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TRV Proper Application
TVR's and their proper application and control philosophy.
The TVR is used in many different ways for many different reasons. It can be used for flash cooling, vapor recovery for energy efficiency for pre-heaters and to drive an evaporators effect. When steam is available at pressures in excess of 45 psig and preferably over 100 psig, thermo vapor recompression can be considered.
In this graphic, a portion of the vapor evaporated from the product is recompressed as a it is pulled into the suction port of the steam jet venture due to the high velocity of the high pressure steam exiting the jets nozzles. The two stream mix and are returned to the steam chest of an evaporator or shell and tube pre-heater. A system of this type can provide a 2 to 1 economy or higher depending on the product, the steam pressure, and the number of effects over which TVR may be applied.
Thermal compressors are somewhat inflexible and do not operate well outside the design conditions. Therefore, if the product is known to foul severely, so that the heat transfer coefficient is significantly reduced, it is best not to use TVR. Read on to the next tip to see the proper application of the TVR.
TVR's and their proper application and control philosophy.
Applying a TVR for energy efficiency is generally applied to multi effect evaporation process’s. As depicted here they can induce product vapor from an effect’s separator vessel and recompress half the volume and sending the second half to second effect. This has two benefits. The first of course is the cost savings on steam usage. Without the TVR this process would have a 1 to 1 efficiency and need a condenser sized to manage that heat load. Caution must be taken however when controlling the TVR.
TVR's are generally designed to operate and be supplied with at a specific motive steam pressure of say 100 psi in order to achieve the desired pick-up ratio. So, most control designers have the motive steam pressure as the input to the control loop as shown in the upper pictorial. This however ignores the temperature sensitivity and fouling tendencies of most products. Therefore, the best practice application in these cases is as shown in the lower pictorial where absolute pressure is sensed at the shell of a shell and tube evaporator or the steam chest of a plate and frame evaporator and is used as the control loops input device. Now you are controlling the temperature that the product will be exposed too and can set the limit. With the other motive steam pressure control philosophy as fouling naturally occurs over the course of a production campaign, the temperature will continue to rise with the pressure and induce greater fouling quicker and subsequent burn on will is inevitable.