Air Coil Roof

Direct-Fired Industrial Makeup Air Units

Canada Blower Co. ia a developer of different types of gas-fired air makeup units for industrial building ventilation.

Gas-Fired Pressurization Units move large quantities of air at low temperature differentials (usually 50° or less), which is a strategy to minimize temperature stratification in the large spaces they are employed to heat. The equipment is typically mounted on the roof of the facility or at grade on elevated supports to ensure that the supply air is delivered high. The high air delivery allows for a longer “throw” of the air, thereby requiring less equipment to cover the required floor area. The most common fuel is natural gas, however, units can be converted to burn propane. There are two primary categories of gas-fired AHUs: indirect-fired and direct-fired.

Indirect-fired units burn the fuel and air mixture inside of a heat exchanger while the air traveling to the space passes over the outside of the heat exchanger. In this design, the products of combustion travel through a vent to the outside of the building.

Direct-fired units utilize air that will be sent to the heated space for combustion without use of a heat exchanger. The products of combustion are mixed directly with large volumes of outdoor air. Such mixing is considered safe because of the high dilution ratio.

In addition, thorough burning of the natural gas takes place so that no harmful products of combustion enter the airstream. One product of combustion is water vapor, which can be problematic with very tight building construction due to the potential for condensation in colder climates. For tight buildings, it is best to consider the use of indirect-fired equipment. There are several common configurations of direct-fired units.

The 80/20 design can vary the quantity of outside air from 100% down to 20%. The burner has a high turndown ratio and only outside air should be drawn across the burner. The discharge air volume is fixed and the quantity of outside air can be adjusted to maintain building pressurization. The makeup air unit configuration is ideal for supplying large quantities of replacement air for facility exhaust systems. Such systems can include paint spray booths and other industrial exhaust applications. These units supply a fixed amount of 100% outside air. Air Recirculation units recirculate a fixed 80% of the air while bringing in 20% outside air that passes over the burner. They supply a minimum amount of ventilation to spaces that do not require large quantities of outside air to maintain building pressurization.

Variable Air Volume VAV units have a varying supply air volume and work similar to the 80/20 design, however, the recirculated air passes through a bypass section instead of through the facility. This type of unit is used in applications where building pressurization is desired and where contaminants located in the space cannot be recirculated.

Facilities that have indoor vehicle operation frequently accumulate carbon monoxide and associated noxious fumes. For such instances, a ventilation sequence can be instituted to limit this potentially harmful buildup. The pressurization units for these buildings could be fitted with a carbon monoxide detector with an initial setpoint of, for example, 50 ppm that would trigger an alarm and energize a time-delay relay.

If the condition still exists after, say, five minutes, then the return air damper would modulate closed and the outside air damper would modulate to the 100% open position. A second setpoint of 100 ppm would initiate an alarm and eventually de-energize the burner. The dampers would then be positioned to enable the exhaust mode. After the carbon monoxide returns to a safe level, the sensor could reset the unit to normal operation.

In addition to emergency ventilation to purge carbon monoxide, the pressurization unit can be used to provide a source of ventilation during warm and hot months of the year. A reverse acting thermostat would modulate the outside air and return air dampers to bring in more outside air to maintain the indoor setpoint. These units can also be fitted with either chilled water or DX cooling coils to provide tempered or conditioned air to the space during the summer.

Direct-fired pressurization units are approximately 93% efficient, with much of the available energy in the burned natural gas being delivered to the space in the form of heat. The supply airstream absorbs the heat that would be normally lost through a flue or vent pipe as in the indirect-fired configuration.

A feature of this type of equipment that is especially important in cold climates is its ability to keep the building slightly pressurized to typically at 0.01 in. wc. This reduces the infiltration of cold air, and when overhead doors are opened, the outside air dampers are modulated open to admit more air to maintain this level of pressurization. This feature also acts as virtual ductwork to distribute air where it needs to go: toward cracks or areas with open doors.

Direct-fired pressurization equipment is capable of delivering plenty of ventilation to a facility when it is configured as the 80/20 system described previously. Infrared systems, however, may need to be supplemented with a separate ventilation system.
With certain classes of facilities, the infrared system would not require additional equipment for ventilation, since large structures with many doors would provide enough natural ventilation through leakage.

Gas-fired pressurization systems each offer good solutions to heating large structures such as warehouses, distribution centers, aircraft hangers, and manufacturing facilities. Situations that require significant ventilation favor pressurization air handlers as do facilities that generate dust, mists, and other contaminants. Cleaner environments with less intensive ventilation needs would do well to go with the infrared radiant heating approach. In very cold climates, it might be beneficial to go with a hybrid solution using radiant heat at the perimeter as the primary heating source and placing pressurization units at the center of the facility for ventilation and for pressurization when doors are open.

Additional information can be found at the Canada Blower company web site http://canadablower.com/index.html.

Susan Terlitski
Industrial Air Make-Up Systems Engineer
Canada Blower
http://canadablower.com/ahu/index.html
http://canadablower.com/oem/index.html

About the Author

Canadian air-handling systems designer.

Mrs. Siouzanna Terlitskaia, nis-co@nis-co.com
Canadian Air Systems

http://nis-co.com

Is there a central air conditioning tech out there willing to listen to story and give my an opinion?

Tenant called 01-02-08 reports no heat in Las Vegas home. Property management called warranty company who sent out tech who cleaned the coils $280.00.
June 3rd tenant called- ac again. Technician went out & replaced 6lbs of freon and fixed a shroud leak. 20 Days later warranty company sends out different technician who says the compressor was installed improperly(not bolted down) "something" is dried out because of this- compressor is blown. Warranty won't cover it. Compressor was replaced 4 years ago and has worked fine. Help! I don't know if warranty company is trying to get out of repairs, if one of first 2 techs made a mistake to cause compressor failure or what. I have not been on the roof, but it's kind of strange that the compressor wouldn't be bolted down and no one would know for 4 years & the first 2 techs wouldn't notice the compressor installed improperly.Second tech said he thinks he would've noticed the bolts gone & that wouldn't cause compressor to fail anyway

Ok, i see where cleaning the coils would cause the heat to malfunction due to reduced air flow. That one's ok.

I can see the 2nd tech did a good job by actually repairing the leak instead of just filling it up and leaving. Ok there.

Why was another tech sent out 20 days later...No cooling again?

A compressor not being bolted down will not kill it. Worst that could happen is a stress crack in the copper tubing connected to it.

Warranty companies suck and they will do anything they can to get out of repairs. All it takes is a tech saying something wasn't installed correctly and they use that to deny paying.

Was the compressor replaced by the same warranty company? Tell them that if they won't pay due to the compressor not being bolted down, make the service company that replaced the compressor pay for it since it was their fault.

I doubt you get very far this way but unfortunatly, the warranty company is holding the cards right now due to the misinformation of the last technician.

Note: Compressors come from the factory with a 5 year or longer warranty. If the compressor was replaced during the manufacturer's warranty period, then only the remainder of the factory warranty applies to the new compressor. Also, the manufacturer doesn't cover labor on the installation of the new compressor. I doubt you have any warranty left on the compressor unless it came with a 10 year factory warranty and you are still within the time allowed. Even still, labor, freight, refrigerant, and other misc parts won't be covered.