Heating Water by Injecting Steam
For some applications a direct heating of water by injecting steam may be convenient. A direct heating system is very simple since it don't require heat exchangers. Fouling of surfaces, which may be a major problem in certain processes, is also avoided.
The required steam flow rate in an steam injection process can be calculated as
ms = P / (hg - (t cp)) (1)
where
ms = steam flow rate (kg/s)
P = heat transfer rate or power from the steam (kW)
hg = specific total enthalpy of the steam before the control valve (kJ/kg)
t = final temperature of the water after the heating (oC)
cp = specific heat water (kJ/kg)
The specific heat of evaporated steam is related to zero degrees Celsius.
Example - Heating a Water Volume by Injecting Steam
100 liter (kg) of water is heated from 20 oC to 80 oC in 5 minutes (300 s). The specific heat of water is 4.2 kJ/kgoC and the power required to heat the water can be calculated as
P = (100 kg) (4.2 kJ/kgoC) ((80 oC) - (20 oC)) / (300 s)
= 84 kW
The absolute steam pressure is 220 kPa and the total enthalpy of the steam is 2711 kJ/kg. The required steam flow rate can be calculated as
ms = (84 kW) / ((2711 kJ/kg) - ((80 oC) (4.2 kJ/kgoC)))
= 0.035 kg/s
= 127 kg/h
The total steam consumption for the heating process can be calculated as
q = (0.035 kg/s) (300 s)
= 10.5 kg
Related Topics
• Thermodynamics
Thermodynamics of steam and condensate systems.
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