Propane Refrigeration Cycle Calculations

[ssg12]

Hello All,

I am working on developing a process to estimate the key refrigerant system performance data (refrigerant compressor shaft work, refrigerant condenser duty) for single stage propane refrigeration system. We are currently applying the Gas Processor Suppliers Association (GPSA) Engineering Databook methodology using standard curves to estimate the above data.

I have been given the task to validate the results for one unit with the hand calculations. I am attaching a PFD sketch for this unit. So here is the data that i know -

The chiller duty (E-3405) or heat load is 0.231MW. The Fluid is getting cooled from 50F to 113F.
The Cooling Water Supply/Return Temp is 89.6/107.6F.

Using an approach temp of 10F, the evaporator temp and condensing temp can be set at 40F and 99.6F respectively. Based on this data, the condenser duty is calculated as 3.26 MMBTU/hr and compressor HP needed is 216HP. This is based on required refrigeration duty of 0.7897MW (2.69 MMBTU/hr). Remind that these values are based on GPSA charts.

I understand the basic refrigeration concept (based on reverse Carnot cycle) involving - Compression, Condensation, Expansion and Evaporation. I have the liquid and vapor Saturation Properties of Propane from NIST. I can get the saturated vapor and liquid enthalpy values on evaporator and condensing temp. I can calculate the refrigeration effect (RE) -

RE = Enthalpy of refrigerant in vapor phase at evap temp - Enthalpy of refrigerant in liquid phase at cond temp

But I am stuck or confused at these points -

1) How do i get the Refrigeration duty (0.7897MW) ?
2) How do i get the Refrigerant mass flow rate ?

I think the relation is, Refrigeration duty = RE * Mass flow rate. Am I Right ?

Once i know these values, i can work out the relations for calculating compressor HP and condenser duty.

I hope i have provided a brief explanation of my work and where i need guidance. I appreciate your help in advance.

Regards,

 

[chicopee]

If you had thermodynamics and studied the refrigeration cycle, I would suggest then that you get your hands on either Kent"Power" handbook, Marks ME handbook or Perry Chem. Eng. handbook and study the appropriate sections. There is data in the refrigeration society that summarizes all the refrigerant types along with their corresponding Low and high pressure values, btu/hp etc... which I may have in my archives and if I find it, I'll post it.

Propane is falling in disfavor in the USA because of its flammability characteristics, therefore, I have not seen any in my area (MA)of the NE probably because of the strict restrictions.
Since it appears that you have the entire diagram, I would suggest that you assign at the intake and outlet of the condensor, evaporator, thermal expansion valve, compressor, storage tank pressure/temperature, enthalpy, entropy, mass flow rates of the vapor/liquid refrigerant all of which will assist you in determining your data.

 

[ssg12]

Thanks Chicopee for the response.

I do have thermodynamic properties of propane (Enthalpy, Sp Heat, Entropy etc) but i am looking for refrigeration formulas or calculation procedure for mass flow rate / refrigeration capacity and consequently compressor HP and condenser duty. May be in a excel spreadsheet or similar form ?

 

[MortenA]

If you are using GPSA engineering handbook why dont you just use the steps they describe?

The duty is the amount of energy you need to remove to get to your desired temperature in your process medium - so unless you are planning to reuse an existing parameter its a design input. The first couple of pages deals with this.

The refigerant mass flow is the amount of refrigerant you need to obtain your desired duty - so thats a result. You need to determine some of the design optin (number of stages etc) and then you can follow their example 14.1 (adapt to your number of stages).

Best regards, Morten

 

[ssg12]

Yes, I am doing exactly the same. Following the steps from GPSA and thermodynamics textbook.

But i am not able to match my figures of compressor power. I am getting 36 kW versus 160 kW. Please see attached. Surely i am doing some error !!

Any help is much appreciated.

Regards,

 

[chicopee]

Propane is classified as a R290 refrigerant for which I have no data. I dug up from my archive some HVAC/R relationships developed by Dupont and these are in the attachment in JPG format. There is one book by Althouse,Turnquist,Bracciano titled modern Refrigeration and Air Conditioning that is used by technical schools for technical people (not engineers)whose livelihood is in this field. This book has practical guidelines on calculating everything including estimating piping loss, refrigerant line sizing, TXV selection, compressor/motor sizing etc..that you want to know in order to assist you in your project.

 

[MortenA]

You know you duty, and then you calculate your cooling water flow (i assume this is the process medium side - and your objective is to cool the water)?

This look OK

But i have the SI verison of GPSA o i cany easily check your table values

You sheet says 216HP=161 kW so maybe its your unit conversion that is wrong? But i dont know if this is the corresponding value?

 

[georgeverghese]

Just went through GPSA and ran the calcs for this operation from (a) first principles and (b) used the fig 14-16 for propane single stage compression, and the two values with these 2 approaches for compression power match up well.

GPSA says "refrigeration duty" is simply the process chiller duty, while refrig. effect is simply the latent heat of vaporisation of propane at the evaporator press.

Refrig effect formula in your first post is not the latent heat of vaporisation

So refrigeration duty = 231kW

But your calcs for compression power are not far off from what I've calculated

Your compression power = 36kW
My calcs from first principles = 46kW ( I've used 70% isen eff for the compressor )
From GPSA figure for single stage compression with isen eff at 75% eff = 47kW

To use GPSA figure, we can convert that 1.0mmbtu/hr of refrig. duty = 293kW

So I dont know how you've got 160kW. I cant see your excel sheet - maybe because I'm running windows XP - sorry

Propane circulation rate = 0.82kg/sec

Compressor discharge press = 12.7bar a, compressor suction press= evap press = 5.5bara

Condensor press = 12.3bar, temp = 100degF

Hope this helps

 

[georgeverghese]

Also, refrigeration duty is not = compression throughput x refrigeration effect.

This is because part of the liquid flashed from 12.3bara to evap pressure turns into sat vap directly even before it gets into the evaporator.

In this particular case, of the 0.82kg/sec liquid flashed across the LCV feeding this evaporator, only 0.62kg/sec remains as liquid to chill the process stream; the rest goes straight into compressor suction. This is understood when you look at the Mollier P-h curves for propane for this isenthalpic flash across this valve.

Thermodynamically, condensor cooling load = compression duty + refrigeration duty = 231 + 46 = 277kW;
this is what the lower graph on fig 14-16 also says ( 231 x 1.2 = 277kW).