Energy conservation measures for air distribution and HVAC systems

Energy conservation measures are modifications that can be made to air distribution and HVAC systems to improve energy efficiency. These modifications may require a significant cost investment and are permanent improvements. The modifications should be considered under any energy management program for existing or proposed air distribution and HVAC systems.
By Thomas E. Mull, PE, Thomas & Mull Associates, St. Louis, MO October 10, 2004
Key Concepts
  • Convert fans from constant volume to variable volume.

  • Use evaporative cooling wherever possible.

  • Convert steam heaters to gas-fired heaters.

    Convert air systems
    Process local exhaust
    Evaporative cooling
    Convert from steam to gas
    Radiant heaters
    More Info:

    Energy conservation measures are modifications that can be made to air distribution and HVAC systems to improve energy efficiency. These modifications may require a significant cost investment and are permanent improvements. The modifications should be considered under any energy management program for existing or proposed air distribution and HVAC systems.

    Convert air systems

    Many air distribution systems are constant volume despite the fact the load varies. Constant volume systems operate continuously at peak airflow, which results in peak energy consumption by the fan motor.

    If the load varies with time, an opportunity exists for significant fan energy savings by converting a constant air volume system to a variable air volume (VAV) system which varies the air flow rate based on the actual heating or cooling load. Existing fans and air distribution systems can be converted from constant volume to VAV with some modifications to the fan and air distribution system.

    To convert a main supply fan from constant volume to variable volume, some type of fan capacity control is required. There are four control methods; discharge dampers, a fan bypass arrangement, fan inlet vanes, and variable speed drives. Discharge dampers and fan bypass arrangements are not recommended because they produce little energy savings (Fig. 1).

    Inlet vanes are an effective air capacity control method for fans (Fig. 2). Other types of fans, such as axial and vane-axial fans, are also available with inlet vanes. Although the fan is still rotating at full speed, energy is saved since the fan is not fully loaded. The inlet vanes also impart a pre-rotation swirl to the air as it enters the fan. The reduction in airflow and the pre-rotation swirl both reduce the load on the fan motor.

    Airflow at the load is typically controlled by some type of terminal unit with an automatically controlled and actuated air damper. The damper opens and closes to control the air flow rate in response to a thermostat at the load.

    To control the airflow rate of the fan, it is advisable to install a static pressure sensor in the main supply air duct. As the terminal units open or close to vary the airflow rate at the load, the pressure in the duct varies accordingly. As a result, the airflow of the fan is readily controlled by measuring the static pressure in the duct. A pressure sensor is typically located about two-thirds of the distance from the fan to the most remote terminal.

    Converting from a constant volume air system to a VAV system could result in an annual fan motor energy savings of almost 52%.

    Process local exhaust

    When a process is hot or discharges pollutants into indoor air, such as fumes, dust, or mist, it is necessary to remove the heat or pollutants from the indoor air. Frequently, the heat and/or contaminants are removed by general building exhaust.

    Allowing the heat or contaminants to be discharged to indoor air results in higher exhaust airflow rates than necessary. As a result, energy is usually wasted in heating or cooling the incoming makeup air. Providing local exhaust for processes can reduce the amount of makeup air that must be heated or cooled.

    Local exhaust systems frequently have a hood located over the process or surrounding the process as much as possible. Occasionally, makeup air is supplied directly at the process to help contain the heat or pollutants (Fig. 3). Local exhaust systems are able to contain heat and contaminants since the systems use the capture velocity of the air rather than volumetric airflow.

    Evaporative cooling

    Many industrial and manufacturing buildings are mechanically cooled. Cooling may be provided for occupant comfort or for a particular process.

    Although cooling may be either required or desirable, cooling large nonoffice-type buildings mechanically can be costly due to the energy consumed by the cooling equipment. An alternative to mechanical cooling of industrial and manufacturing buildings is evaporative cooling.

    Evaporative cooling can be energy efficient and cost effective if current technology and equipment are used. In many applications, evaporative cooling can provide all of the cooling for a facility or it can be supplemented by mechanical cooling. Evaporative cooling is quite energy efficient and cost effective where large amounts of outdoor air are brought into a facility and then exhausted outdoors.

    In many applications, cooling is achieved by passing air, that is cooler than the skin, over the body. Evaporative coolers are well suited to this application. In evaporative coolers, the air is cooled by evaporation of water. When water is evaporated into the air, it results in direct cooling and a reduction in air dry bulb temperature.

    The exiting dry bulb temperature of the air and the amount of cooling is a function of the entering wet bulb temperature of the air. If the system brings in 100% outdoor air, or a large percentage of outdoor air, the performance of the system is quite dependent upon climatic conditions.

    In geographic locations with high dry bulb temperatures and relatively low wet bulb temperatures, evaporative cooling performs best. In locations with high coincident wet and dry bulb temperatures, evaporative cooling is less effective.

    Evaporative cooling equipment can be classified as direct or indirect. Direct evaporative coolers cool air by direct contact with water, either by an extended wetted surface or with a series of water sprays.

    Indirect evaporative coolers cool air by transferring heat with a heat exchanger to either a secondary stream of air which has been cooled by evaporation or to water that has been cooled by evaporation in a heat rejection device, such as a cooling tower. In all cases, indirect evaporative cooling rejects heat to a secondary air stream or water stream through a heat exchanger.

    Direct evaporative coolers evaporate water directly into the conditioned air stream. The heat and mass transfer between air and water lowers the air dry bulb temperature and increases the humidity of the air at a constant wet bulb temperature. When water and nonsaturated air/water mixtures come in direct contact, heat and moisture transfer occurs. The amount of heat and water transfer is a function of the difference between their respective temperatures and vapor pressures.

    In direct evaporative cooling, the air is cooled due to the conversion of sensible heat to latent heat. The conversion of sensible heat continues until the air has reached saturation and the air temperature, water temperature, and vapor pressure equalize. This is called adiabatic saturation since no external heat transfer occurs.

    There are several types of direct evaporative air coolers available. They include random media, rigid media, and slinger packaged air coolers, as well as packaged rotary air coolers.

    Random media air coolers contain pads made of wood or plastic fiber/foam. A pump circulates water from a sump, to a water distribution system. Water flows down through the pads as air passes through the pads.

    Rigid media air coolers have sheets of rigid corrugated material as a wetted surface over which the air passes (Fig. 4).

    A slinger air cooler consists of a water slinger in an evaporative cooling section and a fan section. Outdoor air is drawn through the cooling section that has a water spray, an evaporative filter pad, and a pad that entrains moisture.

    Rotary air coolers wet and wash an evaporative pad by rotating the pad through a water bath (Fig. 5).

    Indirect evaporative air coolers include indirect packaged air coolers and cooling tower/coil systems. Indirect evaporative air coolers pass outdoor air or exhaust air from the conditioned space through one side of a heat exchanger. This air, which is called secondary air, is cooled by evaporation. The surfaces of the secondary side of the heat exchanger are cooled by secondary air.

    On the other side of the heat exchanger, primary air (air supplied to the conditioned space) is cooled by transferring heat through the heat exchanger to the secondary air. Only sensible heat transfer takes place between the two air streams.

    Cooling tower/coil systems employ a water-to-air heat exchanger coil to transfer sensible heat from the primary air to water. The water is cooled by circulating and evaporating it through a cooling tower.

    Since the performance of evaporative cooling systems is dependent upon local weather conditions, among other variables, the analysis of evaporative cooling systems is somewhat involved.

    Convert from steam to gas

    Many older industrial and manufacturing buildings are heated with steam. These systems typically consist of a steam boiler, a steam supply piping system, terminal heating units, and a condensate return system. Terminal heating equipment typically consists of steam unit heaters and air handling units with steam heating coils.

    Heating such buildings with steam can be very energy inefficient. First of all, steam systems are notorious for leaking. Most of the leakage occurs in air vents and steam traps. Producing steam is also quite energy inefficient. Many older boilers have efficiencies of 60%, or less.

    Large buildings and facilities can be more efficiently heated by converting from steam heaters to gas-fired heaters and furnaces. Many gas heaters and furnaces are available with combustion efficiencies over 90%. The energy lost in the combustion process for a high-efficiency gas furnace is a fraction of the losses for a steam boiler. Converting to gas also eliminates the distribution system losses inherent in steam systems. Since heat is produced at the point of use, there are no distribution system losses for gas heating systems.

    Radiant heaters

    Many large industrial buildings and facilities are heated with steam unit heaters, hot water unit heaters, and gas-fired unit heaters and furnaces. These buildings frequently have high roofs, large interior volumes, and high rates of outside air infiltration. Heating these buildings in this fashion is typically inefficient and energy wasteful due to the large volumes of the building, high infiltration rates, and temperature stratification inside the building. A more efficient way to heat such buildings is with infrared radiant heating.

    Infrared heaters may be gas-fired or electric powered and used for spot heating or to heat an entire building. Infrared heaters do not use inside air to provide heat to the building and occupants. Infrared energy warms floors and objects within the building, which releases heat to the air by convection. Human comfort is provided by controlling the mean radiant temperature of the space and the air dry bulb temperature.

    Since infrared heating consists of both radiant and convective heat transfer, air movement and stratification in the heated space is minimized. As a result, radiant heating can reduce heating energy consumption by 30%, or more, with equal comfort. Energy savings also result from the fact that comfort can be achieved with radiant heat at lower indoor air temperatures.

    Radiant heaters are classified by the level of radiation intensity. Low-intensity radiant heaters have source temperatures between 300 F and 1200 F. Medium intensity heaters have source temperatures ranging from 1200 F to 1800 F. High-intensity heaters have source temperatures ranging from 1800 F to 5000 F.

    More Info:

    A related article containing additional information on heat recovery methods (“Heat recovery measures for air distribution and HVAC systems”) is available on the PLANT ENGINEERING website, . Questions about HVAC energy conservation can be directed to author Thomas E. Mull at 636-938-6173. Article edited by Joseph L. Foszcz, Senior Editor, 630-288-8776, .

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