Refrigeration System High Discharge Pressure on a Chilled-Water Skid
A 200-ton centrifugal chiller using R-134a serves a process cooling loop. The condenser is a water-cooled shell-and-tube unit supplied by a cooling tower. Over two weeks the compressor discharge pressure has risen from 145 psig to 185 psig at constant load, and the approach temperature (condenser leaving water minus condensing temperature) has increased from 2°F to 9°F. The compressor amps are up 12%. The cooling tower supply water is 85°F and the return is 95°F, both unchanged. The condenser water flow is 600 gpm, confirmed by the flow meter. The chiller is still meeting the process load, and the plant wants to defer maintenance until the next scheduled outage in six weeks.
1. Diagnose the cause of the rising discharge pressure and approach temperature, and assess whether deferring maintenance is acceptable.
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The signature is unambiguous: rising condensing pressure at constant load and constant cooling-water temperatures, with rising approach temperature and rising compressor amps, points to fouled condenser tubes. The approach temperature - the difference between the condensing temperature and the leaving cooling-water temperature - is the key diagnostic. A clean condenser runs a 1–3°F approach; 9°F means the heat-transfer surface is coated. The 12% amp increase is the compressor working harder to lift the refrigerant from the evaporator pressure to a higher condensing pressure.
The cause is almost certainly scale or biological fouling on the water side, or oil or non-condensable gas on the refrigerant side. The cooling-water flow and temperatures are unchanged, so the water side is delivering the same heat rejection capacity; the problem is in the condenser's ability to transfer it.
Deferring maintenance is not acceptable as stated. At 185 psig discharge, the compressor is operating near the upper end of its envelope, and the consequences of continuing are:
- Higher discharge temperature - at 185 psig condensing, discharge gas temperature rises, which degrades oil and can cause compressor damage.
- Reduced capacity margin - as the approach worsens, the chiller will eventually fail to meet the process load, and the failure will be sudden, not gradual.
- Energy penalty - a 40 psig rise in condensing pressure on a centrifugal chiller is roughly a 10–15% efficiency loss, which is a real operating cost.
The correct judgement is to clean the condenser now, not at the next outage. The cleaning is a routine water-side brush or chemical clean that does not require a major outage, and the risk of waiting is a chiller trip during a process demand period.
2. What would you check to rule out a refrigerant-side cause, and what immediate operating adjustments would you make?
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Refrigerant-side checks:
- Non-condensable gas - check the purge unit run time and the purge discharge. A rising purge run time with a high discharge pressure is the signature of air in the refrigerant. Air raises the condensing pressure for a given condensing temperature and shows up as a high condensing pressure relative to the saturated temperature.
- Refrigerant charge - a low charge shows up as low suction pressure and low subcooling, not high discharge pressure, so it is unlikely here, but verify the sight glass and subcooling.
- Oil in the condenser - oil carried over from the compressor coats the tubes and raises the approach. Check the oil level and the oil return.
- Condenser water-side - pull a tube or check the condenser head for scale and biological growth; confirm the water treatment program is holding the cycles of concentration and the scaling index.
Immediate operating adjustments:
- Clean the condenser - brush or chemically clean the water side, and verify the water treatment is controlling scale.
- Purge non-condensables if the purge run time is elevated.
- Reduce load if the discharge pressure approaches the high-pressure cutout, and confirm the cutout is set and tested.
- Trend the approach temperature daily until it returns to 1–3°F, then weekly.
- Check the compressor oil for degradation from the higher discharge temperature and change it if the lab report shows it.
The exam point: the approach temperature is the condenser's health indicator, and a rising approach at constant water conditions is a fouling diagnosis. The refrigerant side is checked to rule out non-condensables before you commit to a water-side clean.