Chillers
Vapor-compression and absorption systems that remove heat from process fluids.
A chiller is a refrigeration system that removes heat from a liquid (usually water or glycol) via a vapor-compression or absorption cycle. Chilled water is circulated to process equipment or AHUs.
How it works
- 1
Refrigerant absorbs heat in the evaporator, cooling the chilled-water loop.
Low-pressure liquid refrigerant boils inside the evaporator (flooded shell-and-tube or DX plate type) at 2–4 °C, absorbing the latent heat of the returning chilled water and cooling it from 12 °C to 7 °C. Superheat of 3–6 K at the suction confirms full evaporation before the gas reaches the compressor.
- 2
Compressor raises refrigerant pressure and temperature.
The compressor — screw, centrifugal, scroll or reciprocating — lifts the refrigerant from evaporator to condenser pressure. This 'lift' is the whole energy cost of the machine: reduce condenser water temperature or raise chilled-water setpoint and lift falls, so kW/TR falls with it. Capacity is trimmed by slide valve (screw), inlet guide vanes plus VFD (centrifugal) or cylinder unloading.
- 3
Condenser rejects heat to cooling water or ambient air.
Hot discharge gas de-superheats, condenses and sub-cools in the condenser, rejecting the evaporator load plus the compressor work to cooling-tower water (32/37 °C) or ambient air. Sub-cooling of 3–6 K increases the refrigerating effect per kg and protects against flash gas.
- 4
Expansion device drops pressure, restarting the cycle.
The expansion device — electronic expansion valve, float valve or orifice plate — throttles liquid to evaporator pressure, flashing part of it and dropping the temperature. It modulates on superheat so the evaporator stays fully wetted at all loads, then the cycle repeats.
Key components
Screw (100–1500 TR, robust, part-load friendly with VFD), centrifugal (>300 TR, best full-load efficiency, risk of surge at low load/high lift), scroll (small, multiple circuits, good redundancy). Oil temperature, differential pressure and motor winding temperature are the key protections.
Shell-and-tube barrel with enhanced tubes, or brazed-plate for small units. Chilled water must maintain minimum flow — low flow causes freeze-up. Watch the approach (LCHWT minus saturated suction temp); a rise above 2 K means fouled or oil-logged tubes.
Water-cooled shell-and-tube (best efficiency) or air-cooled coil. Condenser approach above 2–3 K signals scaling, biofilm or non-condensable gases; each 1 K rise in condensing temperature costs roughly 2–3 % on power.
Electronic (EEV) with stepper motor and superheat control, thermostatic (TXV) with bulb, or float valve on flooded machines. A hunting valve shows up as swinging suction pressure and unstable chilled-water temperature.
Primary/secondary or variable-primary arrangement. Primary loop keeps constant evaporator flow; the secondary VFD pump matches building/process demand. Poor ΔT syndrome (return water too cold) is usually 3-way valves, dirty coils or bypass, not a chiller fault.
Typical operating parameters
Problem solving videos
Safety notes
- •Refrigerant leak detection — HFC/HFO/ammonia hazards.
- •Never bypass HP/LP cut-outs.
- •PPE for cold surfaces and refrigerant handling.
Maintenance schedule
- •Weekly log: pressures, temps, motor current, approach.
- •Quarterly: tube brushing / eddy current inspection.
- •Annual: oil analysis, vibration analysis, refrigerant purity.
Problem solving matrix
| Issue | Likely cause | Fix |
|---|---|---|
| High condenser pressure | Fouled tubes / low CW flow | Clean tubes, verify CT |
| Low evaporator pressure | Low charge / dirty filter drier | Check subcooling & charge |
| Short cycling | Oversized / low load | Sequence multiple units, VFD |
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