12.1 Condenser Approach and Non-Condensables
Key Takeaways
- Condenser approach is condensing saturation temperature from condenser pressure minus leaving water, or minus entering wet-bulb, depending on what the stem gives you.
- Non-condensable penalty is actual condensing pressure minus saturation pressure at the real liquid or sump temperature, not a wet-bulb subtraction.
- If liquid is 85°F (about 152–154 psig class) and condenser outlet is 181 psig (95°F class), the roughly 27 psi gap is non-condensable or a gauge/location error.
- Purge from the high point at the top of the condenser or receiver vapor space; air collects where ammonia has already condensed and drained.
- Chronic return of the same excess pressure on a vacuum-suction plant is air in-leakage through seals and packing, not a condenser that needs daily venting.
12.1 Condenser Approach and Non-Condensables
CIRO Monitoring System Performance items (about 15% of the sitting) expect you to read a plant screen the way a supervisor reads a log sheet: convert a pressure to a saturation temperature, subtract the right air- or water-side temperature, and decide whether the condenser is dirty, air-bound, weather-limited, or simply loaded. Approach is the first number. Non-condensables are the first trap. Heat-flow chapters already defined UA and wet-bulb limits; this section is the diagnostic use of those numbers on an ammonia industrial plant.
Two approach definitions — pick the one the stem gives you
Condensing saturation temperature (SCT) is not the discharge-gas thermometer. Convert condenser-outlet pressure (or a true condenser pressure tap) through the ammonia P/T chart. That saturation temperature is the refrigerant-side number in every approach formula.
- Water-cooled shell-and-tube: Approach = SCT − leaving condenser water temperature. The water has already picked up the heat; the remaining gap to saturation is the exchanger's approach.
- Evaporative condenser (the usual CIRO screen): Approach = SCT − entering wet-bulb temperature. Wet-bulb is the thermodynamic floor of an evaporative condenser. You cannot condense below wet-bulb, and you should not expect a 2°F approach to a 78°F wet-bulb on a loaded industrial unit.
If a question gives leaving water, subtract leaving water. If it gives wet-bulb on an evaporative condenser, subtract wet-bulb. Mixing those two subtractions is a common wrong answer.
Typical healthy evaporative approaches on ammonia sit in a rough 8–15°F band at design wet-bulb and load. 20°F+ is a problem, not a climate excuse, until you have proven otherwise. Water-cooled ammonia condensers often run 4–10°F to leaving water when clean and fully watered.
How to get SCT — and where the gauge lies
Discharge pressure at the compressor is not condenser pressure. Oil-separator drop, discharge-line loss, and a dirty coalescer all make compressor discharge higher than the condenser. Using discharge pressure as SCT overstates approach and can send you acid-washing a condenser that is not the problem.
Read condenser inlet/outlet or the condenser pressure transmitter the screen labels as head or condensing. Then look up saturation temperature on the on-screen P/T chart. RETA provides that chart in the exam session; do not treat a memorized wall card as more official than the reference in front of you.
Rounded R-717 saturation values in the high-side band (ASHRAE/NIST class, gauge pressure):
| Saturation temperature (°F) | Approx. pressure (psig) |
|---|---|
| 80 | 138 |
| 85 | 152 (plant charts often call this the 154 psig class) |
| 90 | 166 |
| 95 | 181 |
| 100 | 197 |
Ammonia at 95°F saturates at 181.1 psig. That pair is the backbone of the non-condensable example below. 85°F liquid sits near 152 psig on NIST-style tables; many engine-room charts round the same point into the 154 psig class. On the exam, look up the chart you are given and then subtract. The diagnostic is the gap, not a fight over 2 psi of table rounding.
Non-condensables: pressure without matching liquid temperature
Air, nitrogen, and hydrogen do not condense at condenser conditions. They occupy volume, and by Dalton's law they add partial pressure on top of the ammonia vapor pressure. The liquid in the sump still tracks ammonia's saturation, not the total gauge reading.
Diagnostic: compare actual condensing pressure to saturation pressure at the actual liquid or sump temperature.
Worked screen (original numbers, same class as a typical CIRO high-side snapshot):
- Condenser liquid / sump = 85°F → sat ≈ 152–154 psig
- Condenser outlet = 181 psig → that is the 95°F class
- Excess ≈ 181 − 154 ≈ 27 psi (about 29 psi if you use 152 psig). That gap is non-condensable penalty — or a gauge/location error you must rule out before you condemn the condenser.
Apparent SCT from 181 psig is 95°F. True liquid is 85°F. You are carrying about 10°F of phantom condensing temperature. If entering wet-bulb is 78°F, the screen approach is 95 − 78 = 17°F. An operator who only looks at approach will call for washdown. The liquid thermometer tells you to purge, not to tear the basin apart first.
Use sump or condenser-outlet liquid, not liquid after a dedicated subcooler. Extra subcooling makes liquid colder than condensing temperature and inflates the apparent air penalty.
Fouling vs non-condensables vs weather
| Cause | Liquid/sump T vs P-derived SCT | What else you see |
|---|---|---|
| Scale, oil film, plugged spray nozzles | Liquid climbs with SCT — both are hot | High approach and hot liquid; wash/descale helps |
| Non-condensables | Liquid lags SCT — liquid cooler than P implies | Excess P vs sat-at-liquid; purge drops head without a weather change |
| High wet-bulb / hot cooling water | SCT high, but P matches liquid T | Approach to wet-bulb may still be normal |
| Fans off / low water flow / undersized condenser | High SCT, liquid near SCT | Heat-rejection hardware, not air in the vessel |
| Overcharged / liquid-logged condenser | High SCT from lost surface | High receiver level, possible liquid in condenser circuits |
Exam trap: an undersized condenser shows high approach because UA is small. It does not produce a 27 psi gap between 85°F liquid and 181 psig. That mismatch is non-condensables (or the wrong gauge).
Purge — where, why, and what chronic means
Non-condensables collect in the vapor space at the top of the condenser, the last-pass coil header, or the high-pressure receiver. Ammonia condenses and drains; air stays gas. The purge point is the high point, never the sump drain.
- Automatic refrigerated purgers take a gas mixture, condense the ammonia, and vent the residual non-condensable. That is the professional method.
- Manual venting from a condenser can dump ammonia to atmosphere. CIRO is a supervisor exam: use the purger, follow the plant procedure, and treat the vent as a process emission, not a bleed-until-the-gauge-falls habit.
After construction, a pump-down, or opening the high side, a one-time air charge is expected. Purge until liquid temperature and pressure agree, then stop.
Chronic return of the same 20–30 psi penalty — especially on a vacuum-suction low-temperature system — means air in-leakage, not a mysterious condenser disease. Any suction below atmospheric (ammonia colder than about −28°F / 0 psig) will pull air in through open-drive shaft seals, recip packing, recirculator and transfer-pump seals, unused evaporators with leaking solenoids, and loosely packed valve stems. Purge restores the screen for a shift. Finding the leak is mechanical integrity. If the only fix is a daily purge, the plant is feeding air on the vacuum side.
Gauge checklist before you purge
- Same pressure transmitter the screen uses for SCT — not a mismatched compressor-discharge gauge.
- Liquid temperature in the condenser sump or outlet liquid, not after a subcooler.
- Steady wet-bulb and water flow; approach is a steady-state KPI.
- If excess pressure vanishes when you move the gauge to a condenser tap, you had a location penalty, not air.
Treat approach as a heat-rejection health number and the P-versus-liquid-T gap as an air-in-the-high-side number. CIRO will hand you both. Separate them.
An evaporative condenser on anhydrous ammonia shows 85°F liquid in the sump and 181 psig at the condenser outlet. Wet-bulb is 78°F. What is the most defensible first diagnosis of the pressure-temperature mismatch?
Where should an ammonia plant take a purge connection to remove non-condensable gas, and why?
A CIRO-style screen lists evaporative-condenser pressure (converted to saturation temperature), entering wet-bulb, and leaving basin water. Which subtraction is condenser approach on that evaporative unit?
After a successful purge, condenser pressure agrees with 85°F liquid. Twelve hours later the same 25–30 psi excess is back. The plant is a low-temperature system with suction in vacuum. What should the supervisor do next?