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💧 Calculators

Warm-Up Condensate Load (Steam)

Computes the average condensate flow generated while a steam line or piece of equipment is warming up, m = M × c_p × (T_final − T_initial) ÷ (h_fg × t), that is, the sensible heat absorbed by the cold metal divided by the latent heat of the steam and by the time in which the warm-up is to be completed. This average warm-up flow, weighed against the running load, is what sizes the steam trap — take the larger of the two, with a factor of 2 to 3 on the warm-up figure, since the peak in the first minutes runs well above the average: on start-up cold pipework condenses far more steam than it does once hot, and a trap picked from the running load alone floods the line and invites water hammer. Carbon steel has a specific heat around 0.49 kJ/kg·K, and latent heat drops as pressure rises — at 170 °C (about 7 bar gauge) it is roughly 2049 kJ/kg. Enter the metal mass, the specific heat, the initial and final temperatures, the latent heat of the steam and the warm-up time.

Result

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Warm-up condensate load on a steam line, in kg/h

The trap picked from the running load is the one that floods the line at six in the morning. When the isolation valve opens, cold pipework condenses steam at a rate that has nothing to do with normal service: every kilogram of steel has to climb from ambient to steam temperature, and the heat for that comes out of condensation. Whoever sizes a main drip leg, a tank coil or a reactor jacket runs this figure before picking a trap — or pays the difference in water hammer and a waterlogged trap.

m = M × c_p × (T_final − T_initial) ÷ (h_fg × t): the sensible heat the metal soaks up, divided by the latent heat of the steam and by the time allowed for warm-up. With the defaults — 500 kg of steel, c_p = 0.49 kJ/kg·K, from 20 °C to 170 °C, h_fg = 2049 kJ/kg over half an hour — that is 36750 kJ of sensible heat, 17.94 kg of condensed steam and 35.87 kg/h of average flow. Close the loop from the other side: 35.87 × 0.5 × 2049 gives back the same 36750 kJ, matching M × c_p × ΔT worked out separately.

This is the average over the window, never the peak. In the first minute the temperature difference is largest and the instantaneous rate runs well above it; worse, on start-up the line still sits near atmospheric pressure, so the trap discharges against a small differential and delivers a fraction of its catalogue capacity. Add what the arithmetic leaves out: heat loss to the surroundings while warming, the mass of flanges, valves, supports and lagging, and an h_fg that is larger while pressure climbs, 2257 kJ/kg at zero bar gauge against 2049 at seven. The page applies no safety factor and picks no trap.

Frequently asked questions

Is this figure already the capacity the trap needs?
No. Common practice multiplies the warm-up load by a factor of 2 to 3 before selecting a trap, precisely because the figure is a period average and because the pressure differential available at start-up is low. With the 35.87 kg/h of the defaults, look for a trap rated 72 to 108 kg/h at the real start-up differential, not at the running pressure. The page performs no such multiplication and reads no catalogue.
Where do I find the latent heat for my steam pressure?
In a saturated steam table, entering by temperature or by pressure. Latent heat falls as pressure rises: 2257 kJ/kg at 100 °C, 2114 at 150 °C, 2049 at 170 °C, 2015 at 180 °C, reaching zero at the critical point, 374 °C. Because it sits in the denominator, higher-pressure steam condenses more mass to deliver the same heat — raising line pressure raises the warm-up condensate load.
Does it work for equipment or only for pipework?
It works for any metal mass that has to climb in temperature: an exchanger shell, a reactor jacket, a tank coil, a press mould. What the page will not do is add two masses at once, since there is a single mass field and a single specific heat field. Because the formula is linear, run it once for the metal and once for the product it holds, each with its own c_p, and add the two answers by hand.

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The results provided by this tool are for general informational and educational purposes only and do not constitute professional, financial, medical, legal, tax or accounting advice. Always confirm important decisions with a qualified professional and official sources.