7.3 Boost flash pressure using thermocompression and use it in a medium pressure (MP) application

In some cases, low pressure applications may not be available in the plant. However, even in such situations, it is still possible to recover flash steam to the process by boosting its pressure. This can be achieved by using a thermocompressor system.

The thermocompressor is a device that utilises high pressure motive steam to entrain and compress low pressure suction steam including flash steam, boosting its pressure and making it usable in applications that require a higher pressure. The thermocompressor comprises a motive nozzle, a suction chamber and a diffuser. The high pressure motive steam is expanded through the nozzle. The pressure head of the motive steam is converted into velocity head by the nozzle and the low pressure steam is entrained by this high velocity.

The velocity head of the mixture (discharge steam) is reconverted to pressure head by the diffuser (refer to the illustration).

To determine if it is feasible to boost flash steam pressure and use it in an application the following parameters need to be considered:

  • The intended process application for steam usage should have a consistent and uninterrupted steam.
  • The steam consumption of the process should be higher than the sum total of the suction steam (flash steam) and the motive steam required.
  • To be able to boost the pressure of flash steam to the desired level for use in the process application, it is necessary that motive steam is available at a pressure greater than twice the required discharge pressure (in absolute units).
  • The ratio of the discharge pressure to suction pressure should be less than 2.5 (in absolute units).
  • The ratio of the motive pressure to suction pressure should be greater than 2.5 (in absolute units).
  • The entrainment ratio, which is the ratio of the flow rate of motive steam to the flow rate of suction steam should be less than 4.

Illustration:

Take the example below in a plant with two steam consumers, A, which consumes 900kilograms/hour steam at 5barg and B which consumes 500kilograms/hour steam at 2barg.

Both processes put together will generate 103kilograms/hour of flash steam @ 0.5barg.
M (Steam consumption in B) = 500kg/hr b
M (Suction steam flow rate) = 103kg/hr s
P (Suction pressure) = 1.5 bar a (0.5barg) s
P (Discharge pressure) = 3 bar a (2barg) d
Entrainment ratio = 3.5

The required motive pressure Pm can be determined by the following equation:

equation

Pm ≥ Pd
x 2.66 = 3 x 2.66 = 7.98 bar a
Therefore the motive steam pressure should be greater than or equal to 7.98 bar a or 6.98barg
The motive steam quantity (Mm) can be determined by the following equation

Therefore the motive steam quantity (Mm) = 362kg/hour
M (Discharge flow rate) = M + M = 362 + 103 = 465kg/hr

To determine the feasibility of thermocompressing this flash steam and utilising it in equipment B, it needs to satisfy the following conditions.

M (500kg/hr) > 465kg/hr i.e. Ms (103kg/hr) + M (362kg/hr) b m

To be able to boost the pressure of flash steam to the desired level for use in the process application, it is necessary that motive steam (Pm) is available at a pressure greater than 2xP . 2xP =6 and motive steam is available at 11bar a d d

The ratio of the discharge pressure to suction pressure should be less than 2.5 (in absolute units) P /P <2.5 i.e. 3/1.5 = 2<2.5 d s

The ratio of the motive pressure to suction pressure should be greater than 2.5 (in absolute units) Pm/P >2.5 i.e. 7.9/1.5 =5.26>2.5 s

Since it satisfies all conditions it is feasible to thermocompress the flash steam from A and B and utilise the same in equipment B. This will reduce the steam requirement of equipment B by 103kg/hr.