How a Temperature Control Valve Works ~ Learning Instrumentation And Control Engineering Learning Instrumentation And Control Engineering

How a Temperature Control Valve Works

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A temperature control valve is just like any other control valve. The only difference is that the control valve helps to maintain the temperature of a desired process at a specific level. It is often abbreviated as TCV – Temperature Control Valve – in most process drawings and P&IDs.

Temperature Control Schemes
There are two popular temperature control schemes using temperature control valves:
(1) Mixing of a cold process fluid with a hot process fluid to control the temperature of the hot process fluid.
(2) Exchange of heat between the hot process fluid and a cold process fluid as seen in heat exchangers. However, the process fluids do not mix.

Mixing of Cold and Hot Process Fluids
In temperature control valve applications in its most basic form, there are two basic process fluids:

(a) A cold process fluid at temperature T1
(b) A hot process fluid, being fed by the cold process fluid, to be kept at the desired temperature T2
As shown above, the temperature control valve is usually positioned or installed such that it can pass cold process fluid to mix up with the hot process fluid thereby controlling the temperature of the hot process fluid.. It must be noted that mixing in this type of temperature control is mostly physical and should not result in a chemical reaction for control to be smooth and predictable.

Temperature Control Philosophy
To control the temperature of the hot process fluid with that of the cold process fluid, a temperature sensor/transmitter is installed in the hot process fluid. This transmitter measures and transmits the temperature of the hot process fluid to a controller TC (see diagram above) which has a set point, T2.
When the temperature of the hot process fluid goes beyond T2, the controller via the signal of temperature transmitter TT senses it and sends a signal to the TCV to open more so that cold process fluid can mix up with the hot process fluid thereby reducing its temperature. As long as the temperature of the hot process fluid is above T2, the TCV will continue to open and introduce more cold fluid with the hot fluid until the set point temperature T2 is achieved.
For this temperature control scheme to be effective, the temperature of the hot process fluid must always be above the set point T2 so that TCV will always be open. There are other temperature control schemes using temperature control valves but this is one method that is very popular in most process plants.

Exchange of Heat Between Hot Process Fluid and a Cold Process Fluid
In the heat exchanger scheme of temperature control, mixing does not take place but what takes place is energy transfer between the cold and hot fluid. The hot fluid passes through the shell of the exchanger while the cold fluid passes through the tube. The hot process fluid transfers heat energy to the cold process fluid thereby increasing its temperature. However the elevated temperature of the cold fluid is controlled by the controller TC.

As shown above, the same temperature control philosophy applies here with the temperature transmitter TT sensing the temperature of the cold fluid and transmitting it to the temperature controller TC. If the temperature of the cold fluid is above set point, the controller initiates a control action by sending a signal to the temperature control valve, TCV, to either open or close to control the temperature of the cold fluid.