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This answer is coming in from the angle of process control, which I assume the author of the question is also coming from.
Both types of control loop, has a signal that goes into a control loop, which as a result will have a desired control action taken by the controller and deliver to a control valve out in the field. This action will then drive the required process variable to its desired target (a.k.a. Set Value, S.V./S.P.).
Open loop is a control loop that have been left with no feedback mechanism in place. An operator will give input into the control loop via the DCS console, and the intended action will take place by the control loop (i.e. the control valve(s) out in the field will take the action that the controller will require it to do). There is no feedback into the open loop control strategy, rather the operator is the "feedback mechanism" to this control loop via his/her reactions to the changes in the process.
A closed loop has a feedback mechanism to the loop, from the action of the controller. This feedback is normally processed as a single input signal into the control loop along with the process variation from the desired Set Value/Set Point. There is a tuning done on the control loop to arrest noise as well as the actual process feedback to the control loop. The controller's P,I,D tuning will then drive the corrective action taken by controller onto the process via the control valve.
For this to work though, the control loop needs to be in either Auto or Casade mode. Else if left in Manual mode, any control loop (regardless of how well tuned it is), is just a open loop.
An open loop control system acts completely on the basis of input and the output has no effect on the control action. – A closed loop control system considers the current output and alters it to the desired condition. The control action in these systems is based on the output
One of them you used the product as refrigerant and the other you have to use external refrigerant .
