Carrier 37HS Control Applications, System-PoweredControls, Ð High and Low Pressure Pick-UpTubes

Fig. 35 Ð High and Low Pressure Pick-Up Tubes

BELLOWS PRESSURE Ð As the air¯ow through the unit changes, the high and low pressures vary proportionately. Comparing these 2 pressures, the unit volume controller pro- vides a bellows pressure which in turn in¯ates the unit air valve. Bellows pressure varies from near zero at full ¯ow to plenum or duct pressure at shutoff.

UNIT AIRFLOW DELIVERY Ð The relationship between the bellows pressure and the plenum pressure determines the unit air valve position, which controls unit air¯ow delivery. When the pressures become nearly equal, the valve closes and the unit shuts off. Conversely, as the bellows pressure approaches zero, the valve opens completely and the unit

delivers maximum ¯ow, as determined by the existing ple- num pressure. Units normally operate between these ex- tremes. In constant volume applications, the bellows pres- sure is automatically adjusted in proportion to the plenum pressure, providing a constant pre-set ¯ow within the oper- ating limits of the unit. In variable air volume op- eration, the bellows pressure is further modulated to reduce ¯ow below the preset level as load requirements are satis- ®ed. After passing through the valve, the primary air ¯ows down through the diffuser and out to the conditioned space.

CONTROL APPLICATIONS

Introduction Ð The Moduline terminal is offered with a variety of cooling and heating control options which adapt to many building applications. In this section, the various control applications are described and the functions they in- clude are identi®ed. Each application has a function number which corresponds to the function number shown in Tables 14 and 15 in the Control Selection section, page 50. Table 14 describes the functions; Table 15 shows the control packages required for each function. The part numbers shown are found on the Moduline air terminal price pages.

Detailed operating sequences for each control application are found beginning on page 55.

System-Powered Controls

COMPONENTS OF THE SYSTEM-POWERED CON- TROL SYSTEM Ð The 37HS system-powered control con- sists of a set of custom design and manufacturer's compo- nents which provide air¯ow and temperature regulation of a Moduline air terminal. These devices are interchangeable, ®eld-installed components which plug into the Moduline unit without the use of tools.

Figure 36 shows the basic components: Control block (part of the unit); ®lter/manifold; volume controller; and thermo- stat. Figure 37 demonstrates the control air paths in a sec- tional view of the control system. Note that the high pres- sure and low pressure pick-ups of the unit are connected through the control block to the ®lter and from there to the volume controller and thermostat.

Figure 38 shows the control ®lter/manifold; Fig. 39, the air¯ow volume controller; Fig. 40, the diffuser thermostat with aspirator; and Fig. 41, the wall thermostat and alternate to diffuser mounting.

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