I am analysing a heat exchanger system which uses steam travelling through a finned tube to warm air.

I am currently looking at reducing time lags in the system so as to raise the temperature of the air in the unit quicker.

My question is would increasing the velocity of the steam travelling through the tube raise the air temperature on the outside of the tube quicker?

heat exchanger

  • $\begingroup$ Is it saturated steam condensing in the tube, or is it superheated, and with no condensation. $\endgroup$ – Chet Miller Jan 11 '16 at 23:01
  • $\begingroup$ It is not specified so I would assume either could be used $\endgroup$ – user103569 Jan 11 '16 at 23:06
  • $\begingroup$ Will the steam temperature in the tube change significantly as it goes through the loop? Would the velocity change your answer? $\endgroup$ – Daniel Griscom Jan 11 '16 at 23:15
  • $\begingroup$ Well, if it's saturated and the steam flow rate is high enough for all the steam not to condense in the tube, then it shouldn't matter much. If it's saturated and the heat load is such that all the steam would condense before exiting the tube, then increasing the flow rate will increase the heat transfer rate. If it's superheated, and none of the steam condenses in the tube, then increasing the flow rate will increase the heat transfer rate. If it's superheated and some of the steam condenses within the tube, then the situation needs to be analyzed in more detail. $\endgroup$ – Chet Miller Jan 11 '16 at 23:19
  • $\begingroup$ The temperature of the steam would reduce as it transferred its heat through the pipe to the air, so I am thinking that by having more steam travelling traveling through the tube in a shorter period of time more heat could be transferred through the tubing, or is that an incorrect assumption? $\endgroup$ – user103569 Jan 11 '16 at 23:20

If a step in the temperature of the entering air occurs, there will be a lag time before this temperature change appears at the air outlet, can be sensed by the control system, can result in a change in valve position and thus a change in thermal input to the exchanger, and can then change output air temperature. This control response time delay must be considered carefully.

Next, the amount of valve response commanded by the controller in response to a temperature change at the air outlet will also affect the time it takes to achieve enough of a correction to meet needs. Thus, the P,I, and D constants in the controller will also have an effect on the dynamic response time.

Finally, you can not simply change the speed of the steam in the exchanger as the increased heat rate would increase outlet air temperature and the controller would then sense the higher temperature and close down the steam valve lowering steam velocity back toward the original level. To change the steam velocity, you would have to redesign the exchanger for lower internal cross sectional area. If you did that you may have to change PID constants in the control loop to achieve a critically damped control response.

You can not consider this as a simple heat transfer problem as there is an active feedback controller in the system.


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