Why is the refrigerant charge on a capillary tube reach-in weighed in to the nameplate amount?
- The receiver must be filled to exactly half its volume
- There is no receiver, so the charge is critical
- The capillary tube closes if the charge is too small
- The compressor has no way to tolerate any subcooling
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B. There is no receiver, so the charge is critical
A cap tube system has no receiver to hold surplus liquid and no valve to regulate flow, so a few ounces too much or too little changes pressures and superheat noticeably. The charge is therefore weighed in.
Reference: Capillary tube systems; critical charge
Which force acts to open a thermostatic expansion valve?
- Pressure from the evaporator under the diaphragm
- Pressure from the superheat spring under the diaphragm
- Pressure from the sensing bulb on top of the diaphragm
- Pressure from the liquid line at the valve inlet
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C. Pressure from the sensing bulb on top of the diaphragm
Bulb pressure pushes down on the diaphragm and opens the valve. Evaporator (equalizer) pressure and the spring push the other way and close it. The valve settles where the three forces balance.
Reference: Thermostatic expansion valve operating forces
Where should the sensing bulb or sensor of a walk-in cooler's box thermostat be placed?
- In the return air to the evaporator, away from the door
- In the cold air leaving the evaporator fan outlets
- Just inside the door, where warm air enters the box
- Against the suction line at the evaporator outlet
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A. In the return air to the evaporator, away from the door
The return air to the coil best represents average box temperature. A sensor in the discharge air satisfies too quickly and short cycles the system, and one beside the door reacts to every opening.
Reference: Walk-in controls; thermostat sensor location