19.1 Why Two-Stage: Compression Ratio and Discharge Temperature

19.1 Why Two-Stage: Compression Ratio and Discharge Temperature

Quick Answer: A −40°F ammonia freezer against summer condensing cannot be a healthy single-stage machine. The compression ratio in psia is on the order of 16:1 to 19:1, discharge gas is hot enough to cook oil, and volumetric efficiency collapses. Two-stage compression uses a booster (low stage) to lift freezer vapor to an intermediate pressure and a high-stage compressor to finish the lift to the condenser, with interstage desuperheat and liquid subcooling between them.

CIRO Content Area 7 (Two-Stage and Secondary Coolant Systems) is only 15 scored items, but operators who treat two-stage as “two compressors in a row” miss the exam. The reason for two-stage is thermodynamic, not architectural. You are protecting oil, valves, and power cost by splitting compression ratio and by cooling the gas before the second compression.

Compression ratio is an absolute-pressure ratio

Compression ratio (CR) is:

CR = P_discharge ÷ P_suction, with both pressures in psia (pounds per square inch absolute).

Gauge pressure (psig) is what the panel shows above atmosphere. Vacuum in inches of mercury is what the panel shows when the evaporator is below atmosphere. Neither number is the denominator of CR until you convert it.

Standard exam conversions:

• psia = psig + 14.7 (use 14.7 psia as atmospheric pressure unless a problem states another barometer).
• Vacuum to psia: psia = 14.7 − (inches Hg vacuum × 0.491), because 1 inch of mercury equals 0.491 psi.

If you divide 181 psig by “8.7” you have not computed a compression ratio. You have mixed a gauge reading with a vacuum reading and invented a meaningless number. The exam will punish that faster than it punishes a wrong saturation temperature.

The −40°F freezer against an 86°F condenser

Industrial frozen storage, ice cream hardening, and blast freezers commonly hold ammonia evaporators near −40°F. Anhydrous ammonia at −40°F is below atmospheric pressure. A representative saturated suction is about 8.7 inches Hg vacuum, which is 10.4 psia.

Summer condensing on an evaporative condenser is often discussed at +86°F (a textbook “standard” condensing temperature) or 95°F (a hot-day snapshot). Anhydrous ammonia saturation is about 169 psia (≈154.5 psig) at 86°F and about 196 psia (≈181 psig) at 95°F.

Single-stage CR at those two condenser conditions:

A reciprocating or screw compressor asked to swallow vapor at 10.4 psia and spit it out at 170–196 psia is living in a different world from a +20°F dock cooler (suction near 33.5 psig, CR often in the 4–6 range). That is why two-stage is the default architecture for low-temperature ammonia, not an optional upgrade.

How to read a two-stage plant diagram

CIRO study materials show two-stage ammonia plants as a labeled pressure-temperature map, not as a puzzle. A representative snapshot looks like this:

• Condenser: 95°F and 181 psig (high-side saturation).
• Intercooler / interstage: 33.5 psig (intermediate vessel).
• Evaporator: −40°F and 8.7 in Hg vacuum (low-side saturation).

Read it in this order every time:

1. Identify the three pressure levels. High side (condenser/receiver), interstage (intercooler), low side (freezer suction). Two-stage has three, not two.
2. Convert every reading to psia before you talk about ratio, geometric mean, or “is this vacuum real?”
3. Pair each pressure with its saturation temperature on an ammonia P/T chart. 181 psig should sit near 95°F. 33.5 psig should sit near 20°F. 8.7 in Hg vac should sit near −40°F. If the temperature and pressure disagree, you have superheat, subcooling, a bad sensor, or non-condensables — you do not have a new law of ammonia.
4. Name the machines. Vapor leaves the −40°F evaporator into the booster. Booster discharge enters the intercooler. Vapor leaving the intercooler is high-stage suction. High-stage discharge goes to the condenser.

Vacuum conversion you must be able to write from memory:

8.7 in Hg vac × 0.491 ≈ 4.3 psi below atmosphere

14.7 − 4.3 = 10.4 psia

That 10.4 psia is the booster suction pressure for every ratio and geometric-mean calculation that follows. The 8.7 on the screen is a vacuum gauge, not an absolute pressure.

Interstage on that same snapshot: 33.5 psig + 14.7 = 48.2 psia. High side: 181 + 14.7 = 195.7 psia.

Stage ratios for that diagram:

• Booster CR = 48.2 / 10.4 ≈ 4.6
• High-stage CR = 195.7 / 48.2 ≈ 4.1
• Overall CR = 195.7 / 10.4 ≈ 18.8, which equals 4.6 × 4.1

Two-stage did not eliminate the overall ratio. It factored it into two livable stage ratios whose product is the overall ratio. That is the whole point.

Why discharge temperature explodes with ratio

Compression adds enthalpy to the vapor. On the P-h diagram, an isentropic compression is a steep climb in temperature as well as pressure. Ammonia’s isentropes are unforgiving: a large pressure ratio from a cold, low-density suction state produces a very high theoretical discharge temperature.

A back-of-the-envelope isentropic estimate (ideal gas style, k near 1.31 for ammonia vapor) from saturated −40°F suction through a CR of ~19 lands theoretical discharge well above 300°F. Real machines with inefficiency and suction superheat run even hotter. That is not a “warm discharge line.” That is oil coking, carbon on valves, painted discharge pipes that discolor, and a compressor that will not live.

Split the same overall lift into two CRs near 4:1 and each stage’s isentropic discharge from a cooled suction state is in a range oil and steel can survive. The booster still discharges superheated gas — that is why the intercooler exists — but you are no longer asking one set of rotors or valves to do the entire −40°F-to-condenser job.

High discharge temperature also wrecks volumetric efficiency on recips (re-expansion of the clearance pocket) and increases the oil-cooling load on screws. Thermosiphon oil coolers are already working hard at two-stage freezer duty. Single-stage at CR 19 asks the oil cooler to reject heat the refrigerant circuit should have split and rejected at the intercooler.

What two-stage actually is

A CIRO two-stage ammonia plant is not a cascade and it is not “a screw with an economizer port” (related, but a different machine). Typical hardware:

• Booster / low-stage compressor: suction from the low-temperature evaporators (or from the low-temp recirculator / surge drum). Discharge into the intercooler at interstage pressure.
• Intercooler: a vessel (open flash) or a coil-in-bath (closed) at intermediate pressure. It desuperheats booster discharge toward intermediate saturation temperature and usually subcools high-side liquid that will feed the low-temperature loads.
• High-stage compressor: suction from the intercooler (plus any medium-temperature loads that live at interstage). Discharge to the condenser.

The same high-stage machine often also serves +20°F docks, processing rooms, or glycol chillers that sit on the intermediate suction. That is why interstage pressure is an operating choice, not only a formula — covered in the next section.

You gain four things at once:

1. Livable stage ratios and lower discharge temperature on both machines.
2. Denser gas at high-stage suction than at −40°F, so the high-stage moves more mass per cubic foot of displacement.
3. Subcooled liquid to the freezer, which increases net refrigerating effect and cuts flash gas in the low-temp vessel.
4. Flash gas from high-side liquid handled by the high-stage only, not by the booster (the economizer / intercooler benefit).

Two compressors piped in series without an intercooler still split the pressure ratio, but the high-stage suction is still the booster’s hot discharge. You have not done the thing that makes two-stage efficient. Interstage cooling is not optional decoration.

Operator traps on the exam and on the floor

• Using psig in CR. 181 / 33.5 is not a compression ratio. Convert first.
• Treating 8.7 in Hg as 8.7 psia. That understates suction pressure and inflates CR.
• Calling any hot discharge “needs more condenser.” Booster discharge temperature is an interstage and suction story first. The condenser is the high-stage’s problem unless high-side pressure is also high.
• Confusing two-stage with cascade. Cascade uses two different refrigerants (for example ammonia / CO2). Two-stage ammonia is the same refrigerant at three pressure levels.
• Assuming −10°F always needs two-stage. Many plants run single-stage into the teens below zero with screws and good oil cooling. −40°F freezer duty is the classic two-stage case. Know the numbers, not a slogan.

When a CIRO item shows condenser 95°F / 181 psig, intercooler 33.5 psig, and evaporator −40°F / 8.7 in Hg, the first competent move is to write 10.4 psia, 48.2 psia, 195.7 psia in the margin and then talk about ratio. Everything in this chapter starts there.