Introduction to Refrigeration

Caution: This experiment involves systems at elevated pressures and use of refrigerants, which can be toxic at high concentrations. Ensure reasonable safety precautions are followed and that appropriate PPE is worn. Ensure adequate ventilation when working with refrigerants.

1. Fabrication of refrigeration system (see diagram and photograph, Fig. 3)

1. Construct the vapor compressor by first connecting one port of a double-action pneumatic cylinder to a pipe fitting tee. Install a Schraeder valve on the other port of the pneumatic cylinder. Install one-way (check) valves to the two other ports of the tee, one pointing inward and one pointing outward. This allows refrigerant to be drawn in from the evaporator and expelled to the condenser at high pressure.
2. Using two more pipe fitting tees, install pressure gages upstream and downstream of the compressor.
3. A high-pressure bicycle floor pump is used to actuate the compressor. Remove the rubber bead (check valve component) from the bicycle pump plumbing. This will allow the compressor to expand and draw in refrigerant in between pumping strokes. Connect the bicycle pump hose to the Schraeder valve on the compressor.
4. Form a thin (3.2 mm outer diameter) aluminum tubing coil to act as the condenser. In the prototype system (Fig. 3), the coil was formed by helically wrapping the aluminium tubing around a 2.5 cm diameter rigid rubber tube core for four turns (~50 cm total length). The condenser coil length is not critical for this small-scale experiment.
5. Connect one end of the condenser coil to the open port of the pipe fitting tee downstream of the pressure gage using a compression fitting (McMaster Inc. part #5272K291 suggested).
6. Install a short clear PVC pipe into two reducing pipe elbows. This component will act as the high-pressure refrigerant reservoir. Connect the reservoir to the outlet of the condenser tubing.
7. Install a ball valve into a pipe tee with an AN/SAE flare fitting connector. This will be the charging port. Connect a needle flow meter to one side of the pipe tee. This will be the expansion device. Using the narrow aluminium tubing, connect the other port of the pipe tee to the low point of the refrigerant reservoir.
8. Form a second aluminium tubing coil to act as the evaporator. Connect this between the needle valve outlet and compressor inlet.
9. Fill the system with compressed air (550 kPa if available) through the charging port. Use a soapy water spray to identify any plumbing leaks, and make repairs as necessary.
10. Connect thermocouples to the condenser and evaporator coils for temperature measurement.

Figure 3: a. Diagram of components and connections in experimental vapor compression refrigeration system. Please click here to view a larger version of this figure.

Figure 4: T - s (a) and P - h (b) diagrams for experimental R-134a vapor compression refrigeration cycle.

2. Charging the refrigeration system

1. Connect the middle port of a refrigerant charging manifold to the charging port on the refrigerator. Connect a vacuum pump to the low-pressure port of the manifold, and a can of refrigerant to the high pressure port. R134a is the most commonly available refrigerant, and is used here. R1234ze(E) may be a better option because its low saturation pressure would permit easier compressor operation, and its low GWP would reduce the environmental impacts of any leaks.
2. Run the vacuum pump and gradually open all system valves to remove all air. Briefly open the refrigerant canister valve to clear any air from the assembly.
3. Once vacuum is achieved, isolate the vacuum pump and close the low-pressure port on the refrigerant charging manifold. Invert the refrigerant canister, and inject liquid refrigerant into the system until the level in the high-pressure reservoir is slightly above the needle valve level.

3. Operation

1. Adjust the needle valve until it is just barely open.
2. Operate the refrigerator by pumping the bicycle pump connected to the compressor pneumatic cylinder.
3. Track the high- and low-side pressures and evaporator and condenser temperatures until steady state conditions are reached. Record these pressures and temperature values. Note that most pressure gauges report gage pressure. This can be converted to absolute pressure by adding approximately 101 kPa.
4. Indicate the state points (1-4) and approximate connecting curves on T-s and P-h diagrams (Fig. 4).