Ammonia Refrigeration discharge piping 1

[JimEldridge]

Contractor thinks we will save money and energy by combining two refrigeration systems' to use one condenser. One system has four 200hp screws running coolers at 35 psig suction and the other has two 250hp screws running 4 ice makers at 3psig suction. The ice makers are 650 feet away from the proposed new condenser. The cooler system has several evaporators requiring hot gas defrost.15 hours each day something is defrosting.Does anyone foresee any operational problems?Will this really save energy? As an operator I would rather see a separate condenser.

 

[BigInch]

What was I thinking? That will be you question next year.

That's my first thought. Secondly, I can't say I know anything about your process, but it seems doubtful. 650 feet is a new energy loss, so for his statement to be true, you should at least tell us how he proposes to magically create energy to account for that loss PLUS the savings in energy he thinks you will get. UNLESS the horsepower drops for the two units taken togther, you will just have those running at off-design conditions, and probably MORE inefficiently than before, if it is even theoretically possible to do it.

Tell the contractor that he must prove his contentions with calculations. Post them here, then we'll see if he's right. Until then, I'll bet there isn't anything in it. Perhaps a savings in capital cost, if he's doing the 650 feet of pipe for free, but I'll also bet you're looking at a long life of increased operating cost (probably much more significant than an insignificant capital cost savings)... if it works at all. Post a system diagram and the calculations.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[LOWTEMPMAN]

Unless the "one condenser" is new and much larger than the existing condenser, there will be no energy savings. I agree with the past poster. No way can the contractor show energy savings. Unless compressor BHP is reduced from present amount ther is no energy savings. There can be no reduction in compressor BHP unless the condensing pressure is reduced; no condensing pressure can be reduced unless the condensing surface becomes much larger.[ This suggest contractor is taliking about a new large condenser????] The 650 feet of proposed discharge piping is a nice profit for the contractor but does you no good because the pressure drop will be significant and causes the compressor to have to discharge to a higher pressure, [ read this as more BHP] just to keep the same original condensing pressure.


Curious about your hot gas of 15 hours per day. Maybe we can help you on this too. What are the details? Why so much time? This is an awful lot of time on defrost.

 

[MikeHalloran]

Another argument for not combining systems: When, not if, a compressor turns itself into shrapnel, how many other units will it contaminate, and how many other compressors will suffer collateral damage?



Mike Halloran
Pembroke Pines, FL, USA

 

[JimEldridge]

Thanks for the info so far. When I mention the pressure drop losses
they say they will up the size to 6" to overcome that loss. Does that make sense ? Apparently the cost in labor and materials does
not approach the cost for a second condenser located close to the compressors.The existing condensers only have a few years left in them.
Regarding the defrosts. 15 hours is the worst case. part of the
year I don't have to defrost quite that much. Most of the load is
a production area (packaging vegetables). Part of the load is on
the dock (Shipping and receiving). They have a forklift that moves pallets two high and 6 pallets wide. The door they use has no curtains on it and is open quite often.There is no separation from
the dock to the production room. 5 coils on the dock use 20 minutes of hot gas for each defrost every 8 hours. The production room coils defrost 3 times daily at 30 minutes of hot gas. the pipe to these coils is almost 600 feet.

 

[BigInch]

Remember, the contractor's not concerned with operation cost, so he's trading his capital cost for your operating cost later. Assuming that you have a well designed system now, operating costs are balanced with equipment size and cost and what you and the contractor save now in capital cost, you will most surely pay many times over in operating cost. Cost of pipe will be only a minor factor in that. 650 feet of pipe will also have some affect on temperatures too. Most likely to be more important than the cost for upping the diameter.

What was your initial thought to keep them separate all about| Why were you thinking along those lines instead of this proposed modification? Write all the reasons down on paper.
By the way, where's the calculations? I recommend you don't do anything without your engineer's approval first; not the contractor's engineer.

Only put off until tomorrow what you are willing to die having left undone. - Pablo Picasso

 

[JimEldridge]

As soon as I get some calculations I will post them. I am interested
in seeing them as well.
My concerns are more operational than anything. Under normal circumstances I am sure it will run fine. Its the abnormal that seems to creep into the situation that I am worried about. For instance if the pressure at the condenser gets out of balance and
the liquid holds up in the condenser .I lose liquid to the ice machines and the liquid injection cuts the compressor out. Or if some of the discharge turns to liquid and finds its way into a compressor that is off. I agree we have to get an engineer that is
looking at it from our view. Thanks for your help.

 

[LOWTEMPMAN]

Wow! You have some issues. First off I agree with everything the others have posted. Hopoe this additional info helps you some more.
Appears you have ammonia, Right?

3 psig is -21F suction for the ice makers. At 250 BHP the screws each do about 100 TR. The proposed 6" discharge for 2 x 100 TR = 200 TR is more than adequate. Contractor is correct the pressure drop will be small. A 6" line will do roughly 734 TR at a loss of 0.5 psi per 100 feet. If the total equivalent length of the 600 ft of pipe, fittings and valves is say 1000 ft, the PD will be roughly [200TR/734] squared times 0.5 psi per 100 ft times 1000/100 = .4 psi. Not very much.

The velociy of ammonia gas in this line will be small. Subject to condensation, [ you mention this as a problem] especially if you are in a cold climate and the pipe will run outdoors OR if the pipe will run through conditioned spaces. The pipe should pitch downward toward the condenser so any liquid condensed in the pipe will run to the condenser. If you are getting condensed liquid in an idle compressor which I think you are alluding to then perhaps you need to look at the piping arrangement in the compressor room. I have used high pressure floats at the bottom of discharge risers to colect liquid and to drain them to the system low side.

Adding the ice maker screws to the existing condenser will raise the condensing pressure on the condenser and will increase the total compressor system BHP.

Are you buying a new large condenser??? This might make some sense.


Regarding the breakup risk mentioned by one of the posters, yes, screws do break up some times but not often. When they do, debris ends up in the oil separator and does not get out into the rest of the system.

Your hot gas defrost is reasonable, time wise and scheduling are OK.

Am not clear on how many spaces you chill and their temps. Cooler storage?, yes a dock and yes production. Got that. You have an unrestricted doorway from dock to production. If productiuon area not clooed then this is a big cooling loss and a source of moisture. Dock door to a cooler ???? If so is dock and cooler same design temperature. I assume so since you mention only one suction pressure for the screws. Unrestricted door to cooler is probably OK but if truck doors leak then best try to contain moisture in the dock and not let it get in the cooler, so this suggests strip curtains on doors to cooler, [ or better yet fast acting doors but they cost MONEY].

 

[LOWTEMPMAN]

I went back and re-read your reply again and have some more comments. You do say the production area has coils so that means the production area is cooled and if it is to be help at the same temperature as the dock then the open doorway between the dock and the production area is OK. If the two areas are designed to run at more than about 10 F different in temp then you should try strip curtains because alot of warmer air will flow from one area to the other.

You also mention problems about liquid hanging up in the condenser on the ice maker circuit. [More than one condenser??] This could be caused by several factors.
1. Condenser not draining well enough because outlet connection and/or pipe too small.
2. Improper equalization between condenser circuits if more than one circuit is involved.
3. Lack of a high pressure receiver which is to accomodate charge fluctuation in high side when ice maker goes on defrost. Some charge leaves ice maker apparatus in the form of gas from defrost, [ from surge tank, icemaker, piping] and maybe even some slopover and ends up in condenser while at the same time time the ice maker on defrost is not calling for liquid so liuid hangs up in condenser.

4. Improper routing of condenser drain piping and traps if present, be sure no inverse traps, no risers in drain pipe.

Sure hope you have a high pressure receiver on both the ice maker and the cooler systems If you do, then problem is likely item 1 or 2 above.

Almost forgot the main comment. If contractor recommends a common condenser for the ice maker system and the cooler system, how does he propose to be sure that the condensed liquid gets to where it is supposed to go?? He will have to run a new liquid line from the condensers or high pressure receiver on the cooler system back the 600 feet to the ice makers. This raises the question as to whether or not he plans to increase the size of the piping on the cooler system to accomodate the increased flow; and what about the connection sizes on the high presure receiver if you have one/ How will he increase the connection size into and out of the HPR if they are too small.

Having said all this there is a slight possible HP savings that could be attained through all the effort. Depending on your loading condition in winter, fall and spring, it is possible that the ice mkaer compressors could handle the entire plant load, ice making plus coolers. This all depends on your load profile through the year, where you are located, ie Alaska or Florida, and your production profile throughout the year.

The way this could be done is to connecvt the present suction line to the cooler compressors to the suction line to the ice maker compressors and install a back pressure regulating valve in this new line, yes it will be 600 feet long, and I don't know if there is sufficient payback. The idea is that when the load drops off sufficiently that you turn off the cooler compressors and run just the ice maker compressors, and the assupmtion is that the ice maker compresors have enough capacity to handle both the cooler load and the ice maker load. The may work because the part load efficiency of screw compressors is very very bad. Screws should be run at as close to maximum capacity all the time to get best efficiency. The ice maker compresors running at full load at -21 F will probably require less BHP that having all the compressors in the plant running at part load. Of course, an analysis has to be done to check this out, but for what it is worth it may be worthwhile.

Interesting to find out if this is what your contractor had in mind.