R410a liquid line sizing

[The captain]

Just wondering if anyone has a chart?

[DTLarca]

Just wondering if anyone has a chart?

Try this file attached...

[PRESS]

Hi.

DTLarca thanks for your post thats a very good chart. The Captain you might also be interested in the free Danfoss software called DirCulc1.20 it prety much has every refrigerant pipe sizer. You can download it and install it on your computer. It works wonders believe me.

www.isentropictemptech.com (http://www.isentropictemptech.com)

[The captain]

I have a VRV multi system on R410a with 9.52mm - 3/8" liquid line of 40 meters feeding x2 ducted indoor units 13.0 kw and 7.0 kw cooling capacity, total IDU cooling capacity of 20.0kw

The system in question is using a 9.52mm - 3/8'' liquid line to provide 20 kw cooling capacity.

It is my opinion that 20 kw cooling capacity is not possible from a 9.52mm - 3/8'' liquid line?

Opinions please?

[Bigfreeze]

I have a VRV multi system on R410a with 9.52mm - 3/8" liquid line of 40 meters feeding x2 ducted indoor units 13.0 kw and 7.0 kw cooling capacity, total IDU cooling capacity of 20.0kw

The system in question is using a 9.52mm - 3/8'' liquid line to provide 20 kw cooling capacity.

It is my opinion that 20 kw cooling capacity is not possible from a 9.52mm - 3/8'' liquid line?

Opinions please?


Piping length will come into play but from experience at that capacity I would expect the liquid line to be 1/2"

[DTLarca]

Piping length will come into play but from experience at that capacity I would expect the liquid line to be 1/2"

According to the table image I posted above the subcool lost when doing 20kW through a 1/2" liquid line would be 1K per 26m.

The table attached says that through 1/2" pipe the capacity available with 40°C liquid and saturated evaporating at -5°C is 23.5kW when the subcool loss is 0.05K per meter.

If you want to get 20kW down the 1/2" liquid line then the subcool lost would be (20/23.5)² x 0.05 = 0.036K

That is 1/0.036 = 26m/K subcool lost.

VRV systems tend to have built-in subcool circuits at the outdoor units and at all BS boxes which permits the trouble free use of liquid lines smaller than ordinarily seen on custom built systems.

[The captain]

The system is 3 pipe alowing individual heating and cooling to each indoor unit.

In cooling the 7.0kw ducted indoor operates with a good coil TD of 9k while the 13.0kw ducted indoor has a coil TD of 2k.

When the 7.0kw ducted indoor (only) is operating in heating mode suppling additional liquid to the 13.0 kw the coil TD is much better 8-9k.

[DTLarca]

My calcs for the modified liquid line capacities based on subcool lost per meter sacrifice accuracy for expedience.

To be more accurate we should not multiply the tabulated subcool per meter by the square of the capacity ratios but instead we should multiply the tabulated pressure drop.

Between subcool losses of 1 to 3K down the liquid line it does not matter really which way you do it.

For greater losses we must remember that the saturated curve is exactly that - a curve. Where the saturated temperature does not change linearly with the pressure change - the temperature change per pressure change increases with reduced pressures.

Anyway - is the outdoor unit subcool circuit working fine - it sounds like your liquid line is too small but also if the system worked before it could be just that the outdoor subcool circuit is now struggling - it may even simply be that the system does not have enough refrigerant charge to both provide the liquid line with liquid and then also the subcool circuit with the liquid but the BS Boxes subcool circuits are getting liquid.

[DTLarca]

In cooling the 7.0kw ducted indoor operates with a good coil TD of 9k.

You mean the air delta T (dt) is 9K?

TD is temperature difference between two different bodies or substances.

dt (Δt) is the temperature change in one body or substance/medium.

[The captain]

Yes air delta t is 9k. my mistake.

The outdoor subcool control circuit is operating well, 11k ^t liquid in 52 'c and liquid out 41 'c.

Liquid temperature leaving the outdoor unit is 41 'c. Liquid temperature entering BS box is 22 'c. The pipe run is 40 meters.

Refrigerant has been reclaimed and found correct.

[DTLarca]

Yes air delta t is 9k. my mistake.

The outdoor subcool control circuit is operating well, 11k ^t liquid in 52 'c and liquid out 41 'c.

Liquid temperature leaving the outdoor unit is 41 'c. Liquid temperature entering BS box is 22 'c. The pipe run is 40 meters.

Refrigerant has been reclaimed and found correct.

I calculate about 2bar pressure drop and that subcool should give you room for about 4 to 6 bar pressure drop.

If during 100% cooling you have liquid entering the BS Boxes at 22°C then it seems more likely that you have restrictions in the piping much more than just 40m of undersized piping. You would also lose about 1bar for every 10m of climb with R410A.

I have attached my calculations for the expected liquid line pressure drop using 3/8" which drops to about 0.4Bar from 2bar when 1/2" pipe is used instead.

[The captain]

A liquid line restriction would explain why the 13.0 kw indoor is not cooling efficently,EEV fully open with evaporator outlet temp 21'c. The smaller 7.0 kw does operate well and evaporator outlet temp drops to 8-10'c.

The 7.0kw evaporator is smaller and the restricted amount of liquid is enough to supply the coil?

The resulting pressure drop is not enough to cause significant low pressure/ pump down?

[mad fridgie]

Have you derated the system for the pipe run, as VRV/VRF are rated with 0 meter pipe run

[The captain]

Yes Cond unit is precharged with 8 kg and additional charge has been added for liquid line length with formula used correctly.

[bigor_2]

Hi DTLarca
I think about your calculation in #6. You use dt=0.05K/m, but I see dt=5K/m for liquid lines from table 8.
I ask to teach me)

[DTLarca]

I think about your calculation in #6. You use dt=0.05K/m, but I see dt=5K/m for liquid lines from table 8.
I ask to teach me)

Give me a demonstration of your calculation.

[bigor_2]

Give me a demonstration of your calculation.
I offended you? I have never used this table. Teach me.

This a misprint in SI unit version of table 8?
IP unit version this value is 5F.

[DTLarca]

I offended you?

Lol, no, not at all - I am open to the possibility that you are right - but I want to see how you arrive at your numbers, that's all, so that if I am wrong I will see how I am wrong.


I have never used this table. Teach me.

This a misprint in SI unit version of table 8?
IP unit version this value is 5F.

I'll come back as soon as I get a moment.

[bigor_2]

I'll come back as soon as I get a moment.
ok
I will discuss in lonely.
In IP version 5F per 100ft.
100ft->30.48m
delta 5F-> delta 2.78C
2.78/30.48=0.09K per 1 meter
is this correct?

[bigor_2]

DTLarca
Excuse me for my carelessness.
I will not delete their posts.
The other day I watched similar table from ASHRAE HB 2006. Attachment will explain everything!
From what source your table?

[NoNickName]

Hi.

DTLarca thanks for your post thats a very good chart.

You should thank ASHRAE, Refrigeration, System practices for halocarbon refrigerants, from which that chart is infringing copyright.

[DTLarca]

DTLarca
Excuse me for my carelessness.
I will not delete their posts.
The other day I watched similar table from ASHRAE HB 2006. Attachment will explain everything!
From what source your table?

My table is from a powerpoint presentation I downloaded from the web - it was from one of the Eastern Universities online. NoNickName says it is from ASHRAE. It looks identical to yours.

The SI and Imperial charts agree with each other.

The SI is in dt°C per meter and the imperial is in dt°F per 100ft. So they closely agree - the differences are too small to complain about. The tabulated data is very approximate - no rocket science is used. I could produce a spreadsheet with more accuracy if you want.

[DTLarca]

You should thank ASHRAE, Refrigeration, System practices for halocarbon refrigerants, from which that chart is infringing copyright.

yes, it is a bit cheeky :)

[DTLarca]

DTLarca
Excuse me for my carelessness.
I will not delete their posts.
The other day I watched similar table from ASHRAE HB 2006. Attachment will explain everything!
From what source your table?

Ah, okay, I've just had a closer look at your SI chart and I see what you mean. Yours has 2K/m for the middle liquid line capacity column where mine has 0.02K/m

I'm trying to build a spreadsheet version of the table but am having trouble at the moment trying to get a good iteration going which guesses the saturated liquid temperature change down the pipe lengths.

[DTLarca]

DTLarca
Excuse me for my carelessness.
I will not delete their posts.
The other day I watched similar table from ASHRAE HB 2006. Attachment will explain everything!
From what source your table?

Testing...

[DTLarca]

Your SI ASHRAE Table is wrong.

R410A Pressures at:

40°C = 2426000 Pa
39°C = 2367000 Pa

The difference is 59000 Pa

0.02K goes into 1K exactly 50 times.

59000 divided by 50 gives 1180 Pa which means for every 0.02K dt there will be a pressure change of 1180 Pa which agrees with my table but of course not yours.

Your table will work if you assume pipe lengths of 100m a time.

[DTLarca]

DTLarca
Attachment will explain everything!


Bigor, I have also just finished a spread sheet to estimate liquid line subcool loss (dt) and the calculations agree that the subcool lost over 1m for 14.3 kW using 1/2" pipe will be 0.02K per meter.

See the attached snap-shot image...

[mikeref]

40m is a stretch on 3/8 for that capacity for sure, but what is the height difference for indoor and outdoor units?

[DTLarca]

Bigor, I have uploaded a spreadsheet (attached) that calculates the liquid line pressure drop and associated subcool loss for R410A

Because I do not have a formula to calculate saturated temperature from saturated pressure for R410A - only the other way around, I have had to use a crude iteration process.

So you have to turn on manual calculations and iterations. Iterations set to 1 iteration only. Then press and hold down F9 for up to 2 minutes waiting for all the liquid dt numbers to have stopped changing but for the flickering of the last decimal.

Inputs are:

Mass flow from say coolpack as in Q = m dH therefore m = Q/dH

Length of pipe in meters.

Liquid temperature. (The assumption conflates saturation temperature and subcooled liquid temperature)

And finally pipe roughness factor E/D which is generally 0.000001527m/m

The density and viscosity cells use a lookup to find the corresponding values for the temperature input.

Keep holding down F9 until all number have stopped climbing or dropping and only the last decimal place is flickering - then for the particular pipe size in question press F9 a few more times till you get the highest number for that pipe size.

If the liquid velocities are above 5m/s then the line of values for that pipe size are shown in red.

If the pipe pressure drop is above 5 or 6bar, I forget which I chose now, then the final temperature will be defaulted to 0 so as not to bother doing calcs for that pipe size and therefore the dt for that pipe size defaults to 40K meaning it must be ignored.

[DTLarca]

Yes air delta t is 9k. my mistake.

The outdoor subcool control circuit is operating well, 11k ^t liquid in 52 'c and liquid out 41 'c.

Liquid temperature leaving the outdoor unit is 41 'c. Liquid temperature entering BS box is 22 'c. The pipe run is 40 meters.

Refrigerant has been reclaimed and found correct.


When I use the spreadsheet I recently uploaded with 41°C liquid doing 0.1254kg/s flow which I think is about right for 20kW then for 3/8" pipe I get a minimum theoretical subcool requirement of about 4.42K but in fact it would need to be more that that to have at least 3K to eliminate vapour bubbles and then to also overcome joint and turns dynamic losses and any elevation losses. So your 11K subcool very likely is not enough and because your liquid is arriving much lower at 22°C there is certainly too much liquid line frictional pressure drop. Is it from the refrigerant vapourising and thus adding significantly to the line pressure drop so causing the further massive temperature drop or is it merely a pipe retraction somewhere or both.

There is a problem in the pipe or with the pipe size or both - no matter how look at it.

[lawrence1]

The id (mm) of 3/8'' tube varies with the wall thickness.The chart states 3/8'' (assume 9.5mm od)tube has an id of 8.098mm...i believe this is incorrect as it is based on wall thickness of 1.402mm which i have never seen.

3/8'' 21g id is 8.69mm
3/8'' 18g id is 8.28mm

Both guage tubes are rated for R410a

Lawrie

[DTLarca]

The id (mm) of 3/8'' tube varies with the wall thickness.The chart states 3/8'' (assume 9.5mm od)tube has an id of 8.098mm...i believe this is incorrect as it is based on wall thickness of 1.402mm which i have never seen.

3/8'' 21g id is 8.69mm
3/8'' 18g id is 8.28mm

Both guage tubes are rated for R410a

Lawrie

Wow, a reminder that I should not judge by first impressions. I thought you were just a troll - perhaps even a Chinese LG TV salesman. But here suddenly numbers related to fridge spring forth from - well, from somewhere. I haven't checked your numbers and neither whether if they are different their difference makes any difference but WOW - what a sudden and radical change in character. Did perhaps any of my technical posts inspire you towards this character upgrade - such an amazing transition!?

[lawrence1]

This is a refrigeration forum i believe.

[DTLarca]

This is a refrigeration forum i believe.

What is refrigeration?

[lawrence1]

Nope,
Not taking that bait.

[DTLarca]

Nope,
Not taking that bait.

I'm glad you didn't throw the question back at me - because I certainly wouldn't know how to answer it. Well, apart from from throwing in Wittgenstein's family resemblance principle.

http://en.wikipedia.org/wiki/Family_resemblance

I have two copies of his "Philosophical investigations".

But still I wouldn't like to answer the question myself :)

[lawrence1]

You seriously need help.

[DTLarca]

You seriously need help.

Seriously? :)

[bigor_2]

DTLarca
It's amazing. Viewing PowerPoint presentation of our supplier has led me to search the table ASHRAE.
Author of presentation seems too did not know what to do when computing the actual delta T. He also found a only solution: table Le=100m. but why? 100 feet is not equal to 100 meters! It's me about the translation of IP to SI. But if used the formula of your calculations in # 26, then the deltaT 0.05K fit with deltaP 2950Pa

Thanks for the lessons!
Please specify me a link to a university presentation.

Sorry for the frequent editing)

[DTLarca]

DTLarca
It's amazing. Viewing PowerPoint presentation of our supplier has led me to search the table ASHRAE.
Author of presentation seems too did not know what to do when computing the actual delta T. He also found a only solution: table Le=100m. but why? 100 feet is not equal to 100 meters! It's me about the translation of IP to SI. But if used the formula of your calculations in # 26, then the deltaT 0.05K fit with deltaP 2950Pa

Thanks for the lessons!
Please specify me a link to a university presentation.

Sorry for the frequent editing)

You're welcome - I think I got the power point stuff from hku - when using googles advanced search filtering for ppt files.

[bigor_2]

what it is hku?
We also write on the walls three letters, but not every foreigner knows that this:)

[DTLarca]

what it is hku?
We also write on the walls three letters, but not every foreigner knows that this:)

That's hong kong university

http://www.hku.hk/

http://www.google.co.uk/search?q=hku+filetype%3Appt&hl=en&num=10&lr=&ft=i&cr=&safe=images&tbs=

[bigor_2]

Thanks.
Why I had not guessed;)
I will seek...

[The captain]

40m is a stretch on 3/8 for that capacity for sure, but what is the height difference for indoor and outdoor units?

Outdoor unit is elevated, piping is mostly vertical down the building 5 floors.

[The captain]

Is it from the refrigerant vapourising and thus adding significantly to the line pressure drop so causing the further massive temperature drop or is it merely a pipe retraction somewhere or both.

There is a problem in the pipe or with the pipe size or both - no matter how look at it.


Pipe retraction? merely a pipe retraction?

[DTLarca]

Outdoor unit is elevated, piping is mostly vertical down the building 5 floors.

Is that about 15m - you lose another 1.5Bar or so on the liquid line.

That's about 151987 Pa and with 1180Pa per 0.02K that is about 128.8 x 0.02 = 2.57K further drop in saturated liquid temperature or another 2.57K of subcool required.

[DTLarca]

Pipe retraction? merely a pipe retraction?

That would be restriction - sorry :confused:

[The captain]

Is that about 15m - you lose another 1.5Bar or so on the liquid line.

That's about 151987 Pa and with 1180Pa per 0.02K that is about 128.8 x 0.02 = 2.57K further drop in saturated liquid temperature or another 2.57K of subcool required.

The piping on this installation is vertically downward so any static head should become a static gain in terms of presure rise! making my pressure drop smaller? yes?

[DTLarca]

The piping on this installation is vertically downward so any static head should become a static gain in terms of presure rise! making my pressure drop smaller? yes?

Yes, you'll gain about 1bar every 10m and so your liquid should become more subcooled as it drops but not very much colder as it drops. The liquid enthalpy wont change much - only the subcooling will increase.