9.1 Enthalpy, Latent Heat, and Net Refrigerating Effect

9.1 Enthalpy, Latent Heat, and Net Refrigerating Effect

CIRO Heat Flow items reward operators who can turn an on-screen saturated-properties table into capacity. Enthalpy is the property that makes the conversion possible. You do not need a thermodynamics degree. You need a working definition, ammonia's latent-heat range, the net refrigerating effect (NRE) formula, and the mass-flow equation that follows from it.

Enthalpy is the operator's energy label

Enthalpy (h) is heat content per pound of refrigerant, in Btu/lb. On a pressure-enthalpy (P-h) diagram it is the horizontal axis. On the CIRO saturated-properties table it appears as hf (saturated liquid) and hg (saturated vapor). The difference hfg = hg − hf is the latent heat of vaporization—the heat that converts one pound of saturated liquid into saturated vapor at that temperature.

Absolute enthalpy depends on an arbitrary zero. Industrial ammonia tables commonly set hf = 0 Btu/lb at −40°F. CIRO does not grade the reference. It grades differences. Capacity is always mass flow times an enthalpy change. Use one table (or one P-h chart) for an entire problem so every h shares the same zero.

Sensible heat changes temperature. Latent heat changes phase at constant saturation temperature. In an evaporator the useful job is mostly latent: liquid becomes vapor and the heat comes from the room, the product, or the process. Superheat after the coil adds a little extra enthalpy to the vapor, but that slice is small next to ammonia's latent heat.

Ammonia's latent heat is why the pipes can look "small"

At evaporator temperatures, ammonia's latent heat is roughly 560–590 Btu/lb. That is not a slogan. It is what the saturation table shows, and hfg falls as saturation temperature rises. Always quote hfg with its saturation temperature. Saying "ammonia is about 580 Btu/lb" without a temperature is how people grab the wrong row.

Representative on-screen table values (hf = 0 at −40°F):

At −20°F, hfg is 583.8 Btu/lb. At 0°F it is 569.7 Btu/lb. At 20°F it is 555.1 Btu/lb. At a 95°F condensing temperature the latent heat of condensation is only about 492 Btu/lb—still large, but it is not the evaporator number. A typical halocarbon latent heat is on the order of 70–90 Btu/lb, so that plant needs several times the mass flow to move the same tons. Ammonia's large hfg is why a serious heat load still corresponds to a modest lb/h through the compressor.

Net refrigerating effect

NRE is the refrigerating effect actually delivered per pound:

NRE = h_suction vapor − h_liquid entering the evaporator (Btu/lb)

On a P-h diagram that is the horizontal span of the evaporator. The compressor, condenser, and expansion device set the two enthalpies; the evaporator uses them.

DX coil. Expansion through a TXV, EEV, or hand expansion valve is essentially isenthalpic: enthalpy does not change in the valve. The liquid enthalpy leaving the high side (receiver, subcooler, or liquid line) is the enthalpy entering the evaporator. If suction vapor leaves at h1 and high-side liquid is at h3:

NRE = h1 − h3

Flash gas generated in the valve is already vapor when it enters the coil. It occupies evaporator volume and compressor displacement, but it does not pick up latent heat in the coil. That is why DX NRE is smaller than evaporator hfg.

Flooded or liquid-overfeed coil. Liquid entering the coil from a surge drum or recirculator is already at evaporator pressure. If that liquid is saturated and suction vapor is saturated, NRE at the coil is approximately hfg at the evaporator temperature. The plant still pays for high-side liquid enthalpy: liquid arriving from the condenser flashes in the vessel. Compressor mass flow for the cycle is still capacity divided by (h_suction − h_high-side liquid). Do not mix coil-level hfg with cycle-level NRE when you are asked for compressor pounds per hour.

Subcooling lowers h3 and raises cycle NRE. A little suction superheat raises h1 slightly and also raises NRE a little, at the cost of extra compressor work and discharge temperature. Excessive superheat is not a capacity strategy.

Worked NRE — DX, saturated suction, no subcooling

A single-stage ammonia plant evaporates at −20°F. Suction vapor is saturated, so h1 = hg = 605.2 Btu/lb. Condensing saturation is 95°F with no subcooling, so h3 = hf = 149.4 Btu/lb.

NRE = 605.2 − 149.4 = 455.8 Btu/lb

Evaporator latent heat at −20°F is 583.8 Btu/lb. The 128 Btu/lb gap is the flash-gas penalty. Liquid at evaporator pressure would have been hf = 21.4 Btu/lb; the valve admitted liquid at 149.4 Btu/lb, so the flash fraction is (149.4 − 21.4) / 583.8 ≈ 0.22. About 22% of each pound flashed in the valve. The remaining 78% evaporates in the coil: 0.78 × 583.8 ≈ 456 Btu/lb, which matches NRE.

Worked NRE — 10°F of liquid subcooling

Same suction. Liquid is subcooled so its enthalpy matches saturated liquid at 85°F: h3 = 137.7 Btu/lb.

NRE = 605.2 − 137.7 = 467.5 Btu/lb

Subcooling added 11.7 Btu/lb of NRE—about 2.6% more refrigeration per pound—without changing evaporator temperature. That is why a working subcooler, or a condenser drain that actually leaves subcooled, shows up as tons.

Worked mass flow

Mass flow (lb/h) = refrigerating capacity (Btu/h) / NRE (Btu/lb)

1 ton of refrigeration = 12,000 Btu/h = 200 Btu/min.

For 100 tons with NRE = 455.8 Btu/lb:

Capacity = 100 × 12,000 = 1,200,000 Btu/h

ṁ = 1,200,000 / 455.8 ≈ 2,633 lb/h ≈ 43.9 lb/min

If an operator mistakenly divides by evaporator hfg (583.8 Btu/lb):

ṁ_wrong = 1,200,000 / 583.8 ≈ 2,056 lb/h

That understates DX compressor mass flow by about 28%. The extra ~577 lb/h is flash vapor the compressor still has to pump. On a CIRO table item, using hfg where the stem wants NRE is a classic miss.

Same 100 tons at 0°F evaporator, saturated suction (h1 = 612.6 Btu/lb), 95°F saturated liquid (h3 = 149.4 Btu/lb):

NRE = 612.6 − 149.4 = 463.2 Btu/lb

ṁ = 1,200,000 / 463.2 ≈ 2,591 lb/h

Slightly fewer pounds than the −20°F DX case because suction-vapor enthalpy is higher. Do not assume colder always means more pounds. Colder usually means more volume—specific volume of suction vapor rises—which is a displacement and compression-ratio problem. This section is about pounds and Btu.

How to read a CIRO table under the clock

1. Find evaporator saturation temperature, or convert suction psig → psia (add 14.7 psi unless the stem gives another barometer) and enter the pressure table.
2. Read hg for saturated suction, or step into superheat columns if suction temperature is above saturation.
3. Read liquid enthalpy at the actual liquid temperature, or at condensing temperature if the liquid is saturated.
4. Subtract. The difference is NRE in Btu/lb.
5. Convert tons to Btu/h (× 12,000) and divide by NRE to get lb/h. Divide by 60 if the answer is wanted in lb/min.

Exam traps

• Using hfg as NRE on a DX cycle with hot high-side liquid entering the valve.
• Mixing psia and psig when you pick the table row.
• Adding enthalpies instead of subtracting.
• Using condenser hfg (~490–510 Btu/lb at typical heads) as the evaporator effect.
• Treating mass flow as capacity / horsepower instead of capacity / NRE.