will open. Again, this style does not take humidity into account.
(4) The final style is differential enthalpy, and similar to differential dry bulb, it has a sensor outside and a sensor in the return duct and if the outdoor air is better than the air in the building, the economizer opens. The difference is that these are enthalpy sensors and they do take into account humidity. No matter which style you use the result is the same, if the outside air can be used for cooling the economizer becomes the system 1st stage of cooling, if it's not suitable the economizer will energize the compressors on a call for cooling.
Now here's a question that's asked a lot "If I only have one compressor, why do I have to use a two-stage thermostat?" Because when the system is in free cooling mode, the outside air is the 1st stage of cooling, if the thermostat calls for 2nd stage the compressor is energized to supplement the free cooling. If you're using an outdoor dry bulb sensor for free cooling change over, you really wouldn't need refrigeration to supplement because the change over setting is about 50-degrees, the same temperature that you would get if just the compressors are running. A two-stage thermostat should be used with outdoor enthalpy, differential enthalpy, and differential dry bulb because there is a good chance that the discharge air temperature could be in the mid to high 60's.
How does the Free Cooling Mode Work?
When the system enters free cooling mode the outside dampers don't just flop open, they modulate to maintain a mixed air temperature of about 50-degrees. Why? Because you don't want to dump 20-degree air into the space; the occupants would probably be less than comfortable with 20-degree air hitting them on the neck. (Remember, this is about the occupants comfort) The economizer uses a thermistor mounted in the mixed air steam to modulate the return and outdoor air dampers, mixing the two air steams, to provide 50-degree supply air.
That's the sequence for operation for a basic economizer. It may change slightly for equipment with a factory electronic control platform, but the change over strategies will remain the same with only slight variances.
There are four features that all economizers have:
- Spring return, or battery close, damper actuators—for fail-safe operation.
- Return and outside air dampers.
- A mixed or discharge air temperature sensor.
- A change over strategy based on outdoor air conditions.
Keep these four truths in mind whenever you are troubleshooting an economizer. Reading our article on demand ventilation will help round off your knowledge of economizers. It will cover how the ventilation mode works and how to determine outdoor air percentage without special tools or hiring "Windy" the friendly air balancer. (Sorry Air Balancer dude)