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Heat exchangers are commonly used in car radiators and refrigerators to transfer heat from one fluid to another. In a refrigerator, heat exchange occurs between the food in the refrigerator compartments and the refrigerant fluid as well as between the refrigerant fluid and the radiator outside a refrigerator. In effect the heat is transferred from the food to the ambient.
In the figure below, suppose the inlet temperature of the fluid is T1. As the fluid moves on, it loses heat to the fluid entering the exchanger and exits at a temperature T2. The inlet temperature of the refrigerant (or cooling fluid) is T3 and its exit temperature is T4. Suppose further that the target temperature of T2 is T0 and the customer requirement is specified to be T0 ± 10 ° C. In other words, |T2 - T3| < 10 ° C. Is this a good quality characteristic in this example?
No! The problem with this quality characteristic is that both the positive and negative deviations from T0 will be indistinguishable and are regarded similarly. Hence, interactions become important and reproducibility is poor.
How do we do it?
Our strategy is as follows:
T0 is the target temperature desired by the customer.
T1 is inlet fluid temperature, T2 is outlet fluid temperature.
T3 is inlet refrigerant temperature: T4 is outlet refrigerant temperature.
A better method is to formulate T3 as a reference temperature since that is the lowest temperature that can be reached by T2 . Thus we define y = T2 - T3 with a nominal the best target value y = T0 - T3 . This would represent a design effort that aimed to make the heat exchanger efficient. This quality characteristic does not have the same problems as that of the previous quality characteristic with regards to interactions.
Additionally, if a different customer requires a change in T0 , the experimental information obtained from an initial experiment could still be useful and we would merely have to find a suitable way of adjusting to the new target value.
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