2341 Simple controllers
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Simple electric controllers include manual switches that can start or stop HVAC pumps, or they can be switches that are set to open or close an electric circuit at a discrete process value Such switches are operated by pressure, differential pressure, temperature, differential temperature, level, flow, speed, volts, and amperes Most of them have an internal adjustment to control the set point at which the switch either opens or closes an electric circuit Elementary mechanical controllers consist of pressure regulators, pressure-relief valves, balance valves, and self-operated temperature controllers Pressure-relief valves protect water systems from excessive pressures Care should be taken in the use of pressure regulators and balance valves because of the possibilities of energy waste Other basic controllers are of the analog type such as pneumaticallyoperated controllers where the output is directly proportional to control pressure or follows a known curve with respect to control pressure
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2342 Digital controllers
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Most HVAC systems are operated by some form of digital controller There are a number of different digital controllers ranging from simple single-function controllers to complete universal controllers handling all functions of the interrogation and control of a HVAC water system Digital controllers can use either PI or PID control for the operation of HVAC water systems The development of the control loop must recognize the rate of reaction of the water system and the needed rate of response Too often, HVAC control systems for pumps do not provide the necessary rate of response, and the result is hunting of the pump speed and unnecessary starting and stopping of pumps The evolving influence of software-based HVAC control systems has had a pronounced impact on improving total plant energy performance (that is, the energy to heat/cool the water and distribute it around the facility as measured in kilowatts per ton for a chiller plant or British thermal units per thousands of BTU per hour for a boiler plant) Techniques such as adaptive control provide a means to continuously and automatically make incremental changes in equipment sequences
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Instrumentation and Control for HVAC Pumping Systems 542 Installing and Operating HVAC Pumps
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and operating levels that substantially reduce operator involvement Such techniques actually adjust set points and control sequences in real time to adjust for changes in load and system conditions such as differential temperature In addition, the use of standard communication protocols and web technology have greatly aided equipment and control manufacturers in producing well-integrated systems with improved energy performance Care should be taken though; some protocols still do not have well-defined interoperability standards as of this writing The lack of this aspect of a standard may not affect installations where all components are provided by the same supplier However, systems which employ unlike components from different suppliers, still present a significant degree of risk and potential expense for achieving full and seamless integration This must be clarified before using such protocols
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2343 Quality of controllers
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The quality and reliability of a controller for HVAC pump operations should be established by testing facilities such as Underwriters Laboratories or Electrical Testing Laboratories The manufacturer of such controllers should be able to specify the levels of quality used in the development of a particular instrument For example, the United States military standards MIL-STD-810D provides an excellent procedure for testing instrumentation and controllers for environmental stability, including temperature, humidity, and vibration 235 Control Wiring Control wiring should be designed and installed as recommended by the Instrument Society of America Control wiring should be of the sizes recommended by this society; wire sizes between transmitters and pump controller are included in Table 107 Of particular importance is keeping instrument and control wiring away from power wiring Electromagnetic interference (EMI) and radio frequency interference (RFI) should be kept within the levels recommended by the instrument and controller manufacturers that are involved on a specific project 236 Control Valves Great emphasis has been placed in this book on avoiding three-way control valves wherever possible because of the possibility of energy
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Instrumentation and Control for HVAC Pumping Systems Instrumentation and Control for HVAC Pumping Systems 543
waste There are applications of such control valves that are necessary and do not waste energy, such as changing the routing of water streams from chillers to boilers Another application is the blending of two different water temperatures to produce a third temperature The quality of control valves and their actuators for these operations is very important The valve itself should be of a metallurgy necessary for the application and should be the product of a manufacturer who can verify the quality of design and manufacture The characteristic of a control valve, namely, flow versus valve position, should be known before that valve is applied to a specific application For example, a butterfly valve is acceptable on some applications but not on others Valve actuators are available for HVAC water applications that can be either pneumatic or electronic Pneumatic actuators with electronic pilots have proved to be very reliable for HVAC water systems Electronic actuators are now available that have a quality that will provide years of continuous operation without continual service and repair