Future Design MCTB 4.3, User Manual

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MCTB
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4.6.2.6 Advanced Compressor Control Logic Example
Compressor control applications typically involve single or dual stage refrigeration systems. Dual stage
systems are utilized in ultra-low temperature situations such as environmental test chambers or even heat treat
applications where temperatures below -100°F (-73°C) are required. Typically, control schemes for
environmental chambers tend to be more complex. Take the following example in which an environmental
chamber controls both temperature and humidity. For heat/cool operation only, a dual stage refrigeration
system is used to get to ultra-low temperatures. However, when in the temperature/humidity mode of operation
in which the chamber is controlling both temperature and humidity, only the first cooling stage is utilized.
Since humidity can only be controlled between temperatures of 32°F (0°C) and 212°F (100°C), the second
stage refrigeration system is not used for cooling since it operates at too low of a temperature. Also, stage 1 is
required for dehumidification by using what is referred to as a “wet coil” which is used to strip moisture out of
the air within the chamber, much like a home dehumidifier unit strips moisture from the air in damp locations
such as a basement. All of this requires the controls system to know when to use stage 1 compressor vs. stage
1 and 2 compressors as well as when to turn them on and off based on cooling and dehumidification demand.
This can all be accomplished using several logic/timer equations in the MCTB
. Let’s begin with a typical
temperature/humidity system configuration as follows:
Loop 1 (temperature control)
•
Output 1 = heating
•
Output 2 = cooling
•
Output 3 = event output for chamber on/off (enables/disables fans for circulating air)
•
Output 4 = stage 1 compressor (for cooling and dehumidification when humidity control active)
Loop 1 (Humidity control)
•
Output 1 = heating (humidification)
•
Output 2 = cooling (dehumidification)
•
Output 3 = event output for humidity system on/off
•
Output 4 = stage 2 cooling (for cooling when humidity control is not active)
We only require the refrigeration compressors to operate when cooling or dehumidification is required. We also
do not want the compressors turning on and off rapidly, so start and stop delays should be used to prevent
them from turning on and off quickly when the temperature or humidity is hovering around set point. We will
begin by creating an on-delay timer that can be used as the requirement for the compressor(s) to run. We will
define equation 1 and a Timer type. The Trigger will be “Always” as we want it to evaluate the systems operation
at all times. By providing it with a Tagname of “CMP ON DLY” we will know it is for delaying the start of the
compressors.