many systems perform poorly because designers neglected the importance of building controllable HVAC systems and providing well-engineered control systems. It is possible to design good HVAC systems and controls at a reasonable cost. Designers have a duty to provide the owner with the best possible system within budget limits, not necessarily the cheapest. The cheapest may be the most expensive in the long term, in operating cost and owner dissatisfaction. The best system is one that will provide the required degree of comfort for the application with the least expenditure of energy. That degree of comfort is, of course, a function of the application; that is, we should expect closer control of temperature in a hotel bedroom than in the same hotel's kitchen.
What is "CONTROL"?
In simplest term, the control is defined as the starting, stopping or regulation of heating, ventilating, and air conditioning system. Controlling an HVAC system involves three distinct steps:
1- Measure a variable and collect data.
2- Process the data with other information.
3- Cause a control action.
These functions are met through sensor, controller and the controlled device.
Elements of a Control System
HVAC control system, from the simplest room thermostat to the most complicated computerized control, has four basic elements: sensor, controller, controlled device and source of energy.
Sensor measures actual value of controlled variable (the parameter that must be controlled) such as temperature, humidity and provides information to the controller.
Controller receives input from sensor, processes the input and then produces intelligent output signal for controlled device.
Controlled device acts to modify controlled variable as directed by controller.
Source of energy is needed to power the control system. Control systems use either a pneumatic or electric power supply.
Theory of Controls:
Basically, there are two types of controls, open loop control and closed loop control.
Open loop control
Open loop control is a system with no feedback i.e., there is no way to monitor if the control system is working effectively. Open loop control is also called feed forward control.
These types of controls are not suitable for air-conditioning and refrigeration system because it does not provide the facility of comparing the parameters to be controlled.

Closed Loop System:
If the oven in the example had (the difference between the actual value (controlled point) in controlled variable and its set point. A comparison of the sensed parameters is made with respect to the set parameters and accordingly the corresponding signals shall be generated. Closed loop control is also called feedback control.
In general ask a question, does sensor measure-controlled variable? If yes, the control system is closed loop, if not the system is open loop.
HVAC control systems are typically closed loops. Closed loop can be broadly classified into two categories viz. two position controls and continuous controls.
The desired value or setpoint is adjusted at the knob on the front of the thermostat. (Note that the room thermostat contains the sensor, setpoint adjustment, comparison device and the controller, which are shown distinctly in the block diagram above). The temperature sensor measures the actual value and sends a signal back along the feedback path to the comparison device. The comparison device compares the value of temperature at the sensor to that of the desired value or setpoint on the controller. The difference between the desired value and the measured value is known as the error signal. The error signal is fed into the controller as a low voltage signal (e.g., 10 volts) to the actuator.
HEATING
Heating in HVAC systems usually is provided by steam or hot water coils with remote boilers. Electric heating coils, heat pumps, and direct gas-fired duct heaters also are used, and are discussed in other sections of this book.
Heating may be done to preheat outside air or heat mixed air, to heat part of the air stream, or to reheat for humidity control or individual zone temperature control.
1. Preheat
Preheating is used when large percentages of outside air could cause freezing of downstream heating and cooling coils. The main problem in preheating is freeze-up of the preheat coil itself. Several methods are used to prevent this.
Figure 8-9 shows the simplest approach. This is a two-position valve in the steam or hot water supply with an outdoor thermostat that opens the valve whenever the outdoor temperature is below 35or 40oF. (This, incidentally, is an open-loop control.) The filter is downstream of the coil to prevent snow loading in severe winter storm weather. Because no control of leaving air temperature is provided, the preheat coil must be carefully selected to prevent overheating at, say, 30oF outside, while still providing adequate capacity at perhaps -10 or – 20oF outside design conditions. This is a difficult, if not impossible, compromise.
The best solution here is to use hot water with a recirculating pump (Figure 8-11). Now there can always be full flow through the coil with the temperature of the water varied to suit requirements. No air is bypassed; so, there are no mixing problems. Very accurate control of the air temperature is possible. Notice the opposed flow arrangement with the hot water supply entering the side of the coil where the air leaves.
The desired value or setpoint is adjusted at the knob on the front of the thermostat. (Note that the room thermostat contains the sensor, setpoint adjustment, comparison device and the controller, which are shown distinctly in the block diagram above). The temperature sensor measures the actual value and sends a signal back along the feedback path to the comparison device. The comparison device compares the value of temperature at the sensor to that of the desired value or setpoint on the controller. The difference between the desired value and the measured value is known as the error signal. The error signal is fed into the controller as a low voltage signal (e.g., 10 volts) to the actuator.
The desired value or setpoint is adjusted at the knob on the front of the thermostat. (Note that the room thermostat contains the sensor, setpoint adjustment, comparison device and the controller, which are shown distinctly in the block diagram above). The temperature sensor measures the actual value and sends a signal back along the feedback path to the comparison device. The comparison device compares the value of temperature at the sensor to that of the desired value or setpoint on the controller. The difference between the desired value and the measured value is known as the error signal. The error signal is fed into the controller as a low voltage signal (e.g., 10 volts) to the actuator.
COOLING COILS

Cooling coils generally are confined to the air handling unit although occasionally re cooling coils are required, as, for example, with chemical dehumidifiers. There are two types: direct-expansion (DX) coils and those using chilled water or brine.

1 Direct-Expansion Coils
DX coils must, by their nature, use two-position control with its inherently wide operating differential. Nonetheless, this system is often used, particularly in small units and where close control is not required. Figure 8-16 shows a typical DX coil control. The room thermostat opens the solenoid valve, allowing refrigerant liquid to flow through the expansion valve to the coil. The expansion valve modulates according to its setting to try to maintain a minimum refrigerant suction temperature. A low-limit discharge thermostat, T2, keeps the supply air temperature from becoming too cold.
Controllability can be improved by providing face and bypass dampers (Figure 8-17), but this may lead to complications such as lack of humidity control and coil icing at high bypass rates. Maximum bypass rates must be established, and the system may not provide adequate control at very light loads.
The desired value or setpoint is adjusted at the knob on the front of the thermostat. (Note that the room thermostat contains the sensor, setpoint adjustment, comparison device and the controller, which are shown distinctly in the block diagram above). The temperature sensor measures the actual value and sends a signal back along the feedback path to the comparison device. The comparison device compares the value of temperature at the sensor to that of the desired value or setpoint on the controller. The difference between the desired value and the measured value is known as the error signal. The error signal is fed into the controller as a low voltage signal (e.g., 10 volts) to the actuator.
Two-stage direct expansion will often provide adequate capacity control. The stages should be made by rows of coil rather than by sectioning the coil. Otherwise the active section may ice up, forcing most of the air flow through the inactive section and reducing the coil capacity.
In a multi row coil the first stage should be the first row in the direction of air flow and the second stage the rest of the rows, since the first row of a three or four row coil does at least half the cooling. A two-stage thermostat is used (Figure 8-20).
The desired value or setpoint is adjusted at the knob on the front of the thermostat. (Note that the room thermostat contains the sensor, setpoint adjustment, comparison device and the controller, which are shown distinctly in the block diagram above). The temperature sensor measures the actual value and sends a signal back along the feedback path to the comparison device. The comparison device compares the value of temperature at the sensor to that of the desired value or setpoint on the controller. The difference between the desired value and the measured value is known as the error signal. The error signal is fed into the controller as a low voltage signal (e.g., 10 volts) to the actuator.
The recirculating pump arrangement is very useful in two cases: (1) for extremely accurate temperature control and (2) to avoid freezing in those situations where system geometry may make it impossible to avoid stratification of mixed or partially preheated air.
Compensation Control, or Reset
Compensation control, or reset, is a type of control mode in which a compensation sensor is generally used to reset a main sensor to compensate for a variable change sensed by the compensation sensor. The purpose is to achieve operation that is more effective, energy-efficient, or both. In the design of a reset mode, the first things to decide are the control point at which the main sensor will be reset and the variable to be sensed by the compensation sensor. The main sensor, which senses the mixed air temperature in the air-handling unit, is usually reset by a compensation sensor that senses the outdoor temperature, as shown in Fig. below.
اعداد : د. عصام زهير فاضل - جامعة المستقبل