10.1 Cooling Systems: Refrigeration Principles, Chillers & Chilled-Water Loops

10.1 Cooling Systems: Refrigeration Principles, Chillers & Chilled-Water Loops

Quick Summary: Stationary engineers rarely operate only boilers. In institutional plants — hospitals, universities, government buildings, hotels — the same operator runs the chillers, cooling towers, and chilled-water distribution in summer that runs the boilers in winter. The department's reference text for all classes of Montana licensure includes cooling systems, so the material is fair game. Refrigeration is not a different discipline; it is the same thermodynamics run backwards, using latent heat to move energy from a place you want cold to a place you can dump it.

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1. The Vapor-Compression Cycle

Refrigeration exploits the same property that makes steam useful: a fluid absorbs a large amount of heat when it evaporates and releases it when it condenses. The trick is arranging for it to evaporate where you want cooling and condense where you can reject heat.

                     HIGH-PRESSURE SIDE
        +--------------------------------------------+
        |                                            |
   [ COMPRESSOR ] ---> hot high-pressure gas ---> [ CONDENSER ]
        ^                                            |  rejects heat to
        |                                            |  condenser water
   cool low-pressure gas                             v  or ambient air
        |                                       high-pressure liquid
   [ EVAPORATOR ] <--- low-pressure liquid <--- [ METERING DEVICE ]
        |  absorbs heat from                         |
        |  chilled water                             |
        +--------------------------------------------+
                     LOW-PRESSURE SIDE

The two dividing lines are the compressor and the metering device. Everything between the compressor discharge and the metering device inlet is high-pressure side; everything between the metering device outlet and the compressor suction is low-pressure side.

The ton of refrigeration

One ton of refrigeration = 12,000 Btu/hr. The number comes from ice: melting one short ton (2,000 lb) of ice requires 2,000 × 144 Btu/lb = 288,000 Btu, and spreading that over 24 hours gives 12,000 Btu/hr.

Two derived quantities an operator uses daily:

• Cooling load = 500 × gpm × ΔT for water, where 500 comes from 8.33 lb/gal × 60 min/hr. A 1,000 gpm chilled-water loop with a 10 °F rise is carrying 500 × 1,000 × 10 = 5,000,000 Btu/hr, or about 417 tons.
• Chiller efficiency is stated in kW per ton. A modern centrifugal machine may run near 0.55 kW/ton at design and much better at part load; a rising kW/ton at constant load is the earliest sign of fouling or refrigerant loss.

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2. Chiller Types

Surge — the centrifugal failure mode to know

A centrifugal compressor is a dynamic machine. If the pressure it must develop rises (fouled condenser, high condenser water temperature) while flow falls (light load, guide vanes closed down), it reaches a point where it can no longer maintain flow against the head. Flow momentarily reverses, then re-establishes, then reverses again. That oscillation is surge, and it is audible as a rhythmic whoosh with swinging motor amps. Sustained surge damages thrust bearings and impellers. The remedies are the ones that reduce head or restore flow: clean the condenser tubes, lower condenser water temperature, or open hot-gas bypass to add artificial load.

Absorption chillers

An absorption machine replaces the compressor with a thermochemical loop. Water is the refrigerant and lithium bromide solution is the absorbent.

1. In the evaporator, water evaporates under deep vacuum at around 40 °F, chilling the loop.
2. In the absorber, concentrated lithium bromide solution absorbs that vapor, which is what maintains the vacuum.
3. The dilute solution is pumped to the generator, where steam or hot water boils the absorbed water back out, re-concentrating the solution.
4. That vapor goes to the condenser, condenses, and returns to the evaporator.

Absorption chillers earn their place in plants with surplus low-pressure steam or hot water, or where electrical demand charges are punishing. The operational cautions are specific: lithium bromide solution crystallizes if it becomes too concentrated or too cold, which plugs the heat exchanger and requires a decrystallization procedure; and the machine's deep vacuum makes air leakage its chronic enemy, which is why absorption units run a purge system continuously.

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3. Chilled-Water and Condenser-Water Loops

Chilled water

A closed loop, like a hydronic heating loop, and governed by the same rules: it needs an expansion tank, air separation, and correct pump placement relative to the point of no pressure change. Typical design is 44 °F supply, 54 °F return, a 10 °F rise, though larger ΔT designs are increasingly common because they reduce pumping energy.

The chief operating problem is low delta-T syndrome: return water comes back too cool, so the chiller must move more gallons for the same tons and pumping energy climbs. Causes include coil valves stuck partly open, three-way valves bypassing, fouled coils, and improperly set minimum-flow bypasses.

Condenser water

An open loop through a cooling tower, and therefore chemically the opposite of chilled water in every important respect. Open exposure means continuous oxygenation, continuous evaporation, and continuous airborne contamination.

Cooling tower water losses:

Cycles of concentration works exactly as it does on a boiler: COC = tower water conductivity ÷ makeup conductivity, and the required blowdown rate is set by the target COC. Run too few cycles and you waste water and chemical; run too many and you scale the condenser tubes and corrode the system.

Legionella is a boiler-room-adjacent life-safety issue. Cooling towers produce warm aerosol, which is precisely the transmission mechanism for Legionella pneumophila. Control depends on maintaining biocide programs, keeping the basin clean of organic debris and sediment, maintaining drift eliminators, and following the facility's written water management plan. Never enter or work on an operating tower without understanding the aerosol exposure.

Approach and range

Two numbers describe tower performance:

• Range = hot water in temperature − cold water out temperature. Range is set by the load, not by the tower.
• Approach = cold water out temperature − ambient wet-bulb temperature. Approach is set by the tower's capability, and a rising approach at constant load and wet bulb means fouled fill, poor water distribution, reduced airflow, or a failing fan drive.

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4. Refrigerant Safety and Handling

Refrigerants are regulated, and the operating rules are not optional.

• Do not vent. Refrigerant must be recovered into approved cylinders using approved recovery equipment, never released to atmosphere.
• Repair leaks; do not top off. A machine that needs repeated charging has a leak that must be found and repaired.
• Machinery room requirements. Refrigerant machinery rooms require refrigerant detection with alarms, emergency purge ventilation actuated from outside the room, and self-contained breathing apparatus stored outside the room.
• Asphyxiation is the primary hazard. Most common refrigerants are heavier than air and are not toxic in the ordinary sense — they kill by displacing oxygen in a confined space. That is why detection is mandatory and why an alarming machinery room is evacuated, not investigated from inside.
• Frostbite and blindness. Liquid refrigerant at atmospheric pressure is far below freezing. Face shield and gloves are required for any work that can release liquid.
• Never introduce compressed air or oxygen into a system containing refrigerant and oil for leak testing. The combination of oxygen and lubricating oil under pressure is explosive. Use dry nitrogen with a regulator.
• Purge rate is a diagnostic. On a low-pressure machine, a rising purge unit run time means increasing air and moisture ingress, which raises condensing pressure, wastes energy, and forms acids in the oil.

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5. Cooling Plant Fault Matrix