subject: Air Make-up By Commercial Air Handling Units [print this page] Tenderall Fan CoTenderall Fan Co. produces industrial Make-Up Air Unitss for ventilation and heating of large industrial facilities.
Makeup air is vitally important to the health and welfare of a building and its occupants. If the proper amount of makeup air is not introduced into a space, consequences can range from exterior doors that are hard to open or close, to buildups of carbon monoxide in the space, to fume hoods that don't actually control the contaminants they were intended to control. In addition, the outside air that is brought into a building to "make up" the amount of air that has been exhausted has a very significant impact on cooling and heating capacities.
Insufficient make-up air can lead to increased building infiltration and higher energy costs, not to mention unhappy building owners and uncomfortable occupants.
While there are no facts and figures to show how many buildings provide the correct amount of makeup air, it can be stated that there are probably a great many that do not have sufficient makeup air. And there's no one solution that is available for every building. Each building must be analyzed with a careful air balance calculation and evaluated based on the diversity of sources of exhaust air and infiltration.
The type of make up air equipment used, as well as its size, is usually dependent on the climate. Humid areas of the country require considerably larger-capacity units to dehumidify outdoor air. The supply air requirements for occupant comfort are generally air temperatures of 70 degrees to 75 degrees F with an rh of 50%. A good design criteria for air-handlung units engineers is to specify the unit performance at two operating conditions - a design day and an off-peak period such as a 70 degrees rainy day. The off-peak times of the year are actually more critical to evaluate the unit performance because this is the lion's share of the unit operation. Engineers also need to specify the maximum allowable supply air dew points.
Direct gas-fired equipment is the most efficient choice for makeup air. While indirect gas-fired equipment might have a nominal energy efficiency of 75% to 80%, a direct-fired unit will operate at a nominal 93%. Thus, the further north or more extreme the climate, the more valuable a direct-fired piece of equipment becomes.
It is also possible to experience condensation within the heat exchanger sections of indirect- fired units in very cold climates. If the heat exchanger is not properly pitched and drained and not constructed of stainless steel through all sections, then corrosion and premature failure can occur.
That's not to say that indirect gas-fired makeup air equipment doesn't have its place, but again, it comes down to the climate. Climates with high concentrations of combustible or potentially toxic elements in the air might not be suitable for direct-fired equipment as the air is directly exposed to the flame. Many applications allow the recirculation of some of the air from the space and return air that is laden with paint, solvents, or very fine powders would be better handled with an indirect gas-fired piece of equipment.
Additional information can be found at the Canada Blower company web site http://www.tenderall.com/ventilator/index.html.
