I had mentioned in one of my earlier blog entries that the new GPSA Engineering Databook 13th Edition is now available and there have been many changes in it from the previous edition. Refer the link:

http://www.cheresour...ition-si-units/

Compressors have been a focal point of interest for me in the last couple of years and I try to keep myself updated on the subject. While going through Section 13 (Compressors & Expanders) of the new GPSA Databook, I found that they have now introduced new equations for compressor discharge temperature for both centrifugal and reciprocating compressors.

The old texts and literature for compressors used to express the compressor discharge temperature by means of the following equations:

T

_{2}= T

_{1}*(P

_{2}/ P

_{1})

^{(k-1) / k}-------------(1)

T

_{2}= T

_{1}*(P

_{2}/ P

_{1})

^{(n-1) / n}-------------(2)

where:

T

_{2}= discharge temperature, K or R

T

_{1}= suction temperature, K or R

P

_{2}= discharge pressure, kPaa or psia

P

_{1}= suction pressure, kPaa or psia

k = isentropic exponent (also known as the ratio of specific heats, C

_{p}/ C

_{v})

n = polytropic exponent and expressed in equation form as:

n / (n-1) = [k / (k-1)]*η

_{poly}

The GPSA Engineering Databook proposes a new set of equations for the compressor discharge temperature saying that these provide better results compared to equations (1) and (2). These equations can be written as follows:

ΔT

_{isentropic}= T

_{1}* [(P

_{2}/ P

_{1})

^{(k-1)/k}- 1] / η

_{isen}----------------(3)

ΔT

_{polytropic}= T

_{1}* [(P

_{2}/ P

_{1})

^{(n-1)/n}- 1] / η

_{poly}----------------(4)

where:

η

_{isen}= isentropic efficiency expressed as a decimal

η

_{poly}= polytropic efficiency expressed as a decimal

T

_{2}= T

_{1}+ ΔT

_{isentropic}----------(5)

T

_{2}= T

_{1}+ ΔT

_{polytropic}----------(6)

The polytropic efficiency for a centrifugal compressor can be found as a function of the inlet volume flow as per one of my earlier blog entries which correlates the polytropic efficiency as a function of the inlet volume flow at the following link:

http://www.cheresour...et-volume-flow/

Hope the readers of my blog and specially those who have interest in compressors like this blog entry. I hope to have some comments and observations on this blog entry from the readers of my blog.

Regards,

Ankur.

Just one remark and question that I would like to add;

The Isentropic exponent shall be calculated in the average suction and discharge temperature e.g. (Cp/Cv)@((Ts+Td)/2).

The question that I have in this regard,

In process simulation software's' like HYSYS, the exponent is calculated in the mean temperature or just suction temperature is considered?

If the mean temperature is used in HYSYS, then there should be a sort of iteration embedded in the compressor icon of HYSYS.

Regards,

Amir