Refrigeration control for dummies
Appreciate your providing answers on the forum. I have a few of my own maybe you could help me with. Seems I need "refrigeration control 101" for dummies. Our chiller, like most, has a cut in/cut out pressure switch, a refrigerant solenoid, a glycol flow switch, and a thermostat all on the evaporator side. In addition, there are also two pressure switches on the compressor side. My question is what controls the solenoid and what controls the compressor motor contactor? Thanks for your help!
Thanks Phillip for our first real question to the Refrigeration Q&A!!!
Originally Posted by gitchegumee
Generally the compressor is controlled by a pressure control that senses the evaporator pressure.
The liquid line solenoid is controlled by the Thermostat.
The Flow switch is generally tied in to de-energize the solenoid valve in the event of loss of glycol flow.
Here is a typical summary of the sequence of operation:
1) GLYCOL TEMP RISES Your Thermostat senses that the glycol reservoir has warmed up, raising the temperature above the T-Stat Setpoint. The T-Stat then energizes, or opens, the liquid line solenoid valve.
2) After the Solenoid Valve Opens, the Evaporator pressure begins to rise, because we have just introduced the refrigerant into the evaporator. As soon as the evap pressure rises above the cut-in setpoint (generally around 30 PSIG for R22) on the low pressure control, the compressor will then start.
3) GLYCOL TEMP DROPS Your T-Stat then senses the temperature has dropped to the T-Stat Setpoint. The T-Stat then de-energizes, or closes, the liquid line solenoid valve. The compressor will then pump all the refrigerant out of the evaporator, as this occurs the evaporator pressures drop, and after these pressures reach the cut-out setting (Generally around 5 PSIG for R22) on the low pressure control, the compressor will shut off.
4) FAN CONTROLS You mentioned a couple pressure controls on the compressor. These are probably controlling the condenser fans. As your discharge pressure increases, these controls will turn on your fans. As the pressure drops, these will cycle the fans off. On a two fan Unit I like to set the fans as follows:
FAN #1 on at 180 PSIG off at 150 PSIG
FAN #2 on at 225 PSIG off at 175 PSIG
5) FLOW SWITCH If for example the system is running, but due to lack of glycol or a flow issue, the evaporator freezes up. This will then stop the flow of glycol and your flow switch should alarm. Typically this will then open the control circuit, de-energizing the solenoid valve, which would cause the compressor to pump down and shut off.
We do not use flow switches on our systems because they do not protect the system from freeze up, only shut down after the damage has occured. We use a special Pressure Control that shuts the system down, most of the time, before freezing.
I am making some assumptions here and would need to see the control wiring diagram to tell you for sure how your system is controlled. There are mfg's that will even cycle the compressor and solenoid valve off and on with the T-Stat or Flow Switch.
Hope this helps.
Got what I need! Our fans are not controlled by pressure, but are energized through the compressor motor contactor. Should be no problem. And the compressor contactor also has two pressure switches in series that I assume are safetys.