URS Corporation has been fortunate to be involved with evaluating the relative merits of different approaches to heating and ventilating such facilities at a number of geographic locations across the United States. Two of the most common approaches include the use of low-intensity infrared heaters coupled with exhaust fans and the use of direct-fired pressurization units.
Infrared Radiant HeatInfrared is the transmission of energy by electromagnetic waves, which travel in straight lines, and can be reflected and heat solid objects, but which do not heat the air through which it is transmitted. Infrared energy emitted from heaters warms floors and objects that in turn release heat to the air by convection. Re-radiation to surrounding objects also contributes to comfort in the area. Radiant heat can be turned off when not required and can effectively return a space to setpoint quickly. Human comfort is determined by the average of mean radiant and drybulb temperatures.
With radiant heat, it may be possible to set the drybulb temperature lower to provide a given comfort level. This can lead to a reduction in energy loss through the envelope of the building. The nature of radiant heat works to keep temperature stratification and convective air currents to a minimum. There are three categories of infrared radiant heating equipment:
- Low-intensity has a source temperature of 300° to 1,200°F;
- Medium-intensity utilizes source temperatures of 1,200° to 1,800° and
- High-intensity has radiant source temperatures ranging between 1,800° and 5,000°.
The most appropriate and widely used type of infrared heater for the application of heating large spaces such as warehouses, distribution hubs, and aircraft hangers is the gas-fired, low-intensity type. In addition to off-the-shelf systems, the manufacturer of radiant systems may be able to engineer a custom installation. The resulting design would take into account the height of the facility, building geometry, location of occupants, required heat density, and tube spacing.
The gas-fired, low-intensity infrared heating system consists of a series of burners, combustion chambers, steel radiant tubing, and reflectors. The equipment is hung from the structure and is pointed downward toward the facility floor. The burner mixes fuel and air that is burned in the combustion chamber. The hot gases exit into the radiant tube where the energy is liberated to the space in the form of radiant and convective heat.
In some applications, a vacuum pump will aid in drawing the gases down the length of the radiant tube. Reflectors attached to the top of the tube direct much of the radiant energy toward the floor. Reflectors can be installed to reflect at various angles to accommodate specific building geometry. Systems can also be designed to obtain the combustion air directly from the outdoors instead of drawing this air from inside the facility.
Efficiency of gas-fired, low-intensity infrared heating equipment is approximately 78% although it is not specifically listed in the manufacturers' literature. This literature does, however, discuss approaches to design that allow for installation of heating capacity less than the calculated heat loss for the building. The approach cites comfort studies showing how mean radiant temperature correlates to human comfort.
Consequently, these studies suggest that the indoor air temperature can be held at a lower level and achieve the same comfort when the surrounding solid objects are heated through radiant methods.
There are some limitations in using infrared heat due to the nature of this equipment. The following precautions for the application of infrared heaters is taken in part from the 2000 ASHRAE Systems and Equipment Handbook and should be considered before commencing with design:
- Infrared heaters have a high surface temperature and should not be used when the atmosphere contains ignitable dust, gases, or vapors or when the atmosphere contains gases, vapors, and dust that can decompose into hazardous or toxic materials in the presence of high temperature and air.
- Manufacturers' recommendations for clearance between a fixture and combustible material should be followed. Warning notices defining proper clearances should be posted near the fixture.
- Manufacturers' recommendations for clearance between a fixture and personnel areas should be followed to prevent personnel stress due to local overheating.
- Adequate makeup air must be provided to replace the combustion air used by the heaters.
- For comfort, personnel should be protected from substantial wind or drafts. Suitable windshields seem to be more effective than increased radiation density.1
Gas-Fired Pressurization UnitsThis type of equipment moves 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 Rotation™ 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.
- 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 are prone to the accumulation of 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 92% 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 "invisible ductwork" to distribute air where it needs to go, namely toward cracks or areas with open doors.
ComfortA properly designed radiant heating system can provide excellent comfort to building occupants. This is especially true of large, high-bay structures where it can be argued that forced air systems permit stratification with warm air hovering at the roof and cold air pooling down at the occupants on the floor.
However, literature on radiant heat notes that shielding from wind is an important factor in achieving acceptable comfort for these systems. As there can be many doors at the perimeter of a certain large structures, a radiant heating system would be less effective at the side of such a facility facing the prevailing winter winds.
On the other hand, a pressurization system would work to force relatively warm air toward doors that are opened, helping to keep the prevailing winds at bay. As for stratification, pressurization systems work to overcome this tendency by moving very large quantities of air at temperature differentials as low as 50°. Note that interior design temperatures of many of these facilities are kept low during the winter months, with 55° not uncommon.
VentilationDirect-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.
As mentioned previously, the nature of certain facilities requires the installation of a substantial ventilation system due to the use of indoor vehicles. Large quantities of fumes can be generated by these vehicles and must be dealt with by the ventilation system.
MaintenanceThe information shown in the sidebar on maintenance may prove useful in making decisions on choosing the appropriate heating and ventilating systems for large structures.
It is important to review these factors with the client prior to commencing with design so that their needs and expectations can be met. These items are to be done on an annual basis and would require the use of a lift. If machines, conveyors, or other equipment are in the way, then a special articulated lift may be required.
There is controversy as to whether the infrared reflectors need to be cleaned on an annual basis as the ASHRAE handbook indicates, since many facility owners do not perform this cleaning and do not report adverse effects. It has been suggested that if the infrared heaters are mounted at least 20 ft above the floor, that they do not accumulate the type or amount of material that significantly degrades the reflector performance.
ConclusionsInfrared radiant and 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. ES
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Recommended Equipment Maintenance
Infrared Radiant Heat
The following maintenance items for infrared heaters is taken in part from the 2000 ASHRAE Systems and Equipment Handbook:
- Gas-fired infrared heaters require periodic cleaning to remove dust, dirt, and soot.
- Reflecting surfaces must be kept clean to remain efficient.
- An annual cleaning of heat exchangers, radiating surfaces, burners, and reflectors with compressed air is usually sufficient. Chemical cleaners must not leave a film on reflector surfaces.
- The main and pilot air ports should be kept free of lint and dust.
Gas-Fired Pressurization Units
- Inspect overall condition of unit and cabinet.
- Replace filters.
- Clean unit interior.
- Inspect belts, adjust for proper tension, and replace if required.
- Inspect pulleys and alignment.
- Lubricate supply fan motor, and check amp draw against nameplate data.
- Lubricate blower bearings.
- Inspect blower and clean as required. Check for loose or missing weights.
- Inspect burner, and ensure all scale is removed from burner plates. Clean as required. Ensure all gas openings are free from blockage. Use correct size drill to open as required.
- Clean pilot burner, and replace flame/spark rods if damaged.
- Perform a complete operation check of burner: fail test all safeties; light burner and check manifold pressure; visually inspect flame; and calibrate temperature control system.