Lesson 2 - Steam Theory

Steam tables are most commonly used to retrieve thermodynamic data for water when operating conditions have been specified.

Steam table information is commonly provided in even increments, based on a given saturation temperature or saturation pressure. There are also tables for superheated steam and for compressed water. The steam table below provides thermodynamic data for saturated water and saturated steam. One downside of steam tables is that, if we want a temperature or pressure between these increments, then we need to interpolate between the two closest figures – or find a chart with smaller or different increments!

Table: Steam table

The column titled ‘Specific Volume’ refers to the volume occupied by 1 kg of steam at a given pressure and temperature. It is useful to know this when sizing steam pipework systems in the brewery.

It is important to note that the above steam tables (and many Mollier diagrams) are in absolute pressure. However, many pressure readings in a brewery will be gauge pressure. Absolute pressure is the pressure which includes atmospheric pressure, while gauge pressure is relative to atmospheric pressure.

Where P_atm  is atmospheric pressure, which at sea level is 101.325 kPa (0.101 MPa).

Data from steam tables is often used to calculate heat loads in water and steam systems. Let’s look at an example of this now.

What is the total amount of steam and the flow rate required when heating a hot water tank containing 100 hL (10,000 kg) of water from 10°C to 90°C in 30 minutes? Steam is available at 5 MPa (4.9 MPa g). The steam data is available in the steam table above. The specific heat capacity of water is 4.184 kJ kg^-1 K^-1.

To answer this, we need to set up an energy balance to say that the amount of energy from the steam is equal to the amount of energy needed to increase the temperature of water from 10°C to 90°C. This is valid if we assume that there is no heat loss to the surrounding environment.

The energy required to change the temperature of water is the sensible heat. Remember, the sensible heat is calculated as:

Now, we need to determine how much 5 MPa steam we need. The energy available per kg of steam is the enthalpy of vaporisation (taken from the steam table). At 5 MPa, the enthalpy of vaporisation is 1,639.57 kJ kg^-1. Therefore, total energy of steam would be calculated as:

We can equate this to the energy required to heat the water and use it to finally solve for M.

To calculate the steam flow rate, we divide the total steam by the time, and arrive at m.

So far, we have covered the fundamental concepts of steam and we have learned how to calculate a steam mass flow rate for a required brewery operation. Now that we understand these concepts, we can discuss the different types of steam and identify the ideal steam for brewery use.