I have two fluids entering a shell and tube heat exchanger.One enters at 393K and leaves at 323KThe other fluid enters at 353K and leaves at 308K.Is this even possible?
It is not clear if these are measured numbers or calculated numbers. In the case of measured numbers I have seen such things happen due to errors in temperature measurement. In the case of calculated numbers I can only see a case where a fluid can enter an exchanger at a given temperature and leave at the same temperature while the other fluids is cooled down. That is the case of liquid such as a refrigerant liquid that enters the exchanger at its boiling temperature, boils and then leaves the exchanger at the boiling temperature.
Yes. If the fluids were compressed and you expand the tubes, it will force them to evaporate and they will cool down. If you don't compress them on their way out, they stay cooler.
Yes if you lose heat to the air surrounding the heat exchanger, is it well insulated?
Although the other answers on here are technically true, I don't think it is what you are looking for. If the heat enchanger is not well insulated, then the major heat transfer is from the working fluids to the air (and the air temperature would have to increase). Also, you typically don't run gases through a heat exchanger at speeds that make compressibility effects important (~M >0.3) because it doesn't give you much time for heat transfer to take place. If you are assuming that it is perfectly insulated and that the working fluid is water (two common assumptions for a homework problem for example), then one of the temperatures has to go down. Reasons? First, entropy of the universe has to increase. Decreasing the temperature of both fluids would violate this. Also, from a conceptual standpoint, if energy is leaving one fluid (temperature down) then it has to go into the other fluid (temperature up). Both fluids are trying to reach thermal equilibrium: the temperature will continue to change until they are at the same temperature (or if they leave the heat exchanger).
If it was a case of two fluids, where both give up heat, the answer is a definite no. The very name, heat exchanger, means one fluid give up heat and the other pick it up.
