VAV terminal control methods
In order to control the temperature of a space, it is necessary to somehow compensate for the changes in the heating and cooling load. The amount of heating and cooling loads in a building varies and is affected by several factors. These factors include weather changes, seasonality, time of day and night, indoor or outdoor space in the building, geographical orientation of the space, as well as other influencing factors such as the presence of people, mechanical equipment, lighting, computers, etc.
In a ventilation system, to compensate for load changes, air enters the space with a certain temperature and amount. Since the heating and cooling loads of a space are always fluctuating, so in order to compensate for these fluctuations and keep the conditions of the space stable, the ventilation system must react accordingly to these changes. Changing the temperature and amount of air entering the space or a combination of changing the temperature and the amount of air in a controlled manner in response to the change in the load of the space can maintain the desired conditions.
Since central ventilation systems may be designed based on the greatest needs of all the spaces of a building, VAV terminals help reduce energy consumption in the building by controlling the volume or temperature of the incoming air to each space. Of course, the dispersion coefficient (or the possibility of the simultaneous maximum load of spaces) allows the reduction of the capacity of the central ventilation system, and there is no need to calculate the system capacity based on the highest possible demand, which is very unlikely to occur. VAV terminals allow us to change the volume of air or the temperature of the air entering the room according to the type of use of the system based on the feedback taken from the conditions of the room. Variable air volume VAV terminals are divided into two types, pressure-dependent and pressure-independent, according to the type of their control function.
Pressure dependent control
If the rate of air passing through it changes by changing the pressure at the inlet of a terminal, it is called a pressure-dependent system. The flow rate depends on both the inlet pressure and the terminal damper position. A pressure-dependent terminal consists of a damper and a damper actuator that is directly controlled by the thermostat in the room. The damper actuator operates only with room temperature and adjusts the position of the damper. Since in pressure-dependent terminals, the amount of air flow changes based on the input pressure of the terminal, the temperature of the room may fluctuate until the thermostat determines the new position of the damper with the help of the operator. Also, the excess air flow may lead to more noise than the space limit
.
The logarithmic diagram shown in the figure below shows the performance of a pressure-dependent terminal with respect to changes in air pressure in the duct for different damper positions. Line 1a-1b represents one of the damper positions. When the channel pressure increases, the flow rate through the damper changes proportionally to the square root of the pressure drop along the terminal. Lines 2a-2b and 3a-3b show other positions of the damper until reaching the maximum opening point of the damper, i.e. line 4a-4b. Therefore, in pressure-dependent terminals, with the change of the static pressure on the sides of the terminal, the air flow into the room and accordingly the temperature of the room changes, and the thermostat is forced to change the position of the damper by the operator in order to control the temperature of the room. Therefore, the control accuracy in these types of terminals is weaker compared to pressure-independent terminals. Pressure-dependent terminals are suitable for applications where independence from pressure and air flow limitations in the terminal are not considered. For example, for a system that provides a constant volume of air to the space and the static pressure downstream is controlled by another system, or in a system with a central fan with a constant volume of air and bypass dampers (which reacted to static pressure changes) and the excess air is directly returned to the air conditioner by short circuits) this type of terminal can be used.
Pressure independent control
If the rate of air passing through it is constant with the change of pressure at the inlet of a terminal, that system is called independent of pressure. Terminal control independent of pressure is achieved by using a flow sensor and flow controller in the terminal. The terminal controller adjusts the position of the damper by using the amount of air flow entering the terminal as well as the feedback received from the thermostat installed in the space so that the amount of air passing through the terminal remains constant. The amount of passing air can vary between two minimum and maximum values determined in the calibration process.
Schematic figure of the pressure-independent terminal
The logarithmic diagram shown in the figure below shows the flow characteristics and settings in a pressure-independent terminal. The vertical lines 1a-1b and 3a-3b respectively represent the minimum and maximum value of the calibrated current and recorded in the current controller. Line 2a-2b also shows an intermediate flow value based on the needs of the room thermostat. The damper keeps the amount of passing current constant regardless of the pressure difference between the two sides of the terminal on the sides of the vertical lines. The amount of flow is only changed based on the need of the thermostat. The intersection of the vertical lines and the diagonal line 1a-3a shows the minimum static pressure required by the terminal for the specified current value.
Terminal pressure independent operation ensures proper airflow distribution within the space and gives the designer confidence that the minimum and maximum design limits are maintained. The minimum and maximum air flow limits are very important to maintain proper air distribution, which has two benefits:
Limiting the maximum amount of air entering the space prevents excessive cooling as well as creating a lot of noise.
Keeping the minimum air flow through the terminal ensures proper ventilation of the space.
Types of control logic
Proportional control (P). In this type of control, the output signal is proportional to the input signal (often known as the error signal). A significant change in the input signal, for example a sudden increase in static pressure, results in a change in the flow rate read by the flow sensor. This change creates a significant error between this new value and the flow rate set for the controller. This causes a large change in damper position (output signal) to compensate for static gain. The magnitude of the output signal depends on a controller parameter called “Gain”. Depending on the size or smallness of the gain of the controller, this type of control can lead to overshoot or undershoot. Another weakness of proportional control is that it requires a large error signal to produce an output signal. This limitation in proportional control will always have a small error compared to the desired value. This error is called offset error. Pneumatic controllers usually use proportional control.
Proportional-integral (PI) control. This control method is used to improve the proportional control method and it combines proportional control with time-dependent integral control. The main difference between these two control methods is the way the controller reacts in time. In the proportional control method, the controller remains at a fixed output that always has an offset relative to the desired value. Integral control by measuring the offset time, subtracts the offset value and produces another output signal value based on the time. The following example illustrates this in more detail.
For example, if a VAV terminal controller is intended to maintain the room temperature at 70 degrees Fahrenheit, and on the other hand, the thermal load added to the space causes the temperature to increase to 72 degrees Fahrenheit, and the problem leads to the command to increase the amount of cooling input to the space. Now, if we assume that every 2 degrees Fahrenheit of temperature difference, 300 cfm is added to the current 500 cfm flow rate of the system, so the final flow rate reaches 800 cfm. To achieve this new flow rate, the position of the damper must be changed. In the proportional control diagram, the damper opens more than the required amount for the flow rate of cfm800 (overshoot) and then measures the new difference value and then changes the output signal to reach the desired value, i.e. cfm800. At this stage, the damper is placed in a position where the Hebrew flow reaches less than 800 cfm (undershoots). In the proportional-integral control diagram, overshoot and undershoot happen frequently, but in each step, the position of the damper is changed in smaller intervals until the amount of offset error is zero.
Proportional-integral-derivative (PID) control. In addition to having the characteristics of two integral and proportional methods, this control method also has a third level of control called the derivative control method. The derivative control method predicts the way the error signal changes by measuring the rate of change of the error signal with time. The PID control method is used to increase the speed and accuracy of the control. As can be seen in the figure, the PID control of the system reaches the equilibrium condition faster and in fewer steps.
Types of controls and controllers
Various VAV controls can include one or more of the following control elements, which are mentioned below:
flow sensor This sensor measures the speed of the primary air flow at the entrance and sends a signal corresponding to the speed of the primary air flow to the controller in order to adjust the position of the damper. This control cycle is the basis of the operation of pressure-independent systems.
Room thermostat or temperature sensor. The room thermostat measures the room temperature and adjusts the position of the damper by sending a signal to the controller according to the set operating point. Digital control systems use a temperature sensor and any change in the operating point is applied by the digital controller to the air flow rate.
Flow controller. This equipment acts as the main brain of the control and according to the signal received from the flow sensor and the temperature sensor, it processes them and sends the necessary command to the damper operator.
Damper operator. This tool receives commands from the controller to open or close the damper in order to change or keep the air flow constant.
Types of control systems
Electrical systems (pressure dependent). The electrical controls work with 24VAC power provided by a transformer in the control part of the terminal. This type of control system does not have any speed sensor or any controller and cannot control channel pressure fluctuations.
Pneumatic systems (independent of pressure). This type of control system operates with the help of 20-25psi compressed air provided by a central system. The thermostat measures the temperature of the room and if the temperature changes according to the specified operating point with the help of the controller, it changes the air pressure of the actuator and causes the damper to move and be placed in a new position. In the cooling mode, if the room temperature increases, the operator opens the damper, and in the heating mode, when the room temperature drops, the operator does the opposite and closes the damper to some extent.
Analog electronic controls are usually powered by a 24 VAC transformer located inside the terminal control. The electronic control consists of a speed sensor (either a hot wire thermistor or a pneumatic multipoint sensor with an electronic transducer) and an electronic speed controller, and utilizes a proportional control function. Electronic thermostat of one of four types; Cooling, heating, cooling with reheating or cooling-heating are selected. A three-stage reheat (two stages for fan-fan terminal) or automatic heating-cooling changeover relay can be placed inside the control section of the terminal. Analog electronic controls compensate for changes in channel pressure.
Schematic diagram of analog electronic control
Direct digital control (DDC) systems (independent of pressure). This micro-processor is based on electronic controls and is powered by a 24VAC power transformer that is usually located inside the control section of the terminal. The flow signal obtained from a pneumatic or electronic speed sensor as well as the room temperature sensor is converted into digital pulses in control microcomputers. This program usually includes a proportional-integral-derivative (PID) control algorithm to achieve high operational accuracy. These types of controllers not only have the ability to reset and also have volume control functions, but they can also be set to work in local mode or remote control, in addition, the control process can be implemented in a central computer. . Digital controllers directly compensate for changes in channel pressure.
Overview of digital controls
A direct digital controller uses a digital computer to implement control algorithms on one or more control loops. Interface hardware allows input signals from various devices to be processed by a digital computer. The control software calculates the required position of output devices, such as valve and damper actuator and fan starter. The output devices are then moved to the calculated state through the interface hardware.
The basic principles of temperature control in air conditioning systems are well known and can be implemented. These control strategies are implemented using analog, electrical, pneumatic electronic control tools. In the computer age, microprocessor technology is available in certain applications such as HVAC control. In addition to being affordable, microprocessor-based controllers represent the state of the art computing power for controlling VAV terminals, air handling units, package heating and cooling units, and all HVAC equipment. In these controllers, instead of using conventional analog pneumatic or electronic controls, they use direct digital controls. A direct digital controller receives the input signals from the sensors and by processing these digital data, produces the appropriate control action through binary on-off outputs or analog output voltages.
Advantages of direct digital controller. Some control methods, such as resetting the supply pressure operating point for air conditioners or central fans and resetting the temperature operating point for central coils, depend on the feedback sent from the subject space. These control methods can save a lot of energy consumption. In addition to resetting, these types of controllers have volume control functions, and they can also be set to work in local mode or remote control, or run the control process in a central computer. Direct digital controller controls compensate for changes in channel pressure. The use of direct digital controllers has advantages that are valuable for engineers, today almost 100% of the controllers used are direct digital.
The advantages of direct digital controllers besides energy saving are:
Space control problems can be detected remotely and can be solved by engineers and technicians.
Removing the compressed air system and its maintenance costs.
Accurate control of space air temperature.
The ability to limit the operating points of the thermostat in the software to prevent unauthorized people from changing it.
Reduce the calibration frequency.
Ability to coordinate with presence sensors, window switches and Co2 sensors
The ability to observe the performance or adjust the controller through the computer by the residents. Considering the energy saving and other benefits of using direct digital controllers, it is recommended to use this type of controller in new buildings. In existing buildings, these types of controllers can be used in spaces by upgrading the central system.
Reaction of pneumatic speed controllers to changes in room conditions (direct/reverse action)
In direct-acting controllers, any increase in thermostat output pressure will increase the controller’s airflow setting. In reverse-acting controllers, as the output pressure of the thermostat increases, the adjustment of the output air flow of the controller decreases. The damper will open or close to maintain the controller’s settings when the pressure inside the channel changes.
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Speed controller response to room temperature changesت Interaction of thermostat and pneumatic controller
In systems that are responsible for transferring cold air to the space, when a direct-action thermostat sends a signal to a direct-action controller, the amount of air entering the space will increase with the increase in room temperature. The same situation happens in a reverse-action thermostat with a reverse-action controller. In a system that uses a direct-action thermostat with a reverse-action controller or a reverse-action thermostat with a direct-action controller, when the air temperature in the room increases, the air flow into the space will decrease. In hot air systems, this process will be the opposite
Thermostat and controller interaction Damper operator interaction and pneumatic controller
Controllers and actuators work in harmony with each other to control the temperature of the space. DANO and RANC controllers are the most common types of pneumatic controllers. Of course, when electric coils are used for heating, it is more common to use RANO. NO or “normally open” is the best damper position in hot weather areas, because if the damper actuator fails for any reason, the damper remains open, allowing cold passages into the space, and the RA turns off the heating coil. slow
How a pneumatic speed controller works
The set point of the thermostat, which is the desired temperature of the space, is the ideal point of system performance. When the thermostat output is equal to the set point, the system is in equilibrium. Most pneumatic thermostats are factory calibrated at 9 psi. The control point of the controller is actually the value of the flow rate that is set at any moment based on the signal sent from the thermostat and is considered as the actual balance value for the variable under control. The offset is actually the difference between the set point and the actual value of the control point at any instant of time. The damper can be positioned at any angle to compensate for changes in channel pressure as well as maintain a constant flow rate.
A range of controlled variable values, which is proportional to the point of minimum and maximum flow rate set in the controller, is referred to as the regulation range. This range can be adjusted on the controller. The thermostat set point (eg 9 psi) is the point between the minimum and maximum flow rates set on the controller, which is proportional to space load changes. The outlet pressure of the thermostat corresponding to the minimum and maximum flow rate is called the starting point and the ending point, respectively. The thermostat may also be used to control another peripheral unit, such as stepwise regulation of the pressure of a proportional valve in a hot water coil.
How the thermostat works
