High & low control in action: Pressure control

High- and low-control in HVAC pressure control

High pressure control is used to keep components from failing and to prevent the HVAC system from releasing fluids, such as hot water in a boiler, into the surrounding environment. By monitoring a pressure switch or transducer, the nano PLC is able to monitor whether the upper pressure threshold has been surpassed. If it has, the nano PLC can then shut down the system and warn the proper personnel.

Like high pressure control, low pressure control works by having a PLC monitor a pressure switch or pressure transducer to check if the pressure in the system is below a desired threshold. Low pressure in an HVAC system can indicate issues such as low, or leaking coolant in an AC unit. Low pressure control can also be used prevent mechanical failures - for example, shutting off a compressor in an AC unit when the pressure is too low to prevent the system from freezing.

The image depicts how a user can quickly integrate an analog or digital pressure sensor with the easyE4 nano-PLC. The programmer is then able to easily write code to compare the sensor feedback against a pressure threshold setpoint. When the feedback surpasses or does not meet certain values, the programmer is then able to turn off the easyE4’s outputs to shut down the system.

Connection between a pressure sensor to the easyE4 nano-programmable logic controller (PLC)

How does high- and low-pressure controls work

Implementing high- and low-pressure control to shutoff and enable equipment to turn on is very straight forward.

The operator can use an analog pressure transducer to compare the current pressure to the high- and low-pressure thresholds. Alternatively, the operator can use pressure sensors with digital outputs that turn on and off at thresholds programmed into the sensors.

These signals can then be used to disable components in the system that increase or decrease pressure, for instance a compressor in a refrigerator.

How the easyE4 controls high- and low-pressure

The easyE4 nano programmable logic controller (PLC) makes this process very simple. The output from an analog pressure sensor feeds directly to an analog input of the easyE4 nano PLC.

The raw analog signal is then be scaled to a meaningful value in the program. In the example below, the raw 0-4095 count signal is scaled to correspond to 0-50 PSI.

Figure 1: Illustration of how to scale a raw signal into a meaningful pressure feedback value

Comparing scaled pressure feedback to low- and high-pressure thresholds

Next, the scaled pressure feedback is compared to the low- and high-pressure thresholds of the system. If the pressure limits are surpassed, high- and low-pressure fault bits are set. In this example, if the scaled pressure feedback is less than 15 PSI, or if the scaled pressure feedback is greater than 40 PSI, the corresponding pressure fault bits are set.

Figure 2: Illustration displaying a comparison of the scaled pressure feedback against the low-pressure threshold

Figure 3: Illustration displaying a comparison of the scaled pressure feedback against the high-pressure threshold

Lastly, the high- and low-pressure fault bits are used in the run permissive logic. If the pressure fault bits are not active and all other system logic that would enable the system to run are true, the run signal will be set. In this example, if there is no high- or low-pressure fault, and the refrigerator is warmer than the desired temperature, the relay output Q1 of the easyE4 will engage to power the contactor of the compressor.

Figure 4: Illustration of using pressure fault bits in run permissive logic

Explore high/low control capabilities of the easyE4 Nano PLC

Download the easyE4 manual

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PID control for HVAC control applications

PID control can be used to address a variety of control applications. These include:

Pressure control

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