Steam Trap Sizing Calculator

What the steam trap sizing calculator does

A steam trap lets condensate and air out of a steam system while holding the steam in. If the trap is too small, condensate backs up into the heat exchanger or steam main, cutting heat transfer and causing water hammer. If it is far too large, some trap types wear fast and leak steam. The steam trap sizing calculator multiplies the condensate load by a safety factor and states the differential pressure at which the trap must pass that flow, which is how trap capacity charts are read.

Condensate load calculation and trap sizing

When the heat load is known, the condensate load comes from the latent heat of the steam at its pressure:

The trap must then pass the condensate load times a safety factor at the pressure difference across it:

Flash steam forms when hot condensate drops to the lower return pressure: flash % = (h_f1 − h_f2) ÷ h_fg2 × 100, from saturated steam table values.

Steam trap sizing example

A drip leg on a 7 bar g steam main drains 250 kg/h of condensate into a return at 0.5 bar g:

Choose a trap whose capacity chart shows at least 500 kg/h (1,102 lb/h) at 6.5 bar differential and whose pressure rating covers 7 bar g. The latent heat at 7 bar g is 2,048 kJ/kg, and 11.6% of the condensate flashes to steam at the return pressure.

How to use the steam trap size calculator

Choose metric or US units. If you know the condensate load, enter it in kg/h or lb/h; otherwise choose heat load and enter the heat the equipment transfers in kW or BTU/h, and the calculator turns it into condensate with the latent heat of the steam. Enter the steam pressure at the trap inlet and the back pressure in the condensate return, then pick the application to set the safety factor. Press Calculate.

Reading the steam trap capacity result

The required capacity is the flow the trap must pass at the stated differential pressure, not at the steam pressure. Trap capacity falls as the differential drops, so on equipment with a modulating control valve, where the steam pressure can fall close to the back pressure, size at the lowest working differential. The flash steam figure tells you how much vapor the return line must carry; above about 10%, a flash vessel can recover the energy.

When to use steam trap sizing

• Choosing drip traps along a steam main
• Sizing traps for heat exchangers, coils, and unit heaters
• Checking an existing trap after a load increase
• Turning a heat load into a condensate load calculation
• Estimating flash steam for a condensate return design

Common steam trap sizing mistakes

• Sizing at the steam pressure. Use the differential pressure across the trap.
• Using the pipe size. Choose the trap by capacity, not by the connection size.
• Skipping the safety factor. Start-up loads are far higher than running loads.
• Ignoring lift. Each meter of lift adds about 0.1 bar of back pressure.

Tips for steam trap selection

• Trap each piece of equipment on its own, never group traps.
• Fit a strainer and a test point before every trap.
• Use a pump trap where the coil pressure can stall.

Related steam and heating tools

The heat transfer calculator gives the heat a product absorbs, which sets the condensate load. For hot water systems the hydronic flow rate calculator sizes the water flow instead.

Steam trap selection for modulating loads

On a heat exchanger with a temperature control valve, the steam pressure in the coil drops when the load falls. At part load it can fall to the back pressure or below, a condition called stall, and no trap can then discharge by pressure alone. That is why these applications use a higher safety factor and are often fitted with a pump trap. Lifting the condensate into an overhead return adds about 0.1 bar of back pressure for each meter of lift. The heat the exchanger must deliver follows from the product flow and temperature rise, which the heat transfer calculator works out.

Flash steam and the condensate return

Condensate leaves a trap at the saturation temperature of the steam. When it enters a lower pressure return, part of it boils off as flash steam, and the return pipe must be sized for that vapor, not only for the water. Flash vessels and condensate receivers are pressure equipment in their own right, and the pressure vessel calculator gives a first estimate of their shell thickness. On low pressure hot water systems, size the loop with the hydronic flow rate calculator instead.

Steam trap safety factor by application

Latent heat and flash steam to atmosphere

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