All occupied buildings require a supply of outdoor air. Depending on outdoor conditions, the air may need to be heated or cooled before it is distributed into the occupied space. As outdoor air is drawn into the building, indoor air is exhausted or allowed to escape (passive relief), thus removing air contaminants. The term “HVAC system” is used to refer to the equipment that can provide heating, cooling, filtered outdoor air, and humidity control to maintain comfort conditions in a building. HVAC systems range in complexity from stand-alone units that serve individual rooms to large, centrally controlled systems serving multiple zones in a building.
Some buildings use only natural ventilation or exhaust fans to remove odors and contaminants. In these buildings, thermal discomfort and unacceptable indoor air quality are particularly likely when occupants keep the windows closed because of extreme hot or cold temperatures.
In order to exhaust air from the building, make-up air from outdoors must be brought into the HVAC system to keep the building from being run under negative pressure. This make-up air is typically drawn in at the mixed air plenum as described earlier and distributed within the building. For exhaust systems to function properly, the control air must have a clear path to the area that is being exhausted. To prevent operating the building under negative pressures (and limit the amount of unconditioned air brought into the building by infiltration), the amount of make-up air drawn in at the air handler should always be slighter greater than the total amount of relief air, exhaust air, and air exfiltration through the building shell. Excess makeup air is generally relieved at an exhaust or relief outlet in the HVAC system, especially in air economizer systems. In addition to reducing the effects of unwanted infiltration, designing and operating a building at slightly positive or neutral pressures will reduce the rate of entry of soil gases when the systems are operating.
The selection of a specific ventilator should be based on the following factors:
- Job details such as the atmospheric hazard, the size of the confined space, etc
- Airflow required
- Airflow volume required at the end of the duct to control the hazards present; and Breeze to ensure worker comfort in the space.
- Duct friction loss to ensure adequate air volume reaches the end of the duct.
Aims of HVAC Control Systems:
- HVAC Control Systems are used for the following benfits:
- Lower energy cost.
- Lower operations cost.
- Increase flexibility.
- Ensure quality building environment.
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