Ideal/real vapour-compression refrigeration cycle

[crosshair]

I'm trying to get the ideal and the real enthalpy's from the ph diagram. I've got the condensing and evaporation pressure and the various temperatures. I have the condenser entry and exit temps, the evaporator entry and exit temps and the compressor entry temp. Can anyone help in getting the real and ideal enthalpy's for the cycle from the diagram. I'm mainly looking for the real enthalpy's. thanks

 

[UmeshMathur]

According to the API Technical Data Book, the best method of calculating enthalpies is the BWR equation of state. This is the basis for the published P-H charts for virtually all hydrocarbon refrigerants.

For fluorocarbons, there is software for thermodynamic properties on the NIST website that uses the best equations for those substances.

 

[Montemayor]

Crosshairs:

This must be the cuadzillioneth time I’ve said this in these Forums; but what the heck, here it goes again ………

The National Institute of Standards and Technology (NIST), an official branch of the US government, operates a Website (with your tax dollars) at:

http://webbook.nist.gov/chemistry/fluid/

This website gives FREE thermodynamic data on thermophysical Properties of Fluid Systems. Accurate thermophysical properties are available for 34 fluids. These data include the following:

Density
CP
Enthalpy
Internal energy
Viscosity
Joule-Thomson coefficient
Specific volume
CV
Entropy
Speed of Sound
Thermal conductivity
Surface tension (saturation curve only)

I always use this source to calculate all my streams and energy values in working out refrigeration cycles. The list includes the most popular refrigerants in the market. This data is not only accurate, it is totally recognized as authoritative.

API Technical Databook? As Alfonso Bedoya in The Treasure of the Sierra Madre (1948) might have put it: “I don need no stinkin’ API Technical Databook!”

Forgive me UmeshMathur, I felt a need to interject some levity and nostalgia.

 

[UmeshMathur]

Hi, Montemayor:

It's just that I work with a lot of fluids that are not in the NIST database. Also, I tend to imbed the thermodynamic property routines I use into my own process simulator. I have not bothered to do this with the NIST software, which is available for a nominal fee, as it uses a huge number of constants for really complex equations of state.

However, for spot checks or a few hand calculations, the NIST website does suffice. However, I'd hate to do a major engineering study, e.g., designing or optimizing alternative multi-stage commercial refrigeration machines using the web pages to get the property values one by one. In such a situation, the BWR is pretty good, even for fluorocarbons.

Permit me to provide the following quote:

"A computer does not substitute for judgement any more than a pencil substitutes for literacy. But writing without a pencil is no particular advantage.

-Robert S. McNamara"

To this, I would add: when doing highly repetitive computations, avoiding a computerized algorithm implies either that this is a hobby or your time must be getting billed by the hour! (You'll forgive me, as I went through 5 years of undergraduate chemical engineering, 40 class hours a week, with just tables and a slide rule. That's enough hand computation for a lifetime).

By the way, I always enjoy your posts as they are invariably well conceived and researched.

 

[Montemayor]

UmeshMathur:

Well put. Be assured that I share your work outlook and also your 5-year experience as a ChE student with a talcum powder supply for my Post slip stick (I still preserve it and show it to my grandkids. They think it's a back scratcher.)

I consider your reading my posts as a compliment; I follow yours as well and your comments on thread135-149386 – together with Katmar’s – are classic horse sense engineering models. I wanted to enter into that thread but found little I could contribute after you guys. It makes me content about the State-of-Engineering today when all I can offer is humor or levity. Keep up the great posts.