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 cp dT         (1)

where

Q = quantity of energy or heat (kJ)

m = mass of the substance (kg)

cp = 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 cp dT / t         (2)

where

q = mean heat transfer rate (kW (kJ/s))

m = mass of the product (kg)

cp = 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)

Example - Time required to Heat up Water with Steam

The time required to heat 75 kg of water (cp = 4.2 kJ/kgoC) from  temperature 20oC to 75oC with steam produced from a boiler with capacity 200 kW (kJ/s) can be calculated by transforming eq. 2 to

t = m cp dT / q

 = (75 kg) (4.2 kJ/kgoC) ((75 oC) - (20 oC)) / (200 kJ/s) 

 = 86 s

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 = cp dT m / t         (3)

where

q = mean heat transfer rate (kW (kJ/s))

m / t = mass flow rate of the product (kg/s)

cp = 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:

ms = q / he         (4)

where

ms = mass of steam (kg/s)

q = calculated heat transfer (kW)

he = 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 (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:

ms = (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 (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:

ms = (628.5 kW) / (2030 kJ/kg)

    = 0.31 kg/s

    = 1115 kg/h

Related Topics

  • Steam and Condensate - Steam & condensate properties - capacities, pipe sizing, systems configuration and more
  • Thermodynamics - Thermodynamics of steam and condensate applications
  • Heat Loss and Insulation - Steam and condensate pipes - heat loss uninsulated and insulated pipes, insulation thickness and more
  • Pipe Sizing - Sizing steam and condensate pipes - pressure loss, recommended velocity, capacity and more

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