Steam Heating Processes - Load Calculating

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In general steam heating is used to

• change a product or fluid temperature
• maintain a product or fluid temperature

[h=Changing the Product Temperature - Heating up with Steam]3[/h] The amount of heat required to raise the temperature of a substance can be expressed as:

Q = m c_p dT (1)
where
Q = quantity of energy or heat (kJ)
m = mass of the substance (kg)
c_p = specific heat capacity of the substance (kJ/kg ^oC ) - Material Properties and Heat Capacities for several materials

dT = temperature rise of the substance (^oC)

Preferring Imperial Units - Use the Units Converter!

This equation can be used to determine a total amount of heat energy for the whole process, but it does not take into account the rate of heat transfer which is:

• amount of heat energy per unit time

In non-flow type applications a fixed mass or a single batch of product is heated. In flow type applications the product or fluid is heated when it constantly flows over a heat transfer surface.
[h=Non-flow or Batch Heating]3[/h] In non-flow type applications the process fluid is kept as a single batch within a tank or vessel. A steam coil or a steam jacket heats the fluid from a low to a high temperature.
The mean rate of heat transfer for such applications can be expressed as:

q = m c_p dT / t (2)
where
q = mean heat transfer rate (kW (kJ/s))
m = mass of the product (kg)
c_p = specific heat capacity of the product (kJ/kg.^oC) - Material Properties and Heat Capacities for several materials

dT = Change in temperature of the fluid (^oC)
t = total time over which the heating process occurs (seconds)

[h=Flow or Continuous Heating Processes]3[/h] In heat exchangers the product or fluid flow is continuously heated.
The mean heat transfer can be expressed as

q = c_p dT m / t (3)
where
q = mean heat transfer rate (kW (kJ/s))
m / t = mass flow rate of the product (kg/s)
c_p = specific heat capacity of the product (kJ/kg.^oC) - Material Properties and Heat Capacities for several materials

dT = change in temperature of the fluid (^oC)

[h=Calculating the Amount of Steam]3[/h] If we know the heat transfer rate - the amount of steam can be calculated:

m_s = q / h_e (4)
where
m_s = mass of steam (kg/s)
q = calculated heat transfer (kW)
h_e = evaporation energy of the steam (kJ/kg)

The evaporation energy at different steam pressures can be found in the SteamTable with SI Units or in the Steam Table with Imperial Units.

[h=Example - Batch Heating by Steam]3[/h] A quantity of water is heated with steam of 5 bar (6 bar abs) from a temperature of 35 ^oC to 100 ^oC over a period of 20 minutes (1200 seconds). The mass of water is 50 kg and the specific heat capacity of water is 4.19 kJ/kg.^oC.
Heat transfer rate:

q = (50 kg) (4.19 kJ/kg ^oC) (100 ^oC - 35 ^oC) / (1200 s)
= 11.35 kW

Amount of steam:

m_s = (11.35 kW) / (2085 kJ/kg)
= 0.0055 kg/s
= 19.6 kg/h

[h=Example - Continuously Heating by Steam]3[/h] Water flowing at a constant rate of 3 l/s is heated from 10 ^oC to 60 ^oC with steam at 8 bar (9 bar abs).

The heat flow rate can be expressed as:

q = (4.19 kJ/kg.^oC) (60 ^oC - 10 ^oC) (3 l/s) (1 kg/l)
= 628.5 kW

The steam flow rate can be expressed as:

m_s = (628.5 kW) / (2030 kJ/kg)
= 0.31 kg/s
= 1115 kg/h

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