The term Delta T (ΔT) is the difference of temperature between two measuring points. Which differ either in time and/or position. We use it for example, to measure the efficiency of a heat exchanger.
Δ (Delta) is the fourth letter of the Greek alphabet and is used as a mathematical symbol. Δ describes the „difference“ of any changeable quantity. So ΔT is a value to show the difference between two measured temperatures.
Differential temperature is expressed in Kelvin. No matter if the value is measured in °Celsius or in Fahrenheit. Therefore the values can be compared easily. The units are expressed in Kelvins(K) without using any degree symbol. As the lowest temperature is 0 K, no negative numbers exist in the Kelvin scale.
The equation is: ΔT = T2 – T1
To the left there is the drawing of a tubular heat exchanger. There is the cooling water entering at point B and leaving warmer at point A. And there is the liquid stream to be cooled, entering point C and getting out at point D. To get a usable ΔT, one should compare the same liquid going in and out. The entrance temperature in the heat exchanger at B would be T1. And the outlet from the heat exchanger coming out on top at A is T2.
The cooling water entering the heat exchanger will get warmer on its way through the exchanger. The difference is then the ΔT. If T1 and T2 is monitored regularly, one can see the perfomance of the cooling at the delta T. Monitoring this temperature over a longer time, will give indication about the grade of fouling in the heat exchanger.
In pipes differential temperature usually describes the difference between the necessary pipe temperature (Tp) and the expected lowest ambient temperature (Ta). So
ΔT = Tp – Ta
For example, to keep a pipes temperature (Tp) at 30° C in an environment where the ambient temperature (Ta) can drop to minimum temperature of -10° C, it is important to keep the differential temperature (ΔT) at 40K.
Having all 4 temperatures and 1 flow one is able to calculate the Heat Transfer Coefficient.