10.2 Condensers, Approach, and Wet-Bulb Limits
Key Takeaways
- A condenser must reject evaporator heat plus compressor work; evaporative units are limited by outdoor wet bulb, not dry bulb
- Water-cooled approach is condensing saturation minus leaving water; evaporative wet-bulb approach is condensing saturation minus wet bulb
- On a sample-style screen with sump 75°F, liquid 85°F, and 154 psig discharge (~86°F sat), you can compute approach, sump-to-wet-bulb gap, and near-zero liquid subcooling
- Fans off: sump rises toward condensing while head pressure climbs. Pump off: sump may fall toward wet bulb while condensing pressure soars. Scale and fouled coils raise condensing pressure by cutting U
10.2 Condensers, Approach, and Wet-Bulb Limits
The evaporator absorbed Qe. The compressor added W. The condenser must dump Qc ≈ Qe + W to the outdoors (Chapter 9's heat-rejection ratio, typically a bit above 1.2 on a healthy single-stage ammonia plant). If that heat has nowhere to go, discharge pressure rises until a high-pressure cutout, a relief event, or a very expensive new equilibrium. CIRO Heat Flow wants you to read condenser temperatures as a heat-exchanger problem: Q = UAΔT, with ΔT measured to the sink you actually have.
Three condenser families, one ceiling
Water-cooled shell-and-tube or plate units reject to a cooling-tower loop or once-through water. The refrigerant is nearly isothermal at Tsat. The sink is water. Driving difference is LMTD from Tsat to the two water temperatures.
Air-cooled units reject to dry-bulb air. Approach is Tsat minus leaving (or entering, if the stem says so) air. They are uncommon as the main heat dump on large NH3 plants but appear on small packages and some oil coolers.
Evaporative condensers dominate industrial ammonia. Discharge vapor condenses inside tubes (or a coil) while recirculated water wets the outside and fans pull air through the spray. Most of the heat leaves as evaporation of water, so the air's wet-bulb temperature is the thermodynamic floor — not the dry bulb, not the sump, and not 86°F because last summer's setpoint was 86°F.
You cannot condense at the wet bulb. ΔT at that limit is zero, LMTD is zero, and Q is zero. Real units need a positive approach so UAΔT equals the heat of rejection.
Approach, range, and the two definitions CIRO mixes
Memorize both. The stem will tell you which temperatures it gave.
Water-cooled (and any condenser with a measured leaving liquid coolant):
Approach = condensing saturation temperature − leaving water temperature
Range = leaving water − entering water
Approach is the leaving-end ΔT. Range is the water's temperature rise from Qc / (ṁ_water × cp). Tight approach means a large or clean condenser (or a light load). Large approach means not enough UA for today's Q and today's water.
Evaporative condenser versus outdoor wet bulb:
Wet-bulb approach = condensing saturation temperature − outdoor wet-bulb temperature
That is the number energy codes and floating-head strategies talk about (design values often land in a roughly 15–25°F band at design wet bulb; operating plants with extra area may hold something nearer 10–15°F when the condenser can actually get there). CIRO does not publish a single legal approach. Calculate the one on the screen.
A second evaporative gap is sump (basin) temperature versus wet bulb. Recirculated water sits above wet bulb because it has just picked up condenser heat. A typical healthy picture is: wet bulb lowest, sump a few degrees above wet bulb, condensing saturation above the sump, liquid a little below Tsat if the coil actually subcools.
Do not call sump temperature the wet bulb. Do not call liquid temperature the condensing temperature. Saturation comes from discharge (or condenser) pressure through the PT table, not from a well in the drop leg unless that liquid is saturated.
Sample-style screen: sump 75°F, liquid 85°F, 154 psig
RETA-style operating screens for a large screw (think 300 HP class, evaporative condenser, thermosiphon oil cooling) train you to read three condenser-side numbers together. A teaching set — not a copied exam stem — is:
- Condenser water sump: 75°F
- Condensed liquid: 85°F
- Compressor discharge: 154 psig
From Chapter 8's saturation table, 154 psig = 168.7 psia, which interpolates to about 86°F saturated ammonia. Everyone in the trade calls that head ~86°F sat. Use the on-screen PT table on test day; use 86°F here so the arithmetic is visible.
The screen did not print wet bulb. Suppose the weather station or the exam reference gives 70°F wet bulb (state it; do not assume).
| Difference | Arithmetic | What it means |
|---|---|---|
| Wet-bulb approach | 86 − 70 = 16°F | Condensing sat minus outdoor WB — the evaporative ceiling gap |
| Condensing minus sump | 86 − 75 = 11°F | Coil-to-basin gap; water left the coil hotter than the basin average if mixing is imperfect |
| Sump minus wet bulb | 75 − 70 = 5°F | Basin is above WB, as it should be while rejecting heat |
| Liquid subcooling | 86 − 85 = 1°F | Liquid is almost saturated; this condenser is not buying you much subcooling |
1°F of subcooling is a clue, not a crime by itself: maybe the drop-leg sensor is near the coil outlet, maybe the unit is loaded, maybe you have little extra area. It is not 10°F of subcooling. If someone reports '85 degrees of subcooling,' they used the thermometer as the answer (Chapter 8 trap, still live here).
If wet bulb were 78°F on a muggy afternoon with the same 86°F sat, wet-bulb approach would be only 8°F. That may be tighter than the condenser can hold at today's rejection load. Fans will pin at 100% and Tsat will rise until UAΔT matches Qc. That rise is physics, not a failed setpoint. Floating-head control belongs in a later monitoring chapter; here you only need: wet bulb plus a positive approach is the floor under condensing temperature.
Fans off, pump off, scale on
Treat the evaporative condenser as two machines bolted together: a water-side pump that wets the coil, and an air-side fan that carries away humid air.
Fans off, pump still running. Water still recirculates over hot tubes, but air is not removing evaporated vapor. Evaporative heat rejection collapses. The same Qc now tries to leave as sensible heat into a closed water loop and a dead air stack. Sump temperature rises toward condensing temperature — the basin and the coil equilibrate. Condensing pressure climbs because U (air-side) crashed and ΔT to the outdoor wet bulb is no longer being used. High-pressure cutout is the usual ending if nobody notices.
Pump off, fans still running. No water on the tubes. The coil is a crude, undersized air-cooled condenser. UA collapses; condensing pressure soars. The basin is no longer receiving hot coil drain water. Remaining sump water can evaporate toward the air stream or simply sit near outdoor conditions, so sump temperature may fall toward wet bulb while the high side is in trouble. That split — cold basin, screaming discharge pressure — is the pump-failure signature. It is the opposite of the fan-failure signature (hot basin, high head).
Scale, lint, and fouled coils. Mineral scale on the tube exterior, discharge-oil film on the interior, and plugged spray nozzles all lower U. For today's Qc and today's wet bulb, Tsat must rise so ΔT grows enough to keep Q = UAΔT. You read that as higher psig. Cleaning, water treatment, and full spray coverage restore U. Turning the head-pressure setpoint up 'so the TXVs work' without looking at the condenser is how plants cook compressors all summer.
Noncondensable gas (air, hydrogen from corrosion, nitrogen from a sloppy pumpdown) also raises the pressure you read for a given condensing temperature: the gauge sees refrigerant partial pressure plus foreign-gas partial pressure, and gas blankets surface so U falls. That is enough introduction. Full purge diagnosis, hold tests, and 'is it NC or is it scale?' live in the monitoring / heat-exchanger diagnostics chapter. Here you only need: the condenser is a heat rejector; anything that cuts UA or shrinks ΔT to the sink raises head.
Worked rejection check
A plant shows 90 tons on the suction side and 120 BHP shaft (Chapter 9's pairing). Evaporator heat = 90 × 12,000 = 1,080,000 Btu/h. Shaft work = 120 × 2,545 = 305,400 Btu/h. Condenser (plus oil cooler, if that stream is part of W and not double-counted) must reject about 1,385,400 Btu/h.
If the evaporative unit is rated 1,400,000 Btu/h at 15°F wet-bulb approach and today's wet bulb is 70°F, the rating assumes about 85°F sat. At 16°F approach (86°F sat) you are near that rating. At 8°F approach you are asking the same steel for nearly double the UAΔT it was built to give at 15°F — it will not. Head pressure will float up. That is the wet-bulb limit in plant language.
Operator round on the condenser deck
- Read discharge psig, convert to Tsat (table).
- Read sump, liquid, and wet bulb (or tower leaving water on a water-cooled machine).
- Compute wet-bulb approach (or leaving-water approach) and subcooling.
- If head is high, split fan failure (sump up toward Tsat) from pump failure (sump down toward WB, Tsat through the roof) from fouling / undersize / extra load (both fans and pumps proving they run, approach still wide).
- Leave noncondensable hunting for the diagnostics chapter unless the stem already isolated it.
Exam traps
- Approach = Tsat − entering water when the plant definition is leaving water (unless the stem says entering).
- Using dry bulb as the evaporative limit.
- Calling sump the condensing temperature.
- 154 psig treated as 154°F or 154 psia; it is ~86°F sat / ~169 psia.
- Fan-off and pump-off sump stories swapped.
- Assuming 10°F subcooling because the liquid reads 85°F and 85 is a round number — subcooling is Tsat minus liquid temperature.
A sample-style screen shows condenser sump 75°F, condensed liquid 85°F, and discharge 154 psig (~86°F sat). Outdoor wet bulb is 70°F. What is the evaporative wet-bulb approach?
Evaporative condenser spray pump running, fans failed off. What pattern is most consistent with Q = UAΔT on that unit?
Evaporative condenser fans running, spray pump off. What happens to sump temperature and condensing pressure?
Scale on evaporative-condenser tubes raises condensing pressure primarily because: