Steam Heating Processes - Load Calculating

Calculating the amount of steam in non-flow batch and continuous flow heating processes

In general steam heating is used to

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

Changing the Product Temperature - Heating up with Steam

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.

Non-flow or Batch Heating

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)

Flow or Continuous Heating Processes

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)

Calculating the Amount of Steam

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.

Example - Batch Heating by Steam

A quantity of water is heated with steam of 5 bar from a temperature of 35^oC to 100^oC over a period of 20 minutes (1200 seconds). The mass of the 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

Example - Continuously Heating by Steam

Water flowing at a constant rate of 3 l/s is heated from 10^oC to 60^oC with steam at 8 bar.

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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