How do you calculate condensate load for a steam pipe?

Split it into the warm-up (start-up) load and the running load. Warm-up: kg/h = pipe mass (kg) × 0.49 × (steam temperature − ambient) × 60 ÷ (hfg × warm-up minutes). Running: kg/h = heat emission (W/m) × length (m) × insulation factor × 3.6 ÷ hfg. For 100 m of 100 mm Schedule 40 pipe (16.1 kg/m) at 14 bar g from 20 °C in 30 minutes: 1,610 × 0.49 × 178 × 60 ÷ (1,947 × 30) = 144 kg/h of warm-up condensate for the pipe alone.

Why is the start-up load so much larger than the running load?

At start-up the steam must heat the whole mass of steel from ambient to saturation temperature in a short time, and all of that heat comes from condensing steam. Once warm, only the heat lost through the insulation condenses steam. In the Spirax Sarco 100 mm main example the warm-up load is 161 kg/h (with flanges and a valve) against a running load of about 18 kg/h with 75 mm insulation.

What latent heat value should I use?

Use the latent heat of evaporation (hfg) at the steam pressure in the equipment. It falls as pressure rises: about 970 Btu/lb at 0 psig, 946 at 15 psig, 881 at 100 psig and 857 at 150 psig (2,257 kJ/kg at 0 bar g, 2,108 kJ/kg at 4 bar g, 1,947 kJ/kg at 14 bar g). The calculator interpolates it from steam table values.

How do I calculate condensate from an air heater coil?

Heat duty (Btu/hr) = 1.08 × CFM × temperature rise (°F) for standard air, and condensate (lb/hr) = heat duty ÷ hfg. 10,000 CFM heated from 0 to 100 °F needs 1,080,000 Btu/hr; at 15 psig (hfg 946 Btu/lb) that is about 1,140 lb/hr. The tool uses the actual inlet air density, which raises the load for cold inlet air.

What safety factor is used for steam trap sizing?

A factor of about 2 on the calculated load is common for traps on mains and simple heaters, because the pressure is low and the load is high during start-up. Temperature-controlled heat exchangers can need more. The calculator shows 2 × the larger of the start-up and running loads as a starting point; follow the trap maker's sizing data.

How much condensate does an AC cooling coil produce?

Multiply the dry-air mass flow by the drop in humidity ratio across the coil. 4,000 CFM entering at 80 °F and 55% RH (83.7 gr/lb) and leaving at 55 °F and 95% RH (60.9 gr/lb) removes 22.8 gr/lb, about 25.5 kg/hr: 6.75 gal/hr or 162 gal/day of condensate and a latent load of roughly 4.9 tons.

Does altitude affect the AC coil result?

Yes. The coil mode assumes sea-level pressure (101.325 kPa). At lower pressure the humidity ratio for the same temperature and RH is slightly higher, so results above about 1,000 m (3,300 ft) may be a few percent low.

Condensate Load Calculator

Steam condensate load = heat required ÷ latent heat (hfg) at the steam pressure. Pipe warm-up: m = W × cp × ΔT ÷ (hfg × t); running load = heat loss ÷ hfg; air heater: m = airflow × ρcp × ΔT ÷ hfg. A separate mode gives AC coil condensate in gal/hr.

Application

Unit System

Steam pressure (psig)

Equipment

Pipe size (Sch 40)

Pipe weight (lb/ft)

Length (ft)

Flanges & valves (lb)

Start temperature (°F)

Warm-up time (min)

Bare heat loss (Btu/hr·ft)

Insulation factor

Bare heat loss: from a heat emission table for the pipe size and temperature difference (e.g. 4 in / 100 mm pipe at ΔT 178 °C ≈ 1,374 W/m). Insulation factor 1 = bare; 0.07 for 75 mm insulation on 100 mm pipe in the Spirax Sarco example.

Steam Condensate Load

Start-up (warm-up) load

—

Running load

—

Saturation temperature—

Latent heat hfg—

Heat duty—

Trap capacity (2 × peak load)—

Condensate Flow Rate

Gallons per Hour

—

gal/hr

Liters per Hour

—

L/hr

Daily Condensate Production

Gallons per Day

—

gal/day

Liters per Day

—

L/day

Moisture Removal Details

Entering humidity ratio (W₁)—

Leaving humidity ratio (W₂)—

ΔW (moisture removed)—

Dry-air mass flow—

Moisture removal rate—

Latent cooling load—

Drain Flow Planning Reference

Flow Rate	Min Pipe Size	Notes
Up to 2 gal/hr	¾ in (DN20)	Fan coils, small AHUs
2–8 gal/hr	1 in (DN25)	Mid-size AHUs
8–20 gal/hr	1¼ in (DN32)	Large AHUs, RTUs
20+ gal/hr	1½ in (DN40)+	Central plant / chilled water

Planning comparison only—not code sizing. Final pipe and trap design depends on equipment capacity, connection size, pressure, slope, manufacturer instructions and local code.

Saturated steam: temperature and latent heat by gauge pressure

IAPWS-IF97 values, sea-level atmosphere (14.696 psia); the calculator uses the same interpolation.

psig	bar g	Temp °F	Temp °C	hfg Btu/lb	hfg kJ/kg
0	0.00	212.0	100.0	970.1	2,256
5	0.34	227.1	108.4	960.4	2,234
15	1.03	249.7	121.0	945.6	2,199
25	1.72	266.8	130.4	934.0	2,172
50	3.45	297.7	147.6	911.9	2,121
75	5.17	320.1	160.0	895.0	2,082
100	6.89	337.9	169.9	880.9	2,049
125	8.62	352.9	178.3	868.5	2,020
150	10.34	365.9	185.5	857.4	1,994
200	13.79	387.8	197.7	837.8	1,949
250	17.24	406.0	207.8	820.5	1,909
300	20.68	421.9	216.6	804.7	1,872

Warm-up condensate per 100 ft of Schedule 40 pipe (lb, from 70 °F)

Total condensate to heat the pipe only (no flanges, valves or heat loss). Divide by the warm-up time in hours for lb/hr.

Pipe	lb/ft	15 psig	100 psig	150 psig
½ in (DN15)	0.85	1.9	3.0	3.4
¾ in (DN20)	1.14	2.5	4.0	4.6
1 in (DN25)	1.68	3.7	6.0	6.8
1¼ in (DN32)	2.28	5.1	8.1	9.2
1½ in (DN40)	2.72	6.1	9.7	11.0
2 in (DN50)	3.66	8.1	13.0	14.8
2½ in (DN65)	5.80	12.9	20.6	23.4
3 in (DN80)	7.59	16.9	27.0	30.6
4 in (DN100)	10.80	24.0	38.4	43.6
5 in (DN125)	14.63	32.5	52.0	59.1
6 in (DN150)	18.99	42.2	67.6	76.7
8 in (DN200)	28.59	63.6	101.7	115.5
10 in (DN250)	40.53	90.1	144.2	163.7
12 in (DN300)	49.65	110.4	176.6	200.5

Worked examples

Steam main (Spirax Sarco method). 100 m of 100 mm Schedule 40 pipe (16.1 kg/m = 1,610 kg) at 14 bar g (198 °C, hfg 1,947 kJ/kg), from 20 °C in 30 minutes: 1,610 × 0.49 × 178 × 60 ÷ (1,947 × 30) = 144 kg/h for the pipe; Spirax quotes 161 kg/h once nine pairs of flanges and a valve are added. Running load with 75 mm insulation: 1,374 W/m × 103.9 m × 0.07 × 3.6 ÷ 1,947 ≈ 18.5 kg/h.

Air heater. 10,000 CFM heated from 70 °F to 170 °F with 15 psig steam: 1.08 × 10,000 × 100 = 1,080,000 Btu/hr ÷ 946 Btu/lb ≈ 1,140 lb/hr of condensate.

Tank. 500 US gal of water (4,170 lb) heated from 50 °F to 180 °F in 1 hour with 15 psig steam: 4,170 × 1.0 × 130 = 542,000 Btu ÷ 946 ≈ 573 lb/hr.

Sources: Spirax Sarco: steam consumption of pipes and air heaters · TLV steam tables

Summary

Condensate load is the rate at which steam condenses as it gives up its latent heat. For steam systems it sizes steam traps, drip legs and condensate return lines: the start-up (warm-up) load while cold pipe or product heats up is usually several times the running load from heat loss, so traps must handle both. This calculator looks up saturation temperature and latent heat (hfg) from saturated steam table values for the gauge pressure, then applies the Spirax Sarco methods for steam mains (pipe mass × steel specific heat × temperature rise ÷ warm-up time, and heat emission × length × insulation factor), batch tank heating, and air heater batteries. A second mode estimates HVAC cooling-coil condensate (gal/hr and gal/day) and latent load from airflow and entering and leaving air conditions.

How it works

Steam mode: enter the steam pressure (psig or bar g). The tool interpolates saturation temperature and latent heat hfg from IAPWS-IF97 steam table values (log-pressure interpolation).
Pipe warm-up load (kg/h) = W × cp × (Ts − T1) × 60 ÷ (hfg × t), with W the pipe and fittings mass (kg), cp of steel 0.49 kJ/kg·°C and t the warm-up time in minutes.
Pipe running load (kg/h) = bare-pipe heat emission (W/m) × length × insulation factor × 3.6 ÷ hfg.
Tank heat-up load = (liquid mass × cp + vessel mass × 0.49) × (T2 − T1) ÷ (hfg × heat-up hours). Air heater load = airflow × air density × 1.006 × (T2 − T1) × 3,600 ÷ hfg (equivalent to 1.08 × CFM × ΔT ÷ hfg in lb/hr at standard air).
Trap capacity is shown at twice the larger load, a common start-up safety factor; confirm with the trap maker's sizing guidance.
AC coil mode: humidity ratios from the Magnus vapour pressure at sea-level pressure; condensate = dry-air mass flow × (W1 − W2), converted to gal/hr and L/hr.

Use cases

Size steam traps and drip legs on a steam main for cold start-up and running conditions.
Estimate the warm-up time a steam main needs for a given trap or control valve capacity.
Calculate steam consumption and condensate for heating a tank or batch vessel in a set time.
Find the condensate rate from an air heater battery or unit heater for trap and return-line sizing.
Size condensate drain lines and pumps for air-handling unit cooling coils.
Check equipment submittals and compare start-up and running steam demand.

Frequently Asked Questions

Last updated: 2026-10-10 · Reviewed by Nham Vu