Refrigeration Systems and Components
Refrigeration systems and components - this domain tests whether you can follow refrigerant and heat through a real plant and explain what each component does to pressure, temperature and state. Questions focus on compressor types and capacity control, condenser and evaporator styles, expansion devices, receivers, oil management and connecting piping.
The cycle as hardware
The compressor raises pressure and temperature; the condenser rejects heat and condenses; the expansion device drops pressure and meters flow; the evaporator absorbs heat and boils refrigerant. Reciprocating compressors use suction and discharge valves and unload cylinders for capacity control; screw compressors use a slide valve; centrifugal compressors use inlet vanes and must be protected from surge at low load. Expect questions on what happens to capacity and amperage as load falls.
Heat exchangers
- Air-cooled condenser: approach is condensing temperature minus entering air temperature, typically 10–15 K when clean.
- Shell-and-tube condenser: approach is condensing temperature minus leaving water temperature, normally 3–6 K when clean.
- Evaporative condenser: capacity depends on wet-bulb temperature, not dry-bulb.
- Direct-expansion evaporator: the thermostatic expansion valve holds roughly 4–8 K superheat at the outlet.
- Flooded evaporator: liquid covers the tubes, outlet superheat is near zero, and a surge drum or eliminators stop liquid carryover.
Ancillaries
Oil separators keep oil off heat-transfer surfaces and return it to the crankcase. Receivers must keep a liquid seal at the outlet. Filter-driers remove moisture and acid, sight glasses reveal flash gas, suction accumulators protect the compressor, and purge units remove non-condensables.
Worked example
A shell-and-tube ammonia condenser logged 35 °C condensing with 30 °C leaving water - a 5 K approach. Months later, at the same load and water flow, condensing is 39 °C with leaving water still 30 °C, a 9 K approach. The water supply is unchanged, so the condenser is transferring less heat. Water-side scale is the first suspect, then non-condensables, then an oil film on the refrigerant side. Cleaning, purging and restoring oil return are the fixes; adding refrigerant is not.
Traps
Approach is not range - range is water in minus water out. Superheat is compressor protection, not useful refrigeration. Flooded and DX evaporators show opposite superheat signatures. A receiver run dry starves the expansion valve and mimics a low charge.