What is meant by a desuperheating a vapor?

Answer 1

Basic steam desuperheatingDesuperheating is the process by which superheated steam is restored to its saturated state, or the superheat temperature is reduced.

Most desuperheaters used to restore the saturated state produce discharge temperatures approaching saturation (typically to within 3°C of the saturation temperature as a minimum).

Designs for discharge temperatures in excess of 3°C above saturation are also possible and often used.

There are basically two broad types of desuperheater:

• Indirect contact type - The medium used to cool the superheated steam does not come into direct contact with it. A cooler liquid or gas may be employed as the cooling medium, for example, the surrounding air. Examples of this type of desuperheater are shell and tube heat exchangers. Here the superheated steam is supplied to one side of the heat exchanger and a cooler medium is supplied to the other side. As the superheated steam passes through the heat exchanger, heat is lost from the steam, and gained by the cooling medium.

  The temperature of the desuperheated steam could be controlled by either the inlet superheated steam pressure or the flowrate of the cooling water. Control of the superheated steam flow for this purpose is not normally practical and most systems adjust the flow of the cooling medium.

• Direct contact type - The medium used to cool the superheated steam comes into direct contact with it. In most cases, the cooling medium is the same fluid as the vapour to be desuperheated, but in the liquid state. For example, in the case of steam desuperheaters, water is used. A typical direct contact desuperheating station is shown in Figure 15.1.3.

  When the desuperheater is operational, a measured amount of water is added to the superheated steam via a mixing arrangement within the desuperheater. As it enters the desuperheater, the cooling water evaporates by absorbing heat from the superheated steam. Consequently, the temperature of the steam is reduced.

  Control of the amount of water to be added is usually achieved by measuring the temperature of the steam downstream of the desuperheater. The set temperature of the desuperheated steam would typically be 3°C above that at saturation. Therefore, in such arrangements the inlet pressure of the superheated steam should be kept constant.

Desuperheating calculationsThe amount of water added must be sufficient to cool the steam to the desired temperature; too little water and the steam will not have been cooled enough, too much and wet saturated steam will be produced which will require drying through a separator.

Using Equation 15.1.1, which is based on the conservation of energy, the cooling liquid requirement can be easily and quickly determined:Equation 15.1.1

Where:cw=Mass flowrate of cooling water (kg / h)s=Mass flowrate of superheated steam (kg / h)hs=Enthalpy at superheat condition (kJ / kg)hd=Enthalpy at desuperheated condition (kJ / kg)hcw=Enthalpy of cooling water at inlet connection (kJ / kg)

Example 15.1.1

Determine the required cooling water flowrate for the conditions in the following Table:

Solution:

The necessary information can be obtained or interpolated from hard copy steam tables; the relevant extracts are shown in Table 15.1.1 and Table 15.1.2. Alternatively, the Spirax Sarco online steam tables can be used.Table 15.1.1 Extract from steam tables - Saturated water and steam

Table 15.1.2 Extract from steam tables - Superheated steam

The information required to satisfy Equation 15.1.1 is therefore:s=Mass flowrate of superheated steam = 10 000 kg / hhs=Enthalpy at superheat condition (From steam tables 300°C at 10 bar a) = 3 052 kJ / kghcw=Enthalpy of the cooling liquid = 4.2 kJ / kg°C x 150°C =630 kJ / kg

Determining the enthalpy at the desuperheated condition, hd:

From steam tables, the saturation temperature (T s) at 10 bar a is 180°C, therefore at the required desuperheated condition, the temperature will be:

Ts + 5°C = 185°C

Interpolating between the enthalpy of steam at 10 bar a and its saturation temperature, and at 10 bar a and 200°C:

Enthalpy at 10 bar a, T s (saturated steam tables) = 2 778 kJ / kg

Enthalpy at 10 bar a, 200°C (superheated steam tables) = 2 829 kJ/kg

Interpolating for enthalpy at 10 bar a and 185°C:

Finally, applying Equation 15.1.1:Equation 15.1.1

Note that the desuperheated steam is supplied at a rate of:

10 000 + 1 208 kg / h = 11 208 kg / h

Had the requirement been for 10 000 kg / h of the desuperheated steam, the initial superheated steam flowrate can be determined using a simple proportional method: