TEMPERATURE CONTROLLERS
Temperature controllers can be one- or two-pipe. The sensing element is typically bimetal, liquid filled remote bulb, or liquid filled averaging capillary tube. Dimensional change of the element with temperature change results in flapper position change and therefore, pilot and branch pressure change.
HUMIDITY CONTROLLERS
in respect to atmospheric pressure or another pressure source. The low-pressure controller is available in both bleed-type and pilot-bleed designs.
Figure 20 shows a schematic of a bleed-type, low-pressure controller. The direct-acting pressure sensor measures static pressure from a pressure pickup located in a duct. A reference pressure, from a pickup located outside the duct, is applied to the other side of the diaphragm.
Principles that apply to temperature controllers also apply to humidity controllers. The primary difference between temperature and humidity controllers is in the type of sensing element. The sensing element in a humidistat is usually a band of moisture-sensitive nylon. The nylon expands and contracts with changes in the relative humidity of the air.
The humidistat can be used in a one-pipe or two-pipe configuration and is available as either a bleed-type humidistat or a two-pipe capacity humidistat using a capacity amplifier. The humidistat may be direct or reverse acting. The high- capacity humidistat has a capacity amplifier.
PRESSURE CONTROLLERS
Pressure controllers can be divided into two classes according to the pressure range of the measured variable. High- pressure controllers measure and control high pressures or vacuums measured in pounds per square inch or in inches of mercury (e.g., steam or water pressures in an air conditioning system). Low-pressure controllers measure and control low pressures and vacuums measured in inches of water (e.g., pressure in an air duct).
High- and low-pressure controllers have different size diaphragms. In both types, one side of the diaphragm is connected to the pressure to be controlled, and the other side is connected to a reference pressure. Pressures can be measured in respect to atmospheric pressure or another pressure source.
The low-pressure controller is available in both bleed-type and pilot-bleed designs.
Figure 20 shows a schematic of a bleed-type, low-pressure controller. The direct-acting pressure sensor measures static pressure from a pressure pickup located in a duct. A reference pressure, from a pickup located outside the duct, is pplied to the other side of the diaphragm.
On an increase in static pressure, the increased force on the diaphragm exceeds the force of the setpoint spring, pulling the main lever downward. A setpoint adjustment screw determines the tension of the setpoint spring. As the main lever is pulled downward, it moves closer to the nozzle, restricts the airflow through the nozzle, and increases the pressure in the branch. The action continues until the pressure on the feedback bellows balances the static pressure on the diaphragm.
On a decrease in static pressure, or if the static pressure sensor is piped for reverse action (high- and low-pressure pickups reversed), the diaphragm moves upward to move the main lever away from the nozzle and reduce the pressure in the branch.
For differential pressure sensing, the two pressure pickup lines connect to opposite sides of the pressure sensor diaphragm.