4 replies to this topic

[ayan_dg]

What are heating & cooling curves with respect to Heat exchangers . what they actually signify ?

[djack77494]

ayan,
When a process engineer provides data for the design of a heat exchanger, there are many cases where the amount of heat exchanged is not linear along the length of the exchanger. The most obvious case is when there is a phase change. The variation is the amount of heat exchanged versus the resulting temperature change is very important to the exchanger designer. Thus the process engineer must provide this data. He does so in the form of a heating/cooling curve.

[Arvind Iyer]

Dear Sir,

I would think that we could understand whether phase change is taking place or sub-cooling is taking place etc. And this knowledge could help in deciding in whether we need TWO exchangers: one condenser and another for sub-cooling because of the different design considerations required for these two.

What kind of decisions can be taken with the heating/cooling curves?
What are the other information revealed by these curves?
Thanks for your help.

Best Regards,
Arvind Iyer

[djack77494]

Arvind,
It's more complex than you are implying. This is not simply a matter of taking care of any desuperheating and/or subcooling. For a pure substance, the latent heat at a pressure or temperature is specified as a single number. For mixtures, the latent heat caries as the liquid is vaporized. The best "picture" of what is happening for purposes of designing a heat exchanger is provided in the heating and cooling curves.

[knapee]

This following is refer from PRO\II help.

For the heat exchanger, you can generate heating/cooling curves for both hot side and cold side streams. For both of these streams you can perform isothermal and fixed-enthalpy calculations. The temperature, pressure and enthalpy ranges are between the inlet and outlet stream conditions, which are retrieved by PRO/II automatically.


Isothermal Calculation
Select the first or third option to perform isothermal calculation for the hot and cold side streams, respectively. For both sides, the temperature and pressure ranges lie between the inlet and outlet stream conditions. In the case of multiple feed streams, the inlet temperature is taken to be the average inlet temperature and the pressure is taken to be the lowest feed stream pressure.

Fixed-Enthalpy Calculation
Select the second or fourth option to perform fixed-enthalpy calculations for the hot and cold side streams, respectively. For both streams, the enthalpy range is between the total inlet and outlet enthalpies. The pressure range is between the lowest feed stream pressure and outlet stream pressure.

For both calculation types, the first evaluation point is at the inlet conditions. The total number of increments over the pressure, temperature, and enthalpy ranges is equal to the value specified in the Number of evaluation Points field minus one.

Report Options
You can request the results printout to contain additional data such as Liquid Activity Coefficients, K-Values, etc., by selecting the appropriate options. These selections will apply only to the curve for which you are entering the evaluation data. The first of these options (liquid activity coefficients, etc.) will produce meaningful results only if a liquid-activity K-value method is chosen in the thermodynamic data section of input.

Number of Evaluation Points
The temperature, pressure and enthalpy ranges are divided equally into the number of evaluation points where the flash calculations are performed. The default value is eleven. Note that the first evaluation point is at the unit inlet conditions