Compression vs absorption chillers

Positive-displacement compressors (recip, scroll, screw) trap and squeeze refrigerant vapor. Centrifugal compressors impart velocity/pressure with impellers — common on large low-lift water chillers. Capacity control may use unloaders, VFDs, inlet guide vanes, or staging multiple compressors.

Chiller evaporators cool water in shell-and-tube or related exchangers. Condensers may be water-cooled shells or air-cooled coils. Metering devices match machine type — TXVs, orifices, or specialty low-pressure arrangements.

Absorption machines use refrigerant (often water) and an absorbent (often lithium bromide solution) plus a heat source (steam, hot water, gas) to drive the cycle. Electric demand can be lower at the “compressor,” but you need heat and careful solution chemistry. Crystallization and vacuum integrity are classic absorption concerns.

Low-pressure chillers may operate below atmospheric pressure on the refrigerant side and need purge attention for noncondensables. High-pressure machines behave more like “familiar” refrigerant systems at higher pressures — still respect OEM rupture discs, reliefs, and lockouts.

Screw chillers handle larger capacities with slide-valve or VFD unloading. Scroll chillers often appear in modular banks — one module down does not kill the plant. Learn the staging story on site.

Surging, vane positions, and oil return have their own playbook. Low load without proper bypass or VFD strategy causes noise and stress. Do not force a centrifugal into a duty it cannot ride stably.

Steam, hot water, or direct fire must be available and stable. A boiler outage becomes a cooling outage on absorption plants. Coordinate with the heating crew — these systems cross trades.

Nameplates rule. R-134a high-pressure machines, low-pressure specialties, and absorption water/LiBr chemistry do not interchange. Wrong jug = disaster.

Banks of smaller scroll chillers offer redundancy. Troubleshooting includes which modules are enabled, staged, and faulted. A “50% plant” may simply have half the modules locked out on flow or sensor faults.

### Field memory hooks

Write the key temperatures, flows, or mode checks for this topic on your job notes. Teach the helper beside you to repeat the layered order aloud before tools come out. Most callbacks shrink when the first visit follows a written order instead of a parts guess.

When OEM data exists — capacity tables, wiring diagrams, water-flow charts — photograph the rating plate and the page you used. Future you (or the next tech) should not have to rediscover the same facts under pressure.

Situation. New helper tries to “add gas” to an absorption machine like an R-410A rooftop. Machine is down on dilution/crystallization risk after a heat-source outage.

What to think. Wrong mental model. Follow absorption start-up and solution procedures; restore heat and circulation per OEM.

Conclusion. Identify machine type before any cylinder touches a port.

### Teach-back for the helper

Before leaving the topic, have the helper explain in their own words: what is normal, what is measured first, and what must never be skipped for safety. If they cannot teach it back, review the main “How it works” blocks again with a sketch on paper.

Connect this lesson to the previous one and the next one as a chain. Field skill is sequence memory — not isolated trivia. Keep OEM charts and job photos with the work order so the next visit starts smarter.

### Common measurement habits

Use the same meter locations every time so readings compare day to day. Steady the system before judging pressures or temperatures — mid-shift and mid-defrost lie. Write ambient conditions on every note page.

If a reading disagrees with the control display, believe the calibrated handheld until sensors are proven. Controls cannot fix what they cannot see accurately.

### Safety and professionalism

Apply LOTO where energy can hurt you. Wear eye protection around refrigerants, chemicals, and coil fins. On roofs and at window units, respect fall hazards. Explain findings to the owner without jargon soup — clear next steps beat impressive vocabulary.

Dispose of oil, filters, and refrigerant legally. Cutting corners on recovery or tower chemicals is not “saving the customer money”; it is transferring risk.