Economizers

As it name says, an economizer is a device which purpose is to economize by reducing energy consumption. This term is commonly used for Boiler, power plants, heating, ventilating and air-conditioning (HVAC).  We can classify them in temperature economizers and enthalpy economizers, according to the type of sensors used. In HVAC systems, economizers save energy by taking the cold outside air and cool the indoor spaces with it.

Industrial and commercial buildings require a specified quantity of outside air for ventilation. Based on the building’s usage, the outside air quantity must around 15- 25 cubic feet per minute (cfm) per person. In some applications, the HVAC system is supplied with 100% outside air. However, most systems combine outside air with the return air. This combination of air is called mixed air. Any return air not used in the mixed air is exhausted to the outside and is called exhaust air. The economizer control system manages all four different airs (the return air, the outside air, the exhaust air and the mixed air).

In order to have a better temperature control, low limit thermostat modulates outside air, return air and exhaust air dampers to keep the desired mixed air temperature. Moreover, other controls, such as outside air high limit or morning warm up low limit, can be added to make a more efficient system.

Temperature economizers are controlled by temperature; sensors read the outside air temperature, return air temperature and the mixed air temperature providing the required set point temperature. The same concept is applied to enthalpy economizers, but in this case, enthalpy is sensed instead of the temperature.

In order to operate properly, the economizers employ several components; an outside damper, a return air damper, a relief damper, sensors and an electronic controller.  The economizer controls the exhaust air, return air and outside air tamper to keep the mixed air temperature at same level as the set point.

With the correct control system, outside air can be use to assist the heating, cooling and humidifying of the building inside area. It can also provide a positive static pressure that can be used to reduce air infiltrations. Commercial buildings will generally be pressurized at about 0.03 to 0.05 inches of water column static pressure.

Economizer function:

The outside air and the exhaust air damper are normally closed while the return damper is normally opened. When we refer to the term normally closed we mean to its minimum position; ventilation code indicates that the damper should be open to provide a minimum of 20cfm per occupant. These dampers are controlled by the mixed air low limit controller that is direct acting and set for a low temperature. The other control for this system would be a reverse acting outside air high limit; this controller is going to be set for the high temperature. When a rise on the mixed air temperature takes place, and it goes above the low limit controller set point; the controller will send a signal to all dampers changing their normal position.  The outside air and the exhaust air damper will go open, while the return air damper will go closed. This condition will remain as long as the mixed air temperature is above the low limit set point. When the mixed air reaches the set points, the controller causes the damper to return to their normal position.

The follow images will give you a step by step sequence of the function of an economizer.

Let ED be the Exhaust damper, OD will be the Outside Air damper, and RD the Return Air Damper. It’s important to say that the mixed air temperature will cool the building, thus its temperature has to be colder than the one desired inside the building. Let the desire temperature of the building be 75⁰ F, the outside air temperature limit be 72⁰ F and the mixed air temperature be 60⁰ F.

Img.1

As you can see the ED and the OD are normally closed (meaning their lowest position, not totally closed). While the RD is open letting the return air run freely through the fan.

Now, let’s say that the mixed air temperature goes above our set point (60⁰ F); the controllers will send a signal to the ED, OD and the RD, changing them from their normal position. After this change, the OD and the ED will open, while the RD will close.

Img.2

Img.2 shows the RD closed, the outside air pass through the OD, cooling the mixed air; while the ED opens and send air outside to avoid positive pressure inside the building.  This sequence will continue as long as the mixed temperature is above the set point (60⁰ F), and the outside air is lower than its set point (72⁰ F).

If the Outside air temperature reaches set point, the controller will reverse the signal that goes to the dampers, and will cause them return to their normal position.

It’s important to mention, that there is a return air limit that modulates outside, return and exhaust dampers to maintain space temperature. This limit overrides both mixed and outside limits. We have set the space temperature to 75⁰ F; if this temperature changes, going below the set point. The controller will send a signal to all dampers, and they will go back to their normal position (Img.1).  The Dampers should stay in that condition until temperature reaches set point.