HVAC flow control with nano PLCs

Sweltering summer heat at local hotel causes mass guest check out

Samantha received a frantic call from Javier, the building engineer at the Crestar Hotel, a 500-room luxury hotel, downtown Phoenix. Javier received early morning complaints from guests of seeming broken thermostats that kept his team busy.

By mid-morning, when call frequency increased and the complaints were more consistent about the lack of air conditioning, Javier decided to investigate the chiller system. He discovered that the flow rate was inconsistent and refused to remain at the desired set point.

By noon that day, room temperatures soared and a total of 125 guests checked out of the hotel. Some guests and staff suffered severe heat exhaustion as the hotel wasn’t able the keep up with the demand for water and fans. The hotel lost several hundred thousand dollars in one day.

Flow control is critical for HVAC systems

Chillers or boilers and other HVAC systems require circulating coolant to function. If the coolant does not circulate at the proper rate, too much or too little heat will be exchanged, meaning the system will not properly control the temperature, thereby reducing energy efficiency.

Moreover, if the flow rate of an HVAC system is too high or too low it can also cause overheating or freezing damage.

Optimize flow control with PID

Flow control typically requires a more complex control algorithm.

As a result, system integrators and panel builders typically use the Proportional, Integral, Derivative (PID) function within nano programmable logic controllers (PLCs).

PID control is a closed loop mathematical function. It works by:

Applying gains to the error, derivative of the error, and integral of the error between the setpoint and feedback
It sums those values together, and then outputs the sum to the system as a reference
By adjusting the gains, you can adjust the response time, overshoot, and the amount and size of steady state oscillations

Pros and Cons of PID temperature control

Figure 1: An illustration of the PID method of flow control

Nano PLCs for PID flow control

Nano PLCs can easily implement PID to control the flow rate of coolant in Crestar Hotel’s chiller system.

The error between the setpoint and the feedback from an analog device can be fed into a PID function block. The output of the PID can then directly control an analog output to an electronic control valve.

The further the valve is open, the larger the volume of fluid flows throughout the system. As the flow rate fluctuates, the valve position will dynamically adjust to automatically regulate the flow.

Explore the PID capabilities of the easyE4 Nano PLC

Learn more

Select PID control below to delve into the details of how it works

PID control

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