OpenSpecEngineering

Finds how much steam a heater uses, which is also how much condensate it makes, to deliver a heat duty. Enter the duty in kW, or the flow and temperature rise of the stream being heated. Plant and process engineers use it to size steam supply lines, control valves, steam traps and condensate return systems.

Equations

ṁ_s = Q / h_fg     Q = ṁ·c_p·ΔT

ṁ_s is the steam (and condensate) mass flow in kg/s and Q the heat duty in kW [1]. h_fg is the latent heat of the steam in kJ/kg: the heat it gives up as it condenses at its saturation temperature. In the second equation ṁ is the process stream’s flow, c_p its specific heat in kJ/(kg·K) and ΔT its temperature rise.

Inputs

• With Latent Heat of the Steam set to the supply pressure, the page finds the saturation temperature and h_fg from the IAPWS-IF97 steam table [2]. Say whether the pressure is gauge or absolute. Use the pressure in the heater, after the control valve, not the main header pressure.
• Choosing a Process Stream Fluid fills c_p from the shared fluid table [2]–[4]. Pick the entry nearest the stream’s mean temperature.
• h_fg falls as the pressure rises, so the same duty needs a little more steam at a higher pressure.

Limits and common mistakes

• The steam is taken as dry and saturated, and the condensate as leaving at the saturation temperature. Superheat adds a little heat. Subcooling the condensate in the heater adds a little more.
• The result is the running load. Warming up cold pipework and equipment at startup can need several times as much, and traps are usually sized with a margin above the running load [1].
• Heat lost from the heater shell and piping isn’t included. Add it to the duty.
• Wet steam carries less latent heat per kg than h_fg, so it needs more steam.

Worked example

A shell-and-tube heater warms 12,000 kg/h of water from 10 to 60 °C with steam at 7 bar gauge. What steam flow should the control valve and trap be sized for?

1. Choose Duty Basis Heated stream.
2. Enter Process Stream Flow 12000 kg/h.
3. Choose Process Stream Fluid Water, 30°C. c_p fills in as 4.18 kJ/(kg·K).
4. Enter Temperature Rise 50 °C.
5. Choose Latent Heat of the Steam Saturated steam at its supply pressure. Enter Steam Supply Pressure 7 bar, leave Steam Pressure Is at Gauge, and press Solve. The Solution shows: Heat duty Q: 696.667 kW
   Steam saturation temperature: 170.483 °C
   Latent heat hfg used: 2,047.05 kJ/kg
   Condensate / steam load: 1,225.18 kg/h
   Condensate / steam load: 0.340328 kg/s

Check by hand: Q = (12,000/3600) × 4.18 × 50 = 696.7 kW. Steam tables give h_fg = 2048.7 kJ/kg at 170 °C, and 696.7/2048.7 × 3600 = 1224 kg/h. As a rule of thumb, each kW of duty takes about 1.75 kg/h of steam at this pressure.