Chapter 12 HVAC Controls Pneumatic Control Systems • Air Compressor Stations • Transmitters and Controllers • Pneumatic Thermostats • Auxiliary Components • Switching Relays • Controlled Devices • Actuators

Chapter 12 HVAC Controls Pneumatic Control Systems • Air Compressor Stations • Transmitters and Controllers • Pneumatic Thermostats • Auxiliary Components • Switching Relays • Controlled Devices • Actuators

Pneumatic controls must operate properly to control the environment. A pneumatic control system is a control system in which compressed air is used to provide power for the system. Pneumatic control systems were developed in the early 1900s as a primary method of controlling the environment in commercial buildings. See Figure 12-1.

Pneumatic control systems include an air compressor station, transmitters and controllers, additional components, and controlled devices. Pneumatic control systems can be separated into four main groups of components based on their function. The four groups are the air compressor station, transmitters and controllers, auxiliary devices, and controlled devices. See Figure 12-2.

An air compressor station includes the air compressor and auxiliary components such as the air drier, drains, filters, and pressure-regulating valve stations. An air compressor station consists of an air compressor and its auxiliary components. See Figure 12-3. An air compressor takes in air from the atmosphere. The auxiliary components remove particulate matter, mois-ture, and oil and control the air supply pressure and volume.

A bimetallic element is a sensing device that consists of two metals with different expansion and contraction rates that respond to temperature increase or decrease. A pneumatic thermostat is a device that senses room temperature and alters branch line pressure to controlled devices to maintain a space’s temperature. Pneumatic thermostats are available in many shapes and sizes and with many options. All pneumatic thermostats operate on the same basic principles. In a pneumatic thermostat, a sensing device (bimetallic element) is mounted under a cover and exposed to the air in the room. A bimetallic element is a temperature sensing device that consists of two different metals joined together at one end. See Figure 12-4.

Limit thermostats cause a change in a controlled device whenever the set temperature or pressure is reached. A limit thermostat is a pneumatic thermostat that causes a change in a controlled device whenever the set temperature or pressure is reached. Many HVAC systems, such as air-handling units and unit ventilators, require the addition of a limit thermostat to provide HVAC system component safety to the control system. See Figure 12-5. When some type of limit protection is not provided, the system may suffer a freeze-up or some other problem. Limit thermostats are not used for primary building space temperature control but are used only as protection against failure.

Pneumatic humidistats are used to sense and control the humidity in a building space or duct. The construction of a pneumatic humidistat is similar to that of a thermostat, with the exception that the bimetallic element is replaced by a humidity-sensing (hygroscopic) element. See Figure 12-6. The hygroscopic element accepts moisture from, or rejects moisture to, the surrounding air. Pneumatic humidistats are available in both one-pipe and two-pipe versions as well as direct-acting or reverse-acting.

Pneumatic pressurestats are designed to sense and control the pressure inside a duct or building. Pressure inside a duct or building is a major variable that must be controlled in HVAC control systems. A pneumatic pressurestat is a controller that maintains a constant air pressure in a duct or area. Pneumatic pressurestats are designed to sense and control the pressure inside a duct or building, which is measured in inches of water column (in. WC). See Figure 12-7.

Receiver controllers are used for HVAC system control Receiver controllers are used for HVAC system control. A receiver controller accepts one or more input signals from pneumatic transmitters and produces an output signal based on the setup of the controller. A receiver controller is a device that accepts one or more input signals from pneumatic transmitters and produces an output signal based on the setup of the controller. See Figure 12-8. Receiver controllers are used for HVAC system control. Receiver controllers are not normally used to control the temperature, humidity, and pressure in one room of a commercial building.

A single-input receiver controller is designed to be connected to only one transmitter and maintain only one temperature, pressure, or humidity setpoint. A single-input receiver controller is a receiver controller that is designed to be connected to only one transmitter and to maintain only one temperature, pressure, or humidity setpoint. See Figure 12-9. Many air-handling units and central plant systems use single-input receiver controllers to maintain specific desired setpoints in building mechanical systems.

A dual-input receiver controller is designed to take the change of one variable and reset the setpoint of the controller to match the changing condition. A dual-input (reset) receiver controller is a receiver controller in which the change of one variable, commonly outside air temperature, causes the setpoint of the controller to automatically change (reset) to match the changing condition. See Figure 12-10. One transmitter measures the variable that is being controlled, such as hot water temperature. The other transmitter measures the variable that causes the controlled variable to change, such as outdoor air temperature. The controller setpoint change occurs automatically as the outside air temperature changes.

Switching relays are used to change airflow from one circuit to another. A switching relay is a component that switches airflow from one circuit to another. See Figure 12-11. Switching relays are used to change systems from summer to winter operation and create pneumatic logic circuits to enable and disable control system functions.

Minimum-position relays are used to provide fresh air ventilation and prevent outside air dampers from completely closing. Minimum-position relays are low-pressure-limit devices in that the device does not allow the output pressure to fall below a predefined, adjustable low limit. For example, in looking at part of a control system, a pneumatic low-limit thermostat acts as a mixed-air controller with an averaging element in the mixed-air plenum. See Figure 12-12.

Pneumatic positioners ensure damper or valve actuators move to a given position. A pneumatic positioner (pilot positioner) is an auxiliary device mounted to a damper or valve actuator that ensures that the damper or actuator moves to a given position. See Figure 12-13. Sending a certain air pressure to an actuator does not guarantee that an actuator extends a certain length. The extension of an actuator may be opposed by rust, corrosion, or external forces acting on the damper or valve, such as airflow striking the damper blade.

Electric/pneumatic switches enable a pneumatic device such as a damper to respond to the operation of an electrical device such as a fan. One application for electric/pneumatic switches is to control a damper by the operation of a fan. See Figure 12-14. In this application, the electric/pneumatic switch solenoid is wired in parallel with the fan or fan starter. When the fan is ON, the electric/pneumatic switch is energized. When the fan is OFF, the electric/pneumatic switch is de-energized.

Pneumatic/electric switches allow the signal from an outside air transmitter to be used to energize and de-energize a hot water pump. Pneumatic/electric switches are also used to allow the signal from an outside air transmitter to energize and de-energize a hot water pump. See Figure 12-15. In this application, a pneumatic/electric switch is piped to an outside air transmitter. The pneumatic/electric switch energizes a hot water pump when the outside air temperature is below 55°F.

An actuator and linkage control damper regulate the temperature of a building space. An actuator is a device that accepts an air signal from a controller which causes mechanical motion to occur. In HVAC control systems, pneumatic actuators accept airflow from the controller and cause the actuator shaft or valve stem to move. The movement of the actuator shaft or valve stem regulates the flow of air through a damper and the flow of water or steam through a valve. See Figure 12-16. If for any reason the actuators are damaged or are operating incorrectly, valves and/or dampers may not open or close properly. A common damper actuator problem occurs when the linkages slip and the dampers do not fully open or close. Improper valve or damper operation may cause indoor air quality problems such as inadequate airflow, temperature, or humidity conditions. Any actuators found to be damaged must be repaired promptly.

Actuators may be damper or valve actuators Actuators may be damper or valve actuators. Damper actuator components are usually enclosed in the actuator body. Valve actuator components are usually visible from the outside. Actuators may be damper or valve actuators. See Figure 12-17. The basic components of damper and valve actuators are the same, but the physical configuration is different. Damper actuator components are normally enclosed in the actuator body and have a longer stroke than some valve actuators.

Direct-coupled actuators are clamped to the damper shaft. Direct-coupled actuators are high quality, inexpensive, and highly reliable. See Figure 12-18. Direct-coupled actuators are actuators that are directly attached to the damper or valve without the use of a linkage. Direct-coupled actuators are used almost exclusively in most modern control systems. Direct-coupled actuators may use either electrohydraulic or electromechanical methods to create movement.

Dampers are used to control the flow of air and include parallel, opposed, and round blade dampers. A damper is a movable piece of metal in a duct used to control the flow of air. Outside air dampers that are operating improperly can cause indoor air problems. Dampers that are stuck may fail to admit sufficient ventilation air to flush out contaminants. Damper seals that are damaged or missing may allow excessive airflow, causing high humidity levels that allow the growth of microorganisms. Dampers are available in different designs and sizes. Damper designs include parallel, opposed, and round blade dampers. See Figure 12-19.

Control valves control the flow of fluids in an HVAC system Control valves control the flow of fluids in an HVAC system. Control valve components include the valve body, stem, disc, packing, and actuator. The components of a control valve include the valve body, stem, disc, packing, and actuator. See Figure 12-20. The valve body consists of the outer housing through which steam or water passes. The valve body also contains the means for attaching the valve to the piping. Valves are attached to piping using threads, flared fittings, or flanged fittings. The valve body is usually constructed of cast iron, bronze, steel, or stainless steel.

Two-way valves have one inlet and one outlet. A two-way valve is a valve that has two pipe connections. See Figure 12-21. Two-way valves are available in a variety of pipe diameter sizes, from 1/2″ to 27″ or larger. Two-way valves are either normally open or normally closed. A normally open (NO) valve is a valve that allows fluid to flow when the valve is in its normal position. A normally closed (NC) valve is a valve that does not allow fluid to flow when the valve is in its normal position. Normally closed valves have a plug on the bottom of the valve body, which enables the disassembly and removal of the disc and other internal parts.

Mixing valves and diverting valves have similar designs Mixing valves and diverting valves have similar designs. Mixing valves have two inlets and one outlet. Diverting valves have one inlet and two outlets. A three-way valve is a valve that has three pipe con-nections. Three-way valves are used in HVAC systems to control the flow of water because they provide a constant system pressure in the supply piping and through the pumps, boilers, heat exchangers, and chillers. Three-way valves may be mixing valves or diverting valves. See Figure 12-22.