Air System Components

Air system components are typically used in various types of air systems. Each serves a distinct function and are required for a working system.

Listed, find a list of and a simple explanation of it's function in the air system.

Return air duct fan ac components:

The return air fan exists to return air from the conditioned zone and back to the air system. It allows for a good rate of ventilation and adequate air changes. Without the return air fan the mixed air section could not function and we could not make use of the seasonal free cooling provided by outside air.

The total of building return air plus building exhaust air, must not exceed building supply air. If so, the building will be operating at a negative pressure relative to outside. This will allow for unconditioned outside air to enter the building through the building envelope.

This is not desired when very hot or cold outside. It results in unfiltered air entering the building.

Mixed air dampers ac components:

The mixed air dampers consist of return, outside and exhaust air dampers. These dampers are positioned using pneumatic or electric actuators.

The mixed air section is usually controlled by sensing mixed air temperature and modulating the dampers to maintain a 60F air temperature. On an increase in mixed air temperature the outside air dampers open, the return air dampers close and the exhaust air dampers open.

On a decrease in mixed air temperature, the reverse sequence occurs.

The outside air damper should full open before the cooling coil control valve begins to open.

Minimum outside air damper ac components:

AC Component Information

Some air systems are equipped with a separate minimum outside air damper. When the air system is started, this damper opens and stays open while the air system is in operation.

Filter section ac components:

The filters exist to remove dust and dirt particles from the return and outside air streams. There are high efficiency filters and low efficiency filters depending on the size of dirt particles it is desired to remove.

The level of efficiency of filters is measured in percentage. The dirt and dust removed from the air is measured in microns.

There may be high efficiency bag filters which are protected by lower efficiency replaceable filters. The higher the efficiency of the filter the more pressure drop there will be across the filters. The more pressure drop the more fan HP is necessary to develop air flow across the filters.

It is good operating procedure to measure pressure drop across clean filters and when pressure drop increases approximately .3" WC, change them. The dirtier the filter, the less supply air will be developed.

Preheat coil and heat coil ac components:

The air system may have a preheat as well as a heat coil in cold climates, especially for air systems which require a higher percentage of minimum outside air. The preheat and heat coil have similar control configuration.

A controller sensing coil discharge air temperature, modulates the control valve which controls glycol flow through the coil. Most coil control configuration use a three way control valve. When flow is not required through the coil it is diverted to the return. The supply is to the bottom of the coil and the return is from the top of the coil.

There should be a glycol circuit balance valve (CBV) in the control valve common return. The valve should be balanced to the maximum flow rate that the coil is designed for. It may be used for isolation purposes and is equipped with a memory reset, which allows the it to be opened only to the balance position after being closed.

It is not unusual, when a CBV is located at the discharge of a circulating pump, that the pump must be OFF before the it can be full closed.

It has three uses:

  • isolating valve.
  • flow measurement station.
  • balancing valve.

    The purpose of the water balance is to ensure that coils closest to the discharge of the main glycol circulating pump do not allow too much supply glycol to bypass to the return line. This would "starve" coils in air systems further down the line.

    The air system discharge air temperature is limited to a predetermined high temperature. Should the air system shut down, the coil will open to provide full flow through the coil.

    Preheat and heating coil ac components circulating pump:

    These pumps circulate water or a water glycol mix through the preheat coil. In winter, when the air system is in operation, this pump is ON and runs continuously.

    Cooling coil ac components:

    The cooling coil provides for cooling of the mixed air section air. Chilled water is circulated through the cooling coil to provide the cooling effect.

    A controller sensing coil discharge air temperature modulates the control valve, which controls chilled water flow through the coil. Most control configuration use a two way valve. When flow is not required, it is simply stopped.

    If the system has a two way control there must be a pressure differential bypass from the supply line to the return at the end of the line. There should be a CBV installed in the return chilled water line and it serves the same purpose as explained for the preheat and heat coil.

    Cooling coil ac components circulating pump:

    The chilled water coil circulating pump circulates chilled water through the coil. The pump usually runs continuously when the air system is ON.

    Direct expansion cooling system with packaged condensing unit.

    Some smaller air systems make use of a direct expansion cooling system with a packaged condensing unit, instead of a chilled water cooling coil.

    The operation of this system is explained in detail in the cooling section.

    Supply air fan:

    The supply air fan delivers air to the building and provides for temperature control and proper ventilation for carbon dioxide (CO2) control. Carbon dioxide is a result of the human breathing process. The naturally occurring level of CO2 is about 400 parts per million (ppm) and if it increases to 600 to 700 ppm, it can make people drowsy and inattentive.

    The existing standard is supply air should consist of a minimum of 20 cubic feet per minute (cfm) of outside air for each occupant.

    Building supply air which consists of outside air and return air and must exceed the total of building return and exhaust air. It is required that the building be at a neutral or slightly positive pressure relative to outside.

    This will stop infiltration of unconditioned, unfiltered outside air into the building. This outside air will find it's way through the building envelope, doors and windows if the building is at a negative pressure relative to outside.

    On air system start up, the return fan should start before the supply fan. On shut down, the supply fan should stop before the return fan.

    The supply air fan may be constant volume or variable air volume, depending on system design.


    The humidifier adds moisture in the form of steam to the supply air. Usually, the relative humidity of the return air is measured and the amount of moisture added to the supply air is dependent on this measurement.

    A high limit controller with a sensor downstream of the steam grid, limits humidification to no higher than a preset value. This will prevent saturation of the supply air.

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