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

m_s = P / (h_g - (t c_p))                                  (1)

where

m_s = steam flow rate (kg/s)

P = heat transfer rate or power from the steam (kW)

h_g = specific total enthalpy of the steam before the control valve (kJ/kg)

t = final temperature of the water after the heating (^oC) 

c_p = 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/kg^oC and the power required to heat the water can be calculated as

P = (100 kg) (4.2 kJ/kg^oC) ((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

m_s = (84 kW) / ((2711 kJ/kg) - ((80 ^oC) (4.2 kJ/kg^oC)))

     = 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