By: Selele Mashilo
We have now assembled all the major mechanical components and one wonders, how they are going to talk to each other or operate under safe conditions. The safety element is very important for the end user and the public out there.
When the system designers have completed the heat load calculation the manufacturer should understand what that means. The service technician on site, as well, should understand the operational and safety controls. Let us look at a system where the refrigeration heat load was calculated to be 22kW. That means the system should offset this load at the rate of 22kJ/s or better. Remember the heat load will not always be constant. At some stage it will be higher than the design load and at some stage lower. The controllers should be able to tell the system should such conditions exist at any point in time.
The interest is what has been put in the cabinet or space to be cooled. When the system is off, that means more heat gain in the cabinet and the rise in temperature. We have just discovered one variable to be controlled, i.e. temperature.
Temperature is one of the variables to be controlled. Therefore we must have a sensor; this sensor will send a signal for a corrective action to take place. The corrective action is sent to a controller and an actuator. Look at a domestic fridge with a thermostat and settings on it. The thermostat is a sensor. When the temperature rises above set point or gets below set point, the thermostat will send a signal to the controller for a corrective action. The sensor, controller and actuator may be combined and looked as one. If the sensor was not there, under lower temperatures the evaporator will be iced.
The control system should not be design complicated. The design rule is Keep It Simple Sir (KISS).
Other variables to be controlled are pressure (air), flow (water) or humidity (air). There are technical terminologies used in controls. This is for a better understanding of operations of specific controllers e.g. set point like in a domestic fridge, deviation as the difference of the set point and the value of the controlled variable at any instant – if the thermostat is set at 12⁰C and the temperature at a certain time is 15⁰C, the deviation is 3⁰C etc.
Controllers receive signal and act in various ways. The way they act is called control mode e.g. ON-OFF, Floating and proportional controls.
The control instrument have different names e.g. thermostats, pressure stats, defrost control, flow switches etc.
I will look at operations for each control instrument next time
Literature
[1] RACA
[2} ASHRAE HANDBOOKS
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