Suction line designing for ref. systems

[Maxim]

Hello!

I have a question concerning right size of the suction line for multicompressor systems. I know that there is some limitations about minimum refrigerant speed inside pipes. That is the "reason" for double risers and so on.

What actually the correct approach to this problem: to determine minimum possible refrigerant flow rate and calculate pipes, providing the necessary speed for good oil return OR to calculate pipe diameter according to some reasonable pressure losses? The second way could lead to low refrigerant speed inside the pipes in multicompressor systems, when most of the compressors are switched off. What is necessary to take into account while designing long pipelines (e.g. 150-300 m). Are there any limitations regarding maximum actual refrigerant pipeline length? Is it possible, for example, to install refrigeration system with 400-m suction line.

Is it necessary to use double risers (I have never seen such piping on real plants)?

What about sizing discharge and liquid lines?

Another question.

What actually is the best way to calculate condenser capacity for single-compressor and multicompressor refrigeration systems?


Thank you,
Maxim

[Peter_1]

When a multicompressor system can go at very low load, then you certainly need to install double riser where the smallest one is calculated for the smallest possible capacity.

Installing long suction lines is not a problem, as long as they're inclined towards the compressors and you have sufficient speed in the lines.
If you calculate them by max pressure drop that long, then you have a chance that the speed will be too low.

Better solution is to collect several suctions to a central header and rise from this central header, eventually with double risers or triple risers or even more. From the header you can even rise wit all 5/5 lines beside each other, all fitted with an oil slot

We program in such a cases in the PLC a feature where all the SV's are opened (like in a VRV/VRF system) at the same time several times a day - necessary or not to achieve the set temperature - so that the lines have several times/day maximum flow to ensure proper oil return.

For the liquid lines to the evaporators, I don't see a problem, the bigger the better.

Same for discharge lines. Make you install the proper traps to ensure that liquid or oil isn't flowing back to the compressors.
Just check if the oil separator will handle partial load if one main oil separator is fitted

[fridg]

I tend to agree with Peter_1



The minimum velocity on our pipes are 2.5m/s.

As for the condenser at least 30% more capacity , to prevent hydrostatic rupture in case of a blockage.

[Maxim]

To Peter_1 - What is sufficient speed, actually? I know about 6...12 m/s recommendations.

To frifg - As regards to condenser selection my question was is it correct for multicompressor stations just to sum up total power consumption and cooling capacity, obtaining total condenser capacity by that? Are there any difference between low temperature multicompressor station and high temperature multicimpressor station condenser capacity calculation?

And is it necessary to incline discharge pipe towards condenser?

[Peter_1]

6 to 12 m/s seems good, we take 10 m/s but in a multicompressor system, speed varies widely with the load.

Your calculation for the condenser is indeed correct. Take then a DT of 10 K for the condenser to avoid high pressures in summer or/and when condenser coil is partially clogged.
There is no difference in calculation for a low stage or a high stage system: total cooling capacity + absorbed power.

Indeed, incline discharge towards condenser if possible or install a U-bend so that condensed discharged gasses at standstill can't return to the compressor and damage the reeds or the pistons.

Maxim, I read through your questions that you know what you're doing. ;)

[Andy]

To Peter_1 - What is sufficient speed, actually? I know about 6...12 m/s recommendations.

To frifg - As regards to condenser selection my question was is it correct for multicompressor stations just to sum up total power consumption and cooling capacity, obtaining total condenser capacity by that? Are there any difference between low temperature multicompressor station and high temperature multicimpressor station condenser capacity calculation?

And is it necessary to incline discharge pipe towards condenser?


Not in working out the Total Heat Rejection

but you do need to check the THR of the compressors at the highest suction pressure that you will see during pulldown.
say -20 deg c for a -33 deg c system.

you could just let the head pressure float up to 45 deg and design on 40 deg condensing with a 30 deg c ambient, but all this depends on local summer ambient


Hope this helps
Kind Regards Andy:)

[Maxim]

Thank you all, gentlemen.

May be you know if it is possible to find somewhere in Internet the examples of real suction piping systems for different cases and capacities/

Maxim

[Maxim]

To Piter_1 and Andy/

If I have two multicompressor stations

1) 3 pcs compressors R404A

Evaporation temperature -8C

Condensing temperature +40C

Refrigeration capacity 23,3 kW (at the -8C/+40C)
Power input 8,65 kW (at the -8C/+40C)

Then Condenser capacity = 23,3 + 8,65= 31,95 kW
LU-VE air condenser SHVR 43 gives 46 kW (+30/+45 dt15) and 30,7 kW for dt10 (+30/+40). Is this correct choise?

Ambient temperature +28C

May be it's better to take just 32 kW at +28/+40 (e.g. 40 kW at dt=15 (+30/+45))

2) 3 pcs compressors R404A

Evaporation temperature -26C

Condensing temperature +40C

Refrigeration capacity 16,4 kW (at the -26C/+40C)
Power input 11,0 kW (at the -26C/+40C)

Then Condenser capacity = 16,4 + 11,0= 27,4 kW
LU-VE air condenser SHVR 47 gives 40,57 kW (+30/+45 dt15) and 27,04 kW for dt10 (+30/+40). Is this correct choise?

Ambient temperature +28C

May be it's better to take just 28 kW at +28/+40
(e.g. 35 kW at dt=15 (+30/+45))


Maxim

[Andy]

To Piter_1 and Andy/

If I have two multicompressor stations

1) 3 pcs compressors R404A

Evaporation temperature -8C

Condensing temperature +40C

Refrigeration capacity 23,3 kW (at the -8C/+40C)
Power input 8,65 kW (at the -8C/+40C)

Then Condenser capacity = 23,3 + 8,65= 31,95 kW
LU-VE air condenser SHVR 43 gives 46 kW (+30/+45 dt15) and 30,7 kW for dt10 (+30/+40). Is this correct choise?

Ambient temperature +28C

May be it's better to take just 32 kW at +28/+40 (e.g. 40 kW at dt=15 (+30/+45))

2) 3 pcs compressors R404A

Evaporation temperature -26C

Condensing temperature +40C

Refrigeration capacity 16,4 kW (at the -26C/+40C)
Power input 11,0 kW (at the -26C/+40C)

Then Condenser capacity = 16,4 + 11,0= 27,4 kW
LU-VE air condenser SHVR 47 gives 40,57 kW (+30/+45 dt15) and 27,04 kW for dt10 (+30/+40). Is this correct choise?

Ambient temperature +28C

May be it's better to take just 28 kW at +28/+40
(e.g. 35 kW at dt=15 (+30/+45))


Maxim

Hi Maxim
I would not use a 15td or an ambient of 28 deg c;)

10td and an ambient of between 32 and 35 deg c for a condensing of 42 to 45 deg c.

Your selection should always have a little in hand on high temperture systems (tipically 5%) and on low temp systems 25% spare is what I would go for (check the THR at -20 deg c for low temp)

Kind Regards Andy:)

[Maxim]

Hello,Andy.

I have always thought that condenser for multicompressor unit should be smaller that the condenser for the single compressor. The reason (may be I am mistaken) is following. Maximum operating total heat rejection (heat transfer from evaporator + compression energy=power input) of single compressor should theoretically be higher than in case of 3-4 compressor unit with common condenser. That is why it is necessary to add 25% (and to check -20C THR) for single compressor. But is it correct for 3-4 compressor system?

I know that this kind of systems are usually controlled by controllers, maintaining certain suction pressure (for instance -28...-26C). Then you have possibility to automatically restrict maximum suction pressure of system (and THR, which depends on suction pressure).

May be I am mistaken about this/

Maxim

[US Iceman]

I have always thought that condenser for multicompressor unit should be smaller that the condenser for the single compressor. The reason (may be I am mistaken) is following. Maximum operating total heat rejection (heat transfer from evaporator + compression energy=power input) of single compressor should theoretically be higher than in case of 3-4 compressor unit with common condenser.


I disagree.

The maximum heat rejection of the system would be for all compressors running at a single time. This does not matter if it is a single compressor or a dozen compressors.

You have to use the total operating heat rejection when sizing the condenser. If anything less is used, then you are making a guess on how much diversity there is.

By the this I mean; sometimes three compressors may be running, with a fourth only a small percentage of the time. If the condenser is sized for the three compressors at full load, then when the fourth runs, the condenser would be slightly smaller. At this time you can expect to see a slightly elevated condensing temperature.

Someone might say so what! The fourth only runs for short time and the condenser size is smaller (less money).

My reasoning is the condenser should be sized for it's maximum duty. When other compressors are not running, the condensing temperature will decrease.

[TXiceman]

I have to agree with US Iceman here. The one place you do not want to cut corners is on a condenser. Most plants need the most cooling during the hottest weather and this is not when you want o be cutting back on capacity due to insufficient condenser.

On the issue of double and speed risers on suction lines, it is very important that you get this right on DX units that have multiple compressors of unloaders. If you are not sized to carry the oil back to the compressor, you stand a chance of running the compressor low on oil.

It can get to be a balancing act between pressure drop and velocity on jobs with long suction runs. If you get to a situation where the coil or evaporator is below the compressor, you may need to have multiple lifts every 10 to 12 feet of vertical height. Some designers will push it to 15 feet, but I do not like to unless I have good return velocity.

You can also run into the need to have double risers where you have a lift to get to a condenser mounted over the compressor(s).

Ken

[Maxim]

To Andy.



Your selection should always have a little in hand on high temperture systems (tipically 5%) and on low temp systems 25% spare is what I would go for (check the THR at -20 deg c for low temp)


If i select the condenser with dT=10 for compressor unit using THR for working point (-26/+40) that means that at -20 THR will be certainly higher. But condnenser was selected on dT=10 (+30/+40), and in case of -20C evaporation it will just work with dT=15 (+30/+45) (gives more capacity) allowing the system to cope with increased THR.

[Andy]

To Andy.



If i select the condenser with dT=10 for compressor unit using THR for working point (-26/+40) that means that at -20 THR will be certainly higher. But condnenser was selected on dT=10 (+30/+40), and in case of -20C evaporation it will just work with dT=15 (+30/+45) (gives more capacity) allowing the system to cope with increased THR.

Yes if you select for a 10k td it will allow you to float to a 15k (higher condensing). But the extra condenser duty will be required on the warm summer day, when the ambient is localised arround you condener at 35 deg c, with a 15td you will be looking to design a system that has a design pressure of 50 deg c, a bit excessive:D

Better design a system with condenser capacity in hand, aircooled condensers are not expensive, by buying well it will cost nothing extra to have a well sized condenser;)

Having said that my boss still thinks I will design on a 15td (as he did in the past) with the longer warm spells at higher temperature we have possibly due to gobal warming, this will only cause problem;)

I select on a 10 td and check at -20 deg c evaporation, again with a 10td.

Kind Regards Andy:)

[Maxim]

Andy.

What do you think about +48C condensing temperature for commercial R22 refrigeration system. I see no restrictions. For R22 it is only 18,5 bar pressure (most of condensers are tested at 27 bar).

Copeland Scroll ZF-series with liquid injection could work even at 55C.
And even if we have +35 ambient, +48...+50 theoretically should not be the problem.

Maxim

[monkey spanners]

Many Dx milk tank systems run at 65c condensing temperature in the summer, especially on the first milking with an evaporator covered in milk at 20c to 35c depending on how well their plate cooler is working. Always feels odd to put your hand on the suction line and it be warm;) Seem to remember on a factory tour that on the french Serap milk tanks they set thier Hp switches to 450psi on R22. New tanks are on R404A.

Cheers Jon

[Peter_1]

Maxim, think Andy wants to say that it has nothing to do with the allowable pressure and that the condensers were tested at 27 bar.
It's the big savings you make on the electricity bill you make when you condens lower, so a larger condenser, so a lower DT

[TXiceman]

It is always better to design for a bit lower condensing temperature (CT) to save power . In addition, the lower CT lessens the stresses on the compressor which leads to improved realibility.

It is relatively inexpenive to add a bit more condenser during the inital design stages. However, with the competitive nature of most plan and spec jobs, the cheaper bits usually wins unless you can get the owners ear on energy usage. Most consultating engineers will not stick their necks out to push very hard for lower energy usage, but we are seeing more owners demanding an energy analysis for new plants as well as upgrading existing equipment.

On commerical equipment, several manufacturers are now offering a system with oversized condensers for better power levels. Of course you do pay more for the unit, but you do reap the benifits for years to come.

Ken

[Josip]

Hello, Maxim :)


Thank you all, gentlemen.

May be you know if it is possible to find somewhere in Internet the examples of real suction piping systems for different cases and capacities/

Maxim

Your question is not simple, but having us here maybe we can help a little;)

Please follow the link:
http://www.danfoss.com/Pacific/BusinessAreas/Refrigeration+and+Air+Conditioning/Product+Selection+Tools+Details/DIRcalc.htm

download DIRCalc114 and start to play...:)
There are some other programs available too.
Maybe you must register yourself..

Best regards, Josip :)

[Faust]

The design speed in the suction line is ver crucial to maintain enough oil return to the compressors. Even a good oil seperator can return maximum %80 of the oil to the compressor. You should take a desgn speed of 15-10m/s when the compressor is working %100 and when the compressor is unloaded minimum 5 m/s. In addition to this you should keep the pressure drop under 2K.