Primary Air Systems

PRIMARY AIR SYSTEMS

. Some primary air systems operate with 100% outdoor air at all times. Systems using return air should have provision for operating with 100% out< door air to reduce operating cost during certain seasons. In some
systems, when the quantity of primary air supplied exceeds the ventilation or exhaust required, the excess air is recirculated by a return
system common with the interior system. A quality filter is desirable in the central air treatment apparatus. If it is necessary to maintain a given humidity level in cold weather, a humidifier can usually be installed. Steam humidifiers have been used successfully. The water sprays must be operated in conjunction with (1) the preheat coil elevating the temperature of the incoming air or (2) heaters in the spray water circuit.

The cooling coil is usually selected to provide primary air at a dew point low enough to dehumidify the system totally. The air leaves the cooling coil at about 50°F or less, and is almost completely saturated.

The supply fan should be selected at a point near maximum efficiency to reduce power consumption, heating of the supply air, and
noise. Sound absorbers may be required at the fan discharge to attenuate fan noise.

Reheat coils are required in a two-pipe system. Reheat is not required for the primary air supply of four-pipe systems. Formerly, many primary air distribution systems for induction units were designed with 8 to 10 in. of water gage static pressure. With energy use restrictions, this is no longer economical. Good duct design and elimination of unnecessary restrictions (for example, sound traps) can result in primary systems that operate at 4.5 to 6.0 in. of water gage. Primary air distribution systems servicing fan-coil systems can operate at pressures 1.0 to 1.5 in. lower. Careful selection of the primary air cooling coil and induction units for reasonably low air pressure drops is necessary to achieve a medium-velocity primary air system. Distribution for fan-coil systems may be low velocity or a combination of low- and medium-velocity systems. . Variations in pressure between the first and last terminals should be minimized to limit the pressure drop across balancing dampers.

Room sound characteristics vary depending on unit selection, air system design, and the manufacturer. Units should be selected by considering the unit manufacturer’s sound power ratings, the desired maximum room noise level, and the acoustical characteristics of the room. Limits of sound power level can then be specified to obtain acceptable acoustical performance.

PERFORMANCE UNDER VARYING LOAD

Under peak load conditions, the psychrometrics of induction unit and fan-coil unit systems are essentially identical for two-and four- pipe systems. The primary air mixes with secondary air conditioned by the room coil in an induction unit prior to delivery to a room.
Mixing also occurs in a fan-coil unit with a direct-connected air supply. If the primary air is supplied to the space separately, as in fan-coil systems with independent primary air supplies, the same effect would occur in the space. The same room conditions result from two physically independent processes as if the air was directly connected to the unit.

During cooling, the primary air system provides a portion of the sensible capacity and all of the dehumidification. The remainder of the sensible capacity is accomplished by the cooling effect of the secondary water circulating through the unit cooling coils. In winter, primary air is provided at a low temperature, and if humidity control is provided, the air is humidified. All room heating is supplied by the secondary water system. All factors that contribute to the cooling load of perimeter space in the summer, with the exception of the transmission, add heat in the winter. The transmission factor becomes negative when the outdoor temperature falls below the room temperature. Its magnitude is directly proportional to the difference between the room and outdoor temperatures.

For in-room terminal unit systems, it is important to note that in applications where primary air enters at the terminal unit, the primary air is provided at summer design temperature in winter. For systems where primary air does not enter at the terminal unit, the primary air should be reset to room temperature in winter. A limited amount of cooling can be accomplished by the primary air operating without supplementary cooling from the secondary coil. As long as internal heat gains are not high, this amount of cooling is usually adequate to satisfy east and west exposures during the fall, winter, and spring, because the solar heat gain is reduced during these seasons. The north exposure is not a significant factor because the solar gain is very low. For the south, southeast, and southwest exposures, the peak solar heat gain occurs in winter, coincident with a lower outdoor temperature

In buildings with large areas of glass, the transmitted heat from indoors to the outside, coupled with the normal supply of cool primary air, does not balance internal heat and solar gains until an outdoor temperature well below freezing is reached. Double-glazed
windows with clear or heat-absorbing glass aggravate this condition because this type of glass permits constant inflow of solar radiation during the winter. However, the insulating effect of the double glass reduces the reverse transmission; therefore, cooling must be available at lower outdoor temperatures. In buildings with very high internal heat gains from lighting or equipment, the need for cooling from the room coil, as well as from the primary air, can extend well into winter. In any case, the changeover temperature at which the cooling capacity of the secondary water system is no longer required for a given space is an important calculation.