Suction and Discharge Temperatures for Ammonia Compressor Comparison

[US Iceman]

Hi,

Well, the first problem is the compressor is rated for a suction pressure higher than you are currently running. (26 psig is about a 12°F saturation temperature while the quote says the compressor is rated at about 45 psig, which is a saturation temperature of about 30°F).

If the system runs at 26 psig, then the compressor should be rated at that condition.

I will say the rating data you provided looks like a reasonably well stated set of rated operating conditions because they include pressure losses (suction & discharge). However, I question the subcooling. You do not typically get subcooling on a condenser in a properly operating condenser. By adding the subcooling this tends to artificially increase the rated compressor capacity. In all likelihood, you won't get that capacity if you were running at 45 psig suction.

Without getting into a math exercise, the CFM you provided looks more like the gas volume flow at the rated condition. From a quick check I did, that's what it looks like instead of the compressor displacement. For now, I'm not going to worry about it.

I did quickly check the performance for a recip. compressor at the 45 psig & 90° condensing temperature... the BHP/Ton for a recip at these conditions is: 0.92, which is about 10% better than the screw compressor.

We are getting closer....

[NNNNNN]

Hi Iceman,

Below is the reply for the second supplier:

"
1- Capacity Chart? - to follow

2- Evaporating Pressure? - Which evaporator are you asking about?

3- Suction line pressure loss? – This is function of the system piping not the compressor however past tests have shown that there is less than .5 psig between the LPR and the compressor suction. – The balance of the evaporator coils require higher suction pressures. The line losses were designed for approx 2 psig pressure drop.

4- Suction pressure & temperature? - Where are you asking about? The LPR should be around 25 to 26 Psig which would be 11 – 12 °F

5- Discharge pressure? - This is controlled by the size and setting of the condenser. Normally if everything is working properly your peak condensing temperature would be 90F or 165 Psig. I will be higher on start-up and on extreme high humidity days.

6- Discharge pressure loss? – The system was originally designed for 2°F pressure drop in the system discharge piping to the condenser.

7- Condensing temperature? - Original design 90F

8- Amount of subcooling used? – Subcooling is not used in the calculations of a system like this because typically there is none. Anything that appears from time to time under specific operating conditions is a bonus but cannot be counted on for system design.

9- Compressor displacement (CFM - the swept volume of the compressor at it's rated speed of rotation)? - to follow

10-Compressor capacity delivered and the power required at the operation conditions? - 74 Tons @12F SST, 90F SCT, and 98 BHP

11-Operating conditions? - 74 Tons @12F SST, 90F SCT, and 98 BHP
"

What exactly that means?

And Kindly I would ask you what does each mean?

Thanks.

[US Iceman]

Oh Boy, here we go...

These answers revel why the selection and rating of compressors is such a mess. All of those questions I asked originally pertain to how the compressor capacity is determined. The answers tell a lot!

#4) You need to know the suction temperature and pressure because this helps to show the amount of suction superheat, which detracts from the compressor capacity. High suction superheat = bad.

#5) The discharge pressure equals condensing pressure + the discharge line pressure loss. Asking for these helps to tell if the compressor rating is based on actual operating conditions or fictitious examples.

#8) I agree with.

Based on the two examples you provided, each are quoting a 100 HP screw compressor. However, both have quoted this under very different operating conditions.

Based on these quotes, all you can expect to get is a compressor with 100 HP motor.

What I was trying to show with this exercise was, you need to be very specific when you ask a question or for a quote. If not, you get all sorts of strange information back that may not mean anything.

Apparently your system runs about 25 psig suction. OK, then why is the first guy quoting 45 psig?

If the compressor CFM is similar between the two compressors and both have 100 HP motors, they should do about the same capacity at the SAME operating conditions.

My guess here is... Your system is a lot like most others. It was designed to operate at 25 psig and use back-pressure regulators on any evaporator at a higher temperature. This is a nice cheap way to design systems that cost a lot of money to operate.

Your last quote is probably closer to something that represents your actual operating conditions based on the information you have provided.

However, this is such a complex subject I refer you to the above red highlighted text.

I would also suggest you get a quote on a recip. compressor for comparison of cost and efficiency.