View Full Version : hi iam new to RE please clear my doubt

balaji

05-11-2007, 05:12 PM

How to calculate the tonnage of a compressor if the name plate doesnt show any details bcoz most of the unit in my site are very old and they doesnt have any name plate is it possible to calculate the tonnage by measuring the consuming amps of the compressor?pls anybody can clear my doubt based on the compressor how to design the condenser and evaporator and refrigerant pipings

balaji

05-11-2007, 05:14 PM

tonnage of the unit is based on compressor or size of the unit i mean size of condenser coil,evaporator coil etc

The MG Pony

09-11-2007, 04:02 PM

((3.14*R^2)*L*N)\1728)*RPM = CFM

R=Radius

L=length of stroke

N=Number of cylinders

RPM=Compressor speed

Once you know the CFM you can calculate it based on the refrigerant your using. other then that there is no way.

balaji

16-11-2007, 05:02 PM

thanks for your reply but i need more information about thisaccording to you if we calculate the CFM how to convert it in tonnage and i hope there must be some other way to calculate the tonnage that means how they are designing a compressor for the required tonnage pls clear my doubts

The MG Pony

16-11-2007, 10:15 PM

well you find the refrigerants specific volume at the temp you plan it to evapourate then figure out how many pounds the compressor will circulate then figure out the BTU per pound then devide that figure by 12,000.

ultralo1

17-11-2007, 04:25 PM

double tap

ultralo1

17-11-2007, 04:26 PM

MG Pony,

I have similar question, How do you calculate the compression ratio when the suction runs in a vacuum?

example: suction pressure= 15 inches vac

Discharge pressure= 120 psi

Thanks.

The MG Pony

19-11-2007, 03:22 PM

Good question, I'd wager you convert it to psi Absalute and do it as normal?

The whole focus of the course has been you simply do not go into a vacuum!

NH3LVR

19-11-2007, 03:43 PM

MG Pony,

I have similar question, How do you calculate the compression ratio when the suction runs in a vacuum?

example: suction pressure= 15 inches vac

Discharge pressure= 120 psi

Thanks.

Compression Ratio is allways calculated by absolute pressures.

For example;

5" Vacuum (Apx 2.5lbs.), 150lb head

Add 14.7 lbs to Suction=12.2 lbs.

150 divided by 12.2=Compression Ratio 12.3

Hope I got the math right and no time to convert to SI Units. (on the way to work). But you get the idea.

John Hunter

19-11-2007, 10:43 PM

Compression ratios should always be calculated in absolute units. The swept volume per rev = Cylinder area x piston stroke. Volume x number of cylinders x rev/second = gross swept volume per second . Multiply this by the specific volume mass per unit volume , i.e. kg/m3 of the refrigerant , obtained from tables at the suction condition. neglect superheat at this time . A previous calculation using tables or charts will deterring the refrigeration effect in tonnes or kW's obtained per unit mass of refrigerant . This value multiplied by the previously calculated compressor swept volume mass will give you the gross compressor capacity. Then determine the actual compression ratio. Discharge Pressure in (ABS) divided by the Suction Pressure in (ABS).

This will allow the determination of the Volumetric efficiency of the system. Some examples. CR. 3.0 = Vol. Eff 81%. CR. 4.0 = 77% CR. 5 = 72% . CR. 6.0 = 68%. CR. 9 = 55%. If you multiply the CR. by the previously determined compressor gross capacity will give you the actual refrigeration capacity of the compressor at the operating conditions. There is another method if you calculate back using input power by using a factor called Entropic Efficiency but I will not describe it here as it could be more confusing. It is certainly much easier to use makers data or software to undertake this type of work. The (calculated refrigeration load + the motor power) x 20% will gve a reasonable determination of the condenser capacity required. The actual condenser would be selected based on local ambient conditions and the type intended to use.

ultralo1

22-11-2007, 06:04 AM

I did not know that inches of vacuum could be converted to PSI.

Learn something new everyday.

Thank you.

The MG Pony

22-11-2007, 11:33 PM

I did not know that inches of vacuum could be converted to PSI.

Learn something new everyday.

Thank you.

PISa the a is very important as it is an absalute

0 inches vac = 29.92inches absalute = 1atm = 14.696 PSIa

so 15 to PSIa = (29.92-15) / 2.03 = 7.349 PSIa

jim01

16-07-2010, 10:00 AM

thanks to your concern

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