6 replies to this topic

[Moree]

Hi all.

A modification in our process demands to design a heat exchanger for heating nitrogen gas to 90 °C from 30 °C. For nitrogen gas I am using specific heat at constant pressrue (Cp) for calculations. I am not sure about the specific heat at constant volume (Cv). Should I use Cp or Cv? What is the difference between these two? I mean how to apply this in real application.

My question is simple. When to use Cp and when to use Cv?

Edited by Moree, 19 February 2010 - 05:49 AM.

[ankur2061]

Hi all.

A modification in our process demands to design a heat exchanger for heating nitrogen gas to 90 °C from 30 °C. For nitrogen gas I am using specific heat at constant pressrue (Cp) for calculations. I am not sure about the specific heat at constant volume (Cv). Should I use Cp or Cv? What is the difference between these two? I mean how to apply this in real application.

My question is simple. When to use Cp and when to use Cv?

Moree,

For heat balance or enthalpy calculations, always Cp is used & not Cv. Cp values for most elements & compounds are easily available. Mixture Cp's can be easily calculated using the law of proportions.

For basic understanding of specific heat capacity go to the following link:

http://en.wikipedia....c_heat_capacity

Hope this helps.

Regards,
Ankur.

[kkala]

A modification in our process demands to design a heat exchanger for heating nitrogen gas to 90 °C from 30 °C. For nitrogen gas I am using specific heat at constant pressrue (Cp) for calculations. I am not sure about the specific heat at constant volume (Cv). Should I use Cp or Cv? What is the difference between these two? I mean how to apply this in real application.
My question is simple. When to use Cp and when to use Cv?

I quite agree with Ankur, I have not recalled having used Cv ever. Cv could be used when heating e.g. a gas quantity stored in an isolated drum, though I would rather use enthalpies instead.
Probably question was created because there is some frictional pressure loss in the exchanger, so ingoing nitrogen does not have same pressure as outgoing nitrogen. At any case difference in pressures is small compared to the operating pressure of nitrogen, you can also consider Cp at average (ingoing - outgoing) pressure. As far as as nitrogen is flowing, Cp should be used. In a more general case, you can use enthalpies in / out to calculate the heat exchanged.
You have to look into the case of nitrogen isolated in the exchanger, while heating medium circulates. This reminds the "gas stored in an isolated drum", yet using Cv may not be the case: Temperature of isolated nitrogen shall be taken same as max temperature of the heating medium (better to consider design temperature of heating medium for safety), and let calculated Pn be the resulting pressure of isolated nitrogen. If design pressure of nitrogen is higher than Pn, no PRV is needed (high temperature to be considered for design conditions). Otherwise API 520/521 recommends a formula for sizing the PRV (gas expansion) that is remembered not to contain Cv (I am not at work to be more specific looking at API).
Probably Cv is easier to measure in laboratories, then Cp is calculated using known formulas.

Edited by kkala, 20 February 2010 - 04:28 PM.

[VIKRAM007]

A modification in our process demands to design a heat exchanger for heating nitrogen gas to 90 °C from 30 °C. For nitrogen gas I am using specific heat at constant pressrue (Cp) for calculations. I am not sure about the specific heat at constant volume (Cv). Should I use Cp or Cv? What is the difference between these two? I mean how to apply this in real application.
My question is simple. When to use Cp and when to use Cv?

I quite agree with Ankur, I have not recalled having used Cv ever. Cv could be used when heating e.g. a gas quantity stored in an isolated drum, though I would rather use enthalpies instead.
Probably question was created because there is some frictional pressure loss in the exchanger, so ingoing nitrogen does not have same pressure as outgoing nitrogen. At any case difference in pressures is small compared to the operating pressure of nitrogen, you can also consider Cp at average (ingoing - outgoing) pressure. As far as as nitrogen is flowing, Cp should be used. In a more general case, you can use enthalpies in / out to calculate the heat exchanged.
You have to look into the case of nitrogen isolated in the exchanger, while heating medium circulates. This reminds the "gas stored in an isolated drum", yet using Cv may not be the case: Temperature of isolated nitrogen shall be taken same as max temperature of the heating medium (better to consider design temperature of heating medium for safety), and let calculated Pn be the resulting pressure of isolated nitrogen. If design pressure of nitrogen is higher than Pn, no PRV is needed (high temperature to be considered for design conditions). Otherwise API 520/521 recommends a formula for sizing the PRV (gas expansion) that is remembered not to contain Cv (I am not at work to be more specific looking at API).
Probably Cv is easier to measure in laboratories, then Cp is calculated using known formulas.

[VIKRAM007]

Hi

Genrally Cp is specific heat at constant pressure.Cv specific heat at constant volume, Cp-Cv = R. but Suggest you to read basic thermodynamics so this doubt will not come through out your life.also use NIST DATA BASE for CP values at diffrent temperature by shomate equation not in this case in genral.

Regards
Vikram A.Chougule

"MAKE BASIC STRONG" "read Books and apply"

[S.AHMAD]

Hi all.

My question is simple. When to use Cp and when to use Cv?

As the name implies, Cp is referred to sp heat at constant pressure. Example - nitrogen gas flowing through an exchanger. Even though there is some change in pressure, but the change is marginal and assumed negligible.
Cv stands for sp heat at constant volume. Example - if we heat up nitrogen gas in a closed vessel. In this case pressure will change but the volume remains constant.

So, if your exchanger is a closed system, then use Cv otherwise use Cp.

Edited by S.AHMAD, 21 February 2010 - 08:08 PM.

[Moree]

Hi all.

My question is simple. When to use Cp and when to use Cv?

As the name implies, Cp is referred to sp heat at constant pressure. Example - nitrogen gas flowing through an exchanger. Even though there is some change in pressure, but the change is marginal and assumed negligible.
Cv stands for sp heat at constant volume. Example - if we heat up nitrogen gas in a closed vessel. In this case pressure will change but the volume remains constant.

So, if your exchanger is a closed system, then use Cv otherwise use Cp.

I am clear now with the fundamental. Probably I should read more. Thanks to you all for guiding me properly.

Have a sweet day!