Benefit from supplemental heat for industrial spaces
Today’s industrial or warehouse spaces are different from other comfort conditioning applications. These buildings typically have high ceilings and may have large ventilation requirements. Depending on the geographic location and the processes within the building, the building heat requirement may be large or modest, but industrial buildings in all but the warmest climates require some heat for at least part of the year. The building primary heating system could be furnaces, air heaters, radiant floors, a steam hydronic system, or even byproduct hydronic heat from process hot water.
Keeping It Positive
Most industrial buildings can benefit from supplementary heating and airflow equipment. An issue for many facilities is maintaining correct building pressurization. Today’s stringent industrial ventilation requirements can result in a partial vacuum in the building. If there are open-flame processes in the building such as welding, brazing, ovens, boilers or heat-treating equipment, then negative building pressures can create problems.
A partial vacuum could affect flame stability and the ability to maintain exhaust flows. Another issue with negative pressure is that building doors become difficult to open or control, and this may be a safety hazard. Further, negative pressures will draw unfiltered outdoor air into the building that may contaminate manufacturing or food processes and may also be detrimental to employee health. A challenge in maintaining correct pressurization is that building pressure levels can change rapidly with ventilation units being cycled, or large doors being opened.
Makeup Air Heating Needed
When neither heating or cooling are required, problems with negative building pressures may be relieved simply by leaving open large doors, or by continuous operation of propeller fans to supply makeup outdoor air. During the heating season, these solutions won’t work, at least if building comfort and efficiency are issues. The best solution is often the use of air handlers with natural gas-fired heating to supply filtered building makeup air.
Makeup air handlers commonly have heaters that are either direct-fired or indirect-fired. Direct-fired units are more common. These are usually installed on a rooftop, but can be free-standing next to the building on a pad or elevated platform. Vertical makeup units are sometimes mounted on an exterior wall. Makeup air handlers are designed to provide variable volumes of pre-heated filtered outdoor air to the building to offset ventilation losses and building leakage.
Match Flows to Demand
Usually makeup air heating units are a “draw-thru” design, that is, the blower is downstream from the heating section. Some units use a variable-frequency drive (VFD) on the blower to allow the unit to modulate airflow to match building ventilation rates, or match a set building air pressure.
For units that will operate year-round, packages are also available with cooling/dehumidification coils as well as heating sections. This type unit is widely used with facilities such as food processing, wastewater treatment and other industries where it is important to maintain building pressures and control the indoor environment at all times.
Douglas Kosar is an engineer with the Gas Technology Institute, specializing in building energy efficiency. He indicates that for industrial spaces, makeup air handlers are especially important. “They reduce infiltration of cold outdoor air and drafts through open doors, i.e., if negative building pressure and resulting infiltration is allowed it can defeat proper operation of air curtains at large shipping/receiving doors. Further, they can promote proper operation of exhaust systems.”
Warehouses are Different
Kosar points out that warehouse spaces generally have much lower required exhaust flows. “But bonded warehouses may be required to maintain specific temperature and humidity ranges and should employ controlled tempered introduction of any needed makeup/ventilation air. This can be a requirement as well in industrial facilities that must maintain specific temperature and humidity ranges for manufacturing processes such as food and beverage, or pharmaceuticals.”
To install the correct size makeup air system, Kosar says, it is necessary to add up all the exhaust equipment to determine the makeup air requirement, plus ventilation air for workers, and a little extra for some positive pressurization. “That airflow combined with the needed temperature rise from winter design air temperature will determine the heating capacity.”
Focus on Specific Need
Kosar points out that in designing new facilities or retrofitting existing facilities, owners should consider isolating outdoor air treatment to a limited number of specialized, dedicated outdoor air systems. “This approach allows energy efficiency technologies to be more cost effectively applied while delivering improved facility operations.”
Units can be controlled on both a temperature and a building pressure basis to assure maximum comfort and efficiency. The heaters can either be non-ducted or ducted. Ducted air handler outlets are sometimes used to deliver heated air to lower levels or to more distant parts of the building that need extra heat.
Generally the direct-fired heaters are available in the larger capacities – up to 100,000 cfm and sometimes more. Commonly the design temperature rise is approximately 60° to 75°F. Because of the greater weight of indirect-fired units, you may need to install special structural supports for such units in rooftop installations.
Advantages to Direct-Fired
According to John Szymanski from Trane, a supplier of such equipment, the direct-fired models provide lower first cost and a smaller footprint. He adds, “Because they also have a lower internal static pressure, a smaller blower is required.” According to Szymanski, the direct-fired units can also be ordered in a vertical arrangement. “This can assist in air turnover for structures with large floor to ceiling height like warehouses and heavy manufacturing.”
For spaces where outdoor air makeup is not a primary requirement, another solution to improve building comfort is dedicated air turnover units, sometimes called air rotation units. These floor-mounted indoor or outdoor units are designed to overcome inefficiencies caused by air stratification in high-ceiling spaces. They add to building comfort by adding primary or supplementary heating and cooling. In a high industrial or warehouse space without air rotation, ceiling level air temperatures can be as much as 25 degrees higher than at the floor level.
Mixing is the Key
In heating mode, these units draw in air near the floor level and pass it over heating sections as needed and direct it upward in the tall plenum, which can be extended as needed to reach even very high ceilings. Warmed low-velocity conditioned air exits at ceiling level and spreads across the ceiling level, slowly tumbling down the sidewalls. This causes room air to mix and floor level temperatures become much closer to those at the ceiling. In this way, thermostat levels can be set much lower to achieve the same level of comfort, and significant energy is conserved.
A third approach that is widely used in spot industrial applications is unit heaters. These are compact high output units using gas heating with high-velocity air distribution by either propeller or centrifugal fans. Unit heaters are a common solution near high heat-loss areas such as loading docks or openings to unheated warehouse space.
Unit Heaters for Spot Comfort
Unit heaters are available in either direct-fired or indirect-fired styles. For maximum operating efficiency with indirect firing, Modine Manufacturing offers its Effinity93 Model PTC which features a four-pass heat exchanger. The final pass is a condensing coil to extract the maximum heat from the combustion gases. Units are available in sizes from 55,000 to 310,000 BTU/hr — all operating at 93% efficiency. Because it is a condensing unit, it is necessary to pipe condensate to a plant drain.
In addition to these three approaches — makeup air heaters, air turnover units and unit heaters — there are specialty gas-fired heating products such as gas infrared heaters and gas-fired heated air knives for installation at large open doorways. In all cases, owners enjoy the benefits of gas space heating at fuel costs that are highly attractive compared to the other alternatives – oil, propane or resistance electric heat.
Natural Gas a Natural Choice
To achieve a comfortable and efficient industrial environment, it is usually necessary to offset exhaust air flows with makeup air handlers. Air turnover devices assure more even distribution of heat in high-ceiling spaces, and improve overall heating efficiency. Specialty devices such as unit heaters, heated air knives, and gas infrared heaters improve comfort levels in high heat loss areas.
For all of these applications, natural gas is the preferred energy source because it is clean, safe, efficient and affordable. Your engineer can help you improve building comfort and actually reduce heating expense by installing the right kind of supplementary heating equipment.