THROTTLING CALORIMETER

Throttling calorimeter is a device used in determination of dryness fraction of steam. There is a sampling tube which is placed in steam main pipe. It consists of a hole facing upstream to get sample steam. The steam passes through the throttle valve and then flows into the inner cylinder. The main condition is that after throttling steam should be superheated. Normally, the degree of superheat should be 5°C. The pressure after throttling should be few mm of H_g above atmospheric pressure as recorded by manometer. The saturation temperature corresponding to this pressure can be found. If the temperature recorded by the thermometer is more than saturation temperature, it is confirmed that steam is superheated after throttling. Steam flows from top of the inner cylinder to the annular space between inner and outer cylinder. The calorimeter is insulated from surrounding. Before taking temperature reading, the flow of the steam should be in steady state, and all parts to be heated to keep temperature constant. The constructional details of calorimeter are shown in Figure 4.7.

Figure 4.7 Throttling Calorimeter

During throttling enthalpy remains constant, i.e., Enthalpy before throttling = Enthalpy after throttling.

Limitation of the process is that the steam should be superheated after throttling.

Example 4.10: A throttling calorimeter is used to measure the dryness fraction of the steam in steam main which has steam flowing at 10 bar. The steam after passing through the calorimeter is at 1 bar pressure and 120°C. Calculate the dryness fraction of the steam in the steam main. Take c_ps = 2.1 kJ/kg K.

Solution:

From steam table:

At pressure, P₁ = 10 bar, h_f1=762.81 kJ/kg, h_fg1 =2,015.3 kJ/kg.

After throttling, at pressure, P₂ = 1 bar, t_s = 99.63°C, h_g2 = 2,675.5 kJ/kg.

Enthalpy remains constant during throttling process, therefore, h₁ = h₂