How to calculate steam requirements for flow and non-flow applications. Including warm-up, heat losses and running loads.
Methods of Estimating Steam Consumption
Example 2.6.2
A A tank containing 400 kg of kerosene is to be heated from 10 °C to 40 °C in 20 minutes (1 200 seconds), using 4 bar g steam. The kerosene has a specific heat capacity of 2.0 kJ/kg °C over that temperature range. hfg at 4.0 bar g is 2 108.1 kJ/kg. The tank is well insulated and heat losses are negligible.
In some non-flow type applications, the length of time of the batch process may not be critical, and a longer heat up time may be acceptable. This will reduce the instantaneous steam consumption and the size of the required plant equipment.
Flow type applications
Typical examples include shell and tube heat exchangers, see Figure 2.6.2 (also referred to as non-storage calorifiers) and plate heat exchangers, providing hot water to heating systems or industrial processes. Another example would be an air heater battery where steam gives up its heat to the air that is constantly passing through.
Figure 2.6.3 provides a typical temperature profile in a heat exchanger with a constant secondary fluid flowrate. The condensing temperature (T_{S}) remains constant throughout the heat exchanger.
The fluid is heated from T_{1} at the inlet valve to T_{S} at the outlet of the heat exchanger.
For a fixed secondary flowrate, the required heat load (
) is proportional to the product temperature rise (
T). Using Equation 2.6.1:
Mean steam consumption
The mean steam consumption of a flow type application like a process heat exchanger or heating calorifier can be determined from Equation 2.6.6, as shown in Equation 2.6.7.
But as the mean heat transfer is, itself, calculated from the mass flow, the specific heat, and the temperature rise, it is easier to use Equation 2.6.7.
Example 2.6.3
Dry saturated steam at 3 bar g is used to heat water flowing at a constant rate of 1.5 l/s from 10°C to 60°C.
h_{fg}_{}_{}_{}_{}_{} at 3 bar g is 2 133.4 kJ/kg, and the specific heat of water is 4.19 kJ/kg °C
Determine the steam flowrate from Equation 2.6.7:
As 1 litre of water has a mass of 1 kg, the mass flowrate = 1.5 kg/s
At start-up, the inlet temperature, T_{1} may be lower than the inlet temperature expected at the full running load, causing a higher heat demand. If the warm-up time is important to the process, the heat exchanger needs to be sized to provide this increased heat demand. However, warm-up loads are usually ignored in flow type design calculations, as start-ups are usually infrequent, and the time it takes to reach design conditions is not too important. The heat exchanger heating surface is therefore usually sized on the running load conditions.
In flow type applications, heat losses from the system tend to be considerably less than the heating requirement, and are usually ignored. However, if heat losses are large, the mean heat loss (mainly from distribution pipework) should be included when calculating the heating surface area.
Warm-up and heat loss components
In any heating process, the warm-up component will decrease as the product temperature rises, and the differential temperature across the heating coil reduces. However, the heat loss component will increase as the product and vessel temperatures rise, and more heat is lost to the environment from the vessel or pipework. The total heat demand at any time is the sum of these two components.
If the heating surface is sized only with consideration of the warm-up component, it is possible that not enough heat will be available for the process to reach its expected temperature. The heating element, when sized on the sum of the mean values of both these components, should normally be able to satisfy the overall heat demand of the application.
Sometimes, with very large bulk oil storage tanks for example, it can make sense to maintain the holding temperature lower than the required pumping temperature, as this will reduce the heat losses from the tank surface area. Another method of heating can be employed, such as an outflow heater, as shown in Figure 2.6.4.
Heating elements are encased in a metal shroud protruding into the tank and designed such that only the oil in the immediate vicinity is drawn in and heated to the pumping temperature. Heat is therefore only demanded when oil is drawn off, and since the tank temperature is lowered, lagging can often be dispensed with. The size of outflow heater will depend on the temperature of the bulk oil, the pumping temperature and the pumping rate.
Adding materials to open topped process tanks can also be regarded as a heat loss component which will increase thermal demand. These materials will act as a heat sink when immersed, and they need to be considered when sizing the heating surface area.
Whatever the application, when the heat transfer surface needs calculating, it is first necessary to evaluate the total mean heat transfer rate. From this, the heat demand and steam load may be determined for full load and start-up. This will allow the size of the control valve to be based on either of these two conditions, subject to choice.