Introduction
Every piece of equipment that we have introduced in this course requires controls for operation. Some equipment, such as a rooftop package unit, will likely come with factory-installed controls, except for the thermostat. The thermostat has to be mounted in the space and wired to the packaged unit. In other built-up systems, every control component may be specified by the designer and purchased and installed under a separate contract from the rest of the equipment.
Whether the controls are a factory package or built-up on site, well-designed controls are a critical part of any HVAC system. The controls for a system may differ from project to project for a number of reasons. Design considerations for controls choices include availability of expertise in maintenance and operations of the controls, repair and maintenance expense budgets and capital costs of control equipment.
To elaborate, one should always choose controls that are suited to the avail- able maintenance and repair expertise and availability. Find out how the client will be arranging maintenance of the system. As an example, it is generally unwise to choose the latest and greatest high-tech controls for a remote school, unless the school has a maintenance system in place to support the controls. It is generally better to aim for simplicity and reliability in this type of situation.
On the other hand, if the client has experienced, well-trained, controls staff available, on site or by contract, there is an opportunity to specify something quite sophisticated. As always, economics plays a controlling role and the challenge is to demonstrate how the sophisticated computerized system will perform better and save energy compared to a simple off-the-shelf option.
There are several types of controls and each has specific features that make it by far the best choice in particular circumstances. The following is a brief introduction to the main types.
Control Types
Controls fall into broad categories based on a particular feature.
Self-powered Controls require no external power. Various radiator valves and ceiling VAV diffusers have self-powered temperature controls. These units are operated by the expansion and contraction of a bellows that is filled with a wax with a high coefficient of expansion. As the temperature rises, the wax expands, lengthening the bellows. This closes the radiator valve (cuts back on heating) or opens the VAV diffuser (increases the cooling). The advantage of these units is that they require no wiring or other connection so installation cost is minimal.
Electric Controls are powered by electricity. We will introduce two types of electric controls in this course:
On/off Electric Controls are used in almost every system to turn electrical equipment on and off. The electric thermostat is the most common example.
Modulating Electric Controls are based on small electric motors and resistors that provide variable control.
Pneumatic Controls are controls that use air pressure: the signal transmission is by air pressure variation and control effort is through air pressure on a di- aphragm or piston. For example, a temperature sensor may vary the pressure to the controller in the range of 3 psig to 15 psig (pounds per square inch gauge). The controller will compare the thermostat line pressure with the set- point pressure and, based on the difference, adjust the pressure to the heating valve to open, or to close, the valve. The heating valve will typically have a spring to drive it fully open and the increasing air pressure will close the valve against the spring. The valve is called a “normally open” valve, since failure of the air system would have air pressure fall to zero and the spring would open the valve. A “normally closed” valve is the opposite, with the spring holding it closed until the air pressure opens the valve.
Pneumatic controls require a continuous source of compressed air at 15 psig for sensing and controlling. When considering the total cost of the pneumatic system, the provision of the compressor(s), the operation and maintenance cost, and the energy lost with leaks have to be factored into the total cost. However, the pneumatic system does have the advantage of relatively inexpensive and powerful actuators (a device that moves a valve or damper) and it is relatively easy to learn to maintain and service.
Electronic Controls, or more correctly Analogue Electronic Controls, use varying voltages and currents in semiconductors to provide modulating con- trols. They have never found great acceptance in the HVAC industry, since Direct Digital Controls offered much more usability at a much lower price.
Direct Digital Controls, DDC, are controls operated by one, or more, small computer processors. The computer processor uses a software program of instructions to make decisions based on the available input information. The processor operates only with digital signals and has a variety of built-in inter- face components so that it can receive information and output control signals.
There are many instances where the types of controls are mixed. For example a DDC system could have all electric “sensors,” the units that measure temperature, humidity, pressure or other variable properties. This same system may also have pneumatic actuators on all the valves, since pneumatics provide considerable power and control at low cost. A “transducer” creates the interface between the electrical output of the DDC system and the valve. The transducer takes in the DDC signal, say a voltage between zero and ten volts, and converts it to an output of 3psi to 15psi. Thus, at zero volts the output will be 3psi, rising to 15psi at ten volts.
We will spend considerable time on DDC controls later in the chapter. For now, let us consider the basics of controls—what makes them work.