Exercises
Start-up regulators and MOP expansion valves
In the last chapter, we saw the problems resulting from the start-up of a refrigeration system with an LP that was abnormally high in relation to the LP seen in normal operation. We now understand that a MOP expansion valve can sometimes provide a solution to this problem. However, in some instances where, for example, there is a large LP side volume, or where defrosting is performed using hot vapour injection, the time required to return the LP to the MOP point is excessively long. With the compressor being overloaded for too long a period, a safety trip may occur...
To help you answer this question, examine the installation alongside. It is shown operating normally.
The evaporation temperature is therefore -25°C (that is, 1 bar) and the start-up regulator is set to limit the compressor side pressure to a maximum of 1.9 bar (that is -15°C).
The expansion valve with external equalisation is fitted with a removable thermostatic line, with, for example, an adsorption charge. After a defrost, the temperature rises to over 0°C, and the evaporator side pressure rises to above 4 bar.
Imagine that the thermostatic line of the expansion valve is damaged, and that you must replace it. Unfortunately, you don't have a spare thermostatic line of this type in your van. However, you do have 3 MOP thermostatic lines suitable for the expansion valve. You have a line with a MOP point of -30°C, one with a MOP point of -20°C, and one with a MOP point of -10°C.
1) How would this system operate with each of these three lines? Which would you choose?
2) How would you go about adjusting the start-up regulator?
(Solution : Refrepair Manual pages 293 to 304)
