Screw compressor idle tonnage

[zeh0085]

Allright guys Ed here I am doing a little research with my engine room and have come down to the one question that I am having trouble with. I want to know how to calculate how much estimated tonnage a given compressor makes at 0 slide valve. My first compressor in question is a Frick RWF-399-2 this is a 1000 hp unit running R-717 39lbs suction with 145 condensing temp. Is it more than 0? Is there a rule to follow or a link that one could suggest so I can figure this out on other compressors also. Thanks for any input.

[Segei]

Typically, compressor manufacturers suggest that min capacity is 10%.
It is not clear for me. What are you going to achieve?

[zeh0085]

Just getting my facts in order. Nobody ever seams to talk about the the other end of the compressors capacity.

[Josip]

Hi, zeh0085

Just getting my facts in order. Nobody ever seams to talk about the the other end of the compressors capacity.

this minimum capacity is achieved by construction of screw compressor itself and it is inevitable .... but, running screw compressor at this end of capacity is not good for compressor and that capacity is very expensive ..

.... searching forums you can found that is always suggested to run screw compressors at or very close to 100% of capacity... otherwise efficiency is very poor ...

Best regards, Josip

[zeh0085]

Understood there are instances where 100% capacity is not possible. My system in particular is dealing with building cooling loads and constantly changing production loads. The inefficiency begins with the start button being depressed. At idle or 0% I am using 393kwh as opposed to 700kwh @ 100%. Starting 2 completely diffrent compressors would as inefficent as letting one run at a lower slide valve position not to metion the wear and tear on the starters, motor windings, and the fact that some one would be in the engine room starting and stopping compressors on a hourly basis. 4160v motors make a really impressive mess when they go. Does any body know

[Magoo]

Screws are super inefficient at full unload, they will still do around 7 > 10 % rated duty for cooling purposes, you are still pumping oil and moving 100'S kgs of metal round and round for no gain.
Sounds like you need an energy management control system that when lead machine is at 100% and looking for more you force unload lead and start the lag machine, force that to 50% the trim load with lead or lag.
You could also concider VSD's/ and or soft starters.
When 4Kv motors go bang, apart from the mess, it is down right dangerous as well. Fault rating in the 100s of thousands of amps.

[zeh0085]

We priced a VSD for a 1000hp @ 4160v the price was quoted near $800,000 does that sound right? A new compressor is $250,000 I think we'll take our chances. What I usaually do is run the biggest I can balls out and leave a smaller (like 300hp or 500hp)on on to trim it out, but loads can change in the blink of an eye. Thanks I appreciate it Magoo and Segei. I have only had a couple 4160s go and that is good enough for me both times it was another operator not on my shift so I got to see the aftermath not catastophic but ugly never the less.

[Magoo]

Well we should forget about VSD's at that price. Holy for crying out load, have you tried Danfoss, or was it Danfoss that quoted that figure. Sounds more like Allan Bradley to me.
Still concider a management system, a fairly simple PLC program.

[mad fridgie]

Maybe you should look at this from a different angle, without knowing your application and load profile I can not be more specific than to suggest that you look at some form of large thermal buffer, this way you can keep some of your machines loaded at 100%. (and possible make others practically redundent "fault standby only"

[NH3LVR]

Yes they will pump 15% or so when fully unloaded. However I did see a Frick booster once that got very warm when running unloaded. It was a liquid injection cooled and the discharge stared backing up through the compressor. (there were two boosters on the same suction line). My theory is that with the liquid injection on it was in a negative loading. I set up the % of slide valve and all was well.

[McFranklin]

I loaded up a RWF II 399 in Frick's Coolware with 145 discharge and 39# suction. I then changed the slide valve ty from 100% to 0%. That changed the evaporator capacity from 962.4 TR to 116.7 TR. (12.1% of rated). I remember from a conversation about sizing relief valves on separators that you used a percentage of the swept volume of the machine (on poppet machines use 25%) I can't remember the value on slide valve machines. But 0% slide valve is not zero capacity.
Hope this helps.

[Josip]

Hi, zeh0085

Understood there are instances where 100% capacity is not possible. My system in particular is dealing with building cooling loads and constantly changing production loads. The inefficiency begins with the start button being depressed. At idle or 0% I am using 393kwh as opposed to 700kwh @ 100%. Starting 2 completely diffrent compressors would as inefficent as letting one run at a lower slide valve position not to metion the wear and tear on the starters, motor windings, and the fact that some one would be in the engine room starting and stopping compressors on a hourly basis. 4160v motors make a really impressive mess when they go. Does any body know

OK, now we have some better picture about your plant, but can you described it a little more like .... how fast your load is changing, how many times a day/hour, what kind of capacity regulation you have installed, .... sounds like some process cooling plant where you can do something to achieve some better results, but to help you with some good suggestion we need to know a little more about ... otherwise we are guessing ...


Best regards, Josip

[TXiceman]

Generally the machines will do about 10% of flow at minimum slide valve. Power draw will be around 40% of max power. remember that when you are running the plant at reduced capacity and reduced head, the compressor will do more tonnage at the reduced head and the 10% id of that duty, not the duty at the higher design head.

You really can't do an accurate determination of the capacity unless you install some expensive flow instruments in the refrigerant piping if it is a warehouse. On a process cooler, yo will need to measure process flow and temperature across the chiller.

Hopefully the oil cooling system has been properly designed to allow operaion at the lower capacites. On ammonia, the oil cooling load and discharge temperature will increase with reduced ammonia flow.

Ken

[Segei]

Unfortunately, when people design a refrigeration plant little attention was given to the dynamic of this plant. Example. Freezer has 4 evaporators in penthouse. Refrigeration load is 40% of design load. When temperature is satisfied all evaporator cycle off. Why should we cycle all evaporators off? Why don't switch off evaporators one after another with time delay? Wrong design create artificial and unnecessary fluctuation of the ref. load.
For mentioned plant you should determine the reasons of load fluctuation. Is it poor design or change in production? If this is production, I would choose another compressor sequence. 300 HP and 500 HP base compressors and 1000 Hp as a trim. Large compressor will load and unload to handle fluctuation of the refrigeration of the refrigeration loads.

[zeh0085]

My initial question was spurred from cost per ton of refrigeration question I am not experiencing any problems with cooling on my units. We don't typically run unloaded or 0% slide valve and as far cooling performance goes we are doing fine. We were faced with a challenge (not unusual) to save the company $1,000,000 and I presented the question of how much does it cost per year to make a ton of refrigeration. I did a little math and came up with $138,000 per year @ 0%SV & $249,000 per year @ 100%SV that gave me a difference of $111,000. I used McFranklins numbers (thank you) 116.7tons 0%SV & 962.4tons @ 100% difference of 845.7tons. I divided $111,000 by 845.7tons= $131.25 per ton per year. That's where I was going and now I have a better understanding of power consumption.

Now for the whole plant load thing yes I am making refrigeration for production loads primarily I run three pressures (actually multiple pressures with BPR's ) on a two stage system. I can start and stop compressors and change the room around as far as demand is concerned but the cost associated with that would run our eletrical bill up since we get billed on peaks.

[Segei]

My initial question was spurred from cost per ton of refrigeration question I am not experiencing any problems with cooling on my units. We don't typically run unloaded or 0% slide valve and as far cooling performance goes we are doing fine. We were faced with a challenge (not unusual) to save the company $1,000,000 and I presented the question of how much does it cost per year to make a ton of refrigeration. I did a little math and came up with $138,000 per year @ 0%SV & $249,000 per year @ 100%SV that gave me a difference of $111,000. I used McFranklins numbers (thank you) 116.7tons 0%SV & 962.4tons @ 100% difference of 845.7tons. I divided $111,000 by 845.7tons= $131.25 per ton per year. That's where I was going and now I have a better understanding of power consumption.

Now for the whole plant load thing yes I am making refrigeration for production loads primarily I run three pressures (actually multiple pressures with BPR's ) on a two stage system. I can start and stop compressors and change the room around as far as demand is concerned but the cost associated with that would run our eletrical bill up since we get billed on peaks.

I think that energy savings in industrial refrigeration is complicated issue. You done first step in this direction. How are you going to save energy?
Just several remarks. Condensing pressure 145 psig can be lowered. I found that 95% of the plants can operate at condensing pressure below 100 psig. Do not run compressors below 50% load. BPRs are bad for the efficiency. Look at your BPRs. Do you really need them?