Most facilities classified, either commercial or industrial, have a need for some type of make-up air equipment. There are four main reasons:
Contaminated air (contamination can be dust, smoke, or odors) can adversely affect production efficiencies due to absenteeism and slow production rate.
A building under a negative pressure draws dust and dirt into work area. This may affect the performance of machinery and quality of product produced. Accumulation of contaminated air is more likely to occur in buildings that experience a negative pressure.
The effectiveness of exhaust ventilation is substantially reduced in a negative pressure environment.
A building under a negative pressure will starve process heating equipment of combustion air, which can cause inefficient burn or nuisance shut down of the equipment.
Dedicated make-up air is the supplying of make-up air into the immediate area (or the process itself, such as a paint booth) with the supply air dedicated to replacing the air exhausted by that particular device (paint booths, ovens, etc.). In addition to be ducted directly to the device, the make-up air equipment is usually electrically interlocked with the exhaust so both pieces of equipment run at the same time.
General ventilation is the practice of introducing make-up air in a way that it will disperse into the open area of the building. This system typically supplies air to the cleanest area of the building while the exhaust is located in the more contaminated areas.
Quite often, facilities have a requirement for both types of systems.
Usually make-up air needs to be heated in some manner. There are two ways to heat make-up air: indirectly and directly. There are advantages and disadvantages to each of them and some codes will dictate which method should be used.
Indirect heating equipment is any heating device in which the heat medium or combustion is located in a confined chamber (indirect gas fired or hydronic coils are examples). The air does not directly contact the heat medium. It pulls the heat off the heat exchanger or coil fins as it passes through.
Direct heating equipment is that in which the flame or heating element is in direct contact with the air being moved by the air handler (direct gas fired or electric coil).
Direct gas fired equipment is an excellent choice for heating make-up air as it can efficiently raise the ambient air temperature to the design supply air temperature and maintain that temperature throughout the operating range of air flow. Properly designed and operated, a direct gas fired make-up air unit will maintain its high efficiency throughout the life of the equipment which is usually much longer than other methods of heating make-up air.
A properly designed and operated make-up air system will provide business owners many benefits and should not be looked at as an expense but rather a method to save money on a long term basis.