4.- What is minimum change in T? What values are typically used in design?

5.- What is the heat content diagram method? How different is it from the crease point method?

**EXPERT ANSWER**

ANSWER :

(4)

Delta T min is the minimum permissible temperature difference between hot and cold streams entering and exiting an individual heat exchanger (minimum approach temperature). It effectively sets an upper limit on capital cost for an exchanger because small approach temperatures require large heat transfer areas.

The term Delta T (ΔT) is in science, the difference of temperatures between two measuring points. The temperature differs either in time and/or position. We at Merus use it for example, to measure the efficiency of a heat exchanger. Or for checking the performance of a heating system or a cooling system.

Temperature Difference – Delta T (ΔT)

The video at the bottom of this page shows the practical use of delta T. In the case of a cooling tower, we compare the temperature of the warm return line against the colder feed line.

The term Delta T (ΔT) is in science, the difference of temperatures between two measuring points. The temperature differs either in time and/or position. We at Merus use it for example, to measure the efficiency of a heat exchanger. Or for checking the performance of a heating system or a cooling system.

Δ, (Delta) is the fourth letter of the Greek alphabet. Whereas Δ is the symbol of the capital letter and δ is the lower case letter. The Δ is also used as a mathematical symbol. Δ describes the “difference” of any changeable quantity. In a process ΔT is a value to show the difference between two measured temperatures.

If you use ΔT in any kind of formula you have to use the corresponding units. (°Celsius or °Fahrenheit)

To convert use (without +-32!):

ΔT°F = ΔT°C * 9/5

ΔT°C = ΔT°F * 5/9

THE EQUATION OF DELTA T IS: ΔT = T2 – T1

To the left is a drawing of a tubular heat exchanger. The cooling water is entering at point B and leaving warmer at point D. The liquid stream to be cooled, entering point C and getting out at point A. The entrance temperature in the heat exchanger at B would be T1. And the outlet from the heat exchanger coming out at D 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. Having all 4 temperatures and 1 flow rate of a heat exchanger, one is able to calculate the Heat Transfer Coefficient.

In a heating or a cooling system for a building the delta T is used to compare the cool water send into the house and the warmer water, which comes back. So one can see if and how much the Air Condition is used.

(5)

Removing moisture in the wet farm products through the open air ventilation is a traditional postharvest treatment method. However, its overall energy consumption is high and the drying time is long. Here, a combined system is proposed through integrating a bottom organic Rankine cycle (ORC) to a top closed farm products air drying cycle to save energy and decreasing the drying time. Based on a steady state thermodynamic model without any losses, a theoretical work on thermal performance the proposed system has been conducted. The key operation parameters and suitable operation conditions for both the prominent energy saving ratio and high moisture extraction characteristics have been analyzed. The calculation results showed both the prominent energy saving and high moisture extraction performances can be achieved at the low ORC evaporating pressure condition, but the optimal energy saving performance and the moisture extraction performance could not be achieved simultaneously. Increasing dew point temperature of the humid air leaving the drying chamber improves both the energy saving and moisture extraction performances of the drying system significantly. Under the working conditions in this research, with the dew point temperature value of the humid air leaving the drying chamber over 323 K, the optimal energy saving performance is achieved at a fixed low ORC evaporation pressure. But, the prominent energy saving and high moisture extraction performances could not be accomplished simultaneously, with the dew point temperature value of the humid air leaving the drying chamber lower than 323 K.