Compressors, condensers, evaporators and metering devices

Northfield's shop keeps a shelf of failed parts, and Dana uses it as a classroom. On a slow Friday she set out four of them in a row on the bench: a cut-open hermetic compressor from a walk-in, a section of condenser coil with bent fins, an evaporator from a reach-in cooler and a thermostatic expansion valve with its sensing bulb still attached. "These are the four parts every system has," she said. "Each one comes in a dozen designs. You will spend years learning them all. Today you just need to know what each one does, what kinds there are and how each fits the cycle."

This lesson introduces the four main components and the accessories around them. RAC-210 covers compressors, condensers and evaporators in depth, and RAC-220 covers metering devices.

Compressors: the pump of the system

The compressor draws vapour from the evaporator and raises it to condensing pressure. Its capacity depends on how much vapour it can move, its displacement, and on how dense that vapour is. Compressors are grouped in two ways: by how they compress the vapour, and by how the motor is housed.

By housing, a hermetic compressor has its motor and compressor sealed together in a welded steel shell. It cannot be opened for repair, and the motor is cooled by suction vapour. Most small systems use hermetic compressors. A semi-hermetic compressor has a bolted shell that can be opened to repair valves, pistons or the motor; it is common in commercial refrigeration. An open compressor has its motor outside, driving the compressor through a shaft seal; it is used mainly in large industrial ammonia systems, where the refrigerant would attack copper motor windings.

Many modern compressors run at variable speed, driven by an inverter that changes the motor's frequency. Instead of cycling on and off at full capacity, the compressor slows down to match the load. That saves energy, holds temperature more steadily and handles humidity better in air conditioning. Most ductless heat pumps and many new central systems use variable-speed compressors.

A compressor's capacity falls as the evaporating temperature falls, because the suction vapour gets thinner. The compressor's displacement, the volume it sweeps each revolution, stays the same, but each litre of vapour contains less refrigerant. A volumetric efficiency, below 1, accounts for vapour that re-expands from the clearance space and for leakage; it also falls as the compression ratio rises.

# Compressor capacity against evaporating temperature (approximate R-134a data)
displacement = 92e-6        # m3 per revolution
rpm = 3500
conditions = [  # evaporating C, suction density kg/m3, volumetric efficiency, refrigerating effect kJ/kg
    (0, 13.9, 0.92, 153.0),
    (-10, 9.8, 0.90, 148.0),
    (-20, 6.6, 0.85, 141.5),
    (-30, 4.3, 0.78, 135.0),
]
swept = displacement * rpm / 60          # m3/s
for t, rho, eta, re in conditions:
    m = swept * eta * rho                 # kg/s
    print(f"evaporating {t:4d} C: {m * 3600:5.0f} kg/h, capacity {m * re:4.1f} kW")

evaporating    0 C:   247 kg/h, capacity 10.5 kW
evaporating  -10 C:   170 kg/h, capacity  7.0 kW
evaporating  -20 C:   108 kg/h, capacity  4.3 kW
evaporating  -30 C:    65 kg/h, capacity  2.4 kW

The program extends the example across four evaporating temperatures. Capacity falls steadily as the evaporator gets colder, from the thinning vapour, the falling volumetric efficiency and the shrinking refrigerating effect all at once. Compressor manufacturers publish this as a table or set of curves of capacity and power against evaporating and condensing temperature, and technicians use those curves to check whether a compressor is pumping properly.

Condensers: getting rid of the heat

The condenser rejects the heat absorbed in the evaporator plus the heat of compression. Inside, the refrigerant passes through three zones: desuperheating, where the hot discharge vapour cools to its condensing temperature; condensing, the largest part, where it turns to liquid at constant temperature; and subcooling, near the outlet, where the liquid cools below saturation. Condensers are grouped by what carries the heat away.

• Air-cooled condensers use fans to blow outdoor air across finned tubes or flat microchannel tubes. They are simple and need no water, and they are used on almost all residential and light commercial systems. Their weak point is the air side: dirt, debris, bent fins, blocked clearances and recirculating hot air all raise the condensing temperature.
• Water-cooled condensers pass refrigerant and water through a shell-and-tube, tube-in-tube (coaxial) or brazed plate heat exchanger. Water carries heat far better than air, so they run at lower condensing temperatures, but they need a water supply, usually a cooling tower that rejects the heat to the air by evaporation, and water treatment to prevent scale and biological growth.
• Evaporative condensers spray water over a coil while fans blow air through, combining the two. They are common in industrial ammonia plants.

An air-cooled condenser is usually designed to condense at about 8 to 17 K above the outdoor air temperature, depending on its size and efficiency. The airflow it needs comes straight from the sensible heat equation for air: the heat rejected divided by the air density, its specific heat and the air's temperature rise across the coil.

Evaporators: absorbing the heat

The evaporator is where the useful work happens. Most evaporators in air conditioning and commercial refrigeration are finned-tube coils with fans, called forced-air or forced-convection evaporators: unit coolers in walk-ins, cooling coils in air handlers and furnaces, indoor units of ductless systems. Liquid chillers use plate heat exchangers or shell-and-tube evaporators to cool water or glycol. Some older or simple equipment uses plate or bare-tube evaporators cooled by natural convection, such as the shelves of an old chest freezer.

Evaporators are also classified by how they are fed. In a dry-expansion, or direct-expansion, evaporator, the metering device feeds just enough liquid that it all boils off before the outlet, leaving the vapour superheated; this is by far the most common. In a flooded evaporator, the evaporator is kept full of boiling liquid by a float or level control, and vapour is drawn off the top; flooded evaporators transfer heat very well and are used in large chillers and industrial systems.

An evaporator's capacity depends on its temperature difference, TD: the difference between the air or fluid entering it and the refrigerant's saturation temperature. Walk-in coolers often run with a TD of about 5 to 8 K, which keeps the coil from drying out the stored product. A larger TD gives more capacity from the same coil but removes more moisture, which dries out produce and meat, and it lowers the suction pressure, which costs efficiency. When an evaporator runs below 0 °C, frost builds up on the fins and must be melted off regularly by a defrost cycle.

Metering devices: controlling the flow

The metering device drops the pressure and controls how much refrigerant reaches the evaporator. There are two broad kinds: fixed restrictions, which cannot adjust, and valves, which can.

• Capillary tubes are long, very small-bore copper tubes. The length and bore set the flow. They are cheap, reliable and have no moving parts, and they are used in household refrigerators, window units and small commercial equipment. They need a precise charge, because they cannot adjust to changes in load.
• Fixed orifices, or pistons, are short restrictions with a calibrated hole, used in many older residential air conditioners and heat pumps. Like capillary tubes, they are charged by superheat, using the manufacturer's chart for the indoor and outdoor conditions.
• Thermostatic expansion valves (TXVs) adjust their opening to hold a steady superheat at the evaporator outlet. A sensing bulb clamped to the suction line contains a charge that turns its temperature into a pressure; that pressure pushes on a diaphragm to open the valve, while evaporator pressure and an adjustable spring push to close it. When superheat rises, the bulb warms, the valve opens and feeds more refrigerant; when superheat falls, it closes. Large evaporators, or ones with a pressure drop, use an external equalizer line to sense the outlet pressure.
• Electronic expansion valves (EEVs) are driven by a stepper motor under the control of a circuit board that reads pressure and temperature sensors. They hold superheat more precisely across a wide range of loads and are standard in variable-speed systems.
• Float valves and automatic expansion valves are used in particular applications: floats in flooded evaporators, and constant-pressure automatic valves in a few small systems.

Dana's failed expansion valve had lost the charge in its sensing bulb through a crack in the capillary. With no bulb pressure pushing it open, the spring closed it, the evaporator was starved, superheat was very high and the reach-in cooler warmed up. That is one of the faults the superheat measurement in Lesson 7 is designed to catch.

Accessories in the refrigerant circuit

Around the four main components, most systems have several accessories, and technicians need to recognize them.

The filter drier deserves a special mention. Moisture and acid are the main chemical enemies of a refrigeration system, and the drier is the system's defence. Whenever a system is opened for repair, the drier is replaced, because it may already be saturated and because it is open to the air during the work. A drier with a large temperature drop across it is restricted, which shows up as high subcooling and high superheat, the restriction pattern from Lesson 7.

Matching the components

The four components must be matched. The compressor's pumping capacity must equal the evaporator's ability to boil refrigerant at the design temperature, the condenser must reject the total heat at an acceptable condensing temperature, and the metering device must pass the right flow at the pressures that result. When the parts are balanced, the system settles at a steady evaporating and condensing temperature where all four agree. When a part is undersized or fails, the balance point moves. A dirty condenser raises the condensing temperature until the condenser can again reject the heat; an iced evaporator lowers the evaporating temperature until the compressor's lower capacity matches the coil's reduced output. A technician who understands this balance can read the gauges and see which part has moved it.