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puddleboy3
13-01-2008, 11:11 PM
I now have the pleasure of carrying out maintenance on a Mitsi water cooled VRF system in a shopping centre. On my first visit I was inspecting the pipe work to the outdoor units and found that the as the flow and return came in to the unit they joined together with two equal tees. I would have expected to find a commissioning valve in between to allow balancing! Can some one explain the reason for this.

Thanks in advance

taz24
14-01-2008, 01:10 AM
I now have the pleasure of carrying out maintenance on a Mitsi water cooled VRF system in a shopping centre. On my first visit I was inspecting the pipe work to the outdoor units and found that the as the flow and return came in to the unit they joined together with two equal tees. I would have expected to find a commissioning valve in between to allow balancing! Can some one explain the reason for this.

Thanks in advance


Shoddy work!!!

taz

Brian_UK
14-01-2008, 01:22 AM
Is this the condenser cooling circuit of the centre that you are talking about?

Is the pipework to your condenser the ones going through the wall?

If so, does your system have its own pump to circulate the water?

frank
14-01-2008, 11:40 AM
I would expect a seperate flow & return to the condenser ideally through a Danfoss or similar http://www.danfoss.com/United_Kingdom/Products/ProductFinder/ProductFinder.htm

The existing pipe layout seems very wrong.

Does the equipment operate OK piped up like this?

puddleboy3
14-01-2008, 02:19 PM
Frank/Brian the pipe work in the picture is entering the store from the shopping centre and is maintained @ 23 degrees C. When I attended site the system was shutting down due to the low limit sensor which was set @ 20 degrees C. After checking the pipe work I thought this doesnt look right. Checked another store in the shopping centre and found it to be the same but the equipment was different Hushion. Have a look at the document ive attached. Ive marked the pipe layout on it.

Cheers

frank
14-01-2008, 05:28 PM
Looking at the piping diagram, the strainer should be on the inlet side to afford some protection to the unit. What is proving water flow?
From the installation manual, it says that the cooling water circuit should be maintaind between 30C in summer to 20C in winter. There is no electric heater frost protection fitted to these units apparently.
The unit will shut down when the entering water is 5C or below, or 50C or above. From the diagrams, this is done by TH6 and TH9 which are shown in the heat exchanger. (See page 138 Service Manual)
What are the sensors in the external pipework? (Low & High Limit?)
Which way is your pump running? If it is running in reverse and forcing water into the return then I can see short cycling problems
Here's a link to the manuals - the service manual is 12mb http://www.eatlime.com/download.lc?sid=822AE8C5-5EB6-9D4B-5E74-D3AD4F03BD61

Thermatech
14-01-2008, 05:41 PM
I have seen a common header used for flow & return to water cooled City Multi.
But you can cheak the shopping centre ring main temperature. trace the pipe's out into the service corrador at the back of the shop. Then run the City Mutli system flat out cooling & see what off coil temp you get & monitor on coil temp. If on coil temp goes up but centre ring main flow temp remains stable then you are getting some water recirculating in the common header.
Thats not nessessarily a problem when the system is operating in cooling which most shopping centre shop do most of the time due to large heat load form halogen lights all over the shop provided the unit on coil temp does not approach 50 deg C.
But if the City multi does some early morning warm up heating then the outdoor unit off coil temp will be lower as the water heat exchanger is now the evaporator.
City Multi PQRY protects the water heat exchanger with 2134 fault if temperature is lower than 5 deg C or higher than 50 deg C.
So the external high & low temperature sensors are installed extra to the City Multi. There may be scope to reduce the low limit below 20 deg C to prevent nuisance faults.Bacause the unit will protect itself with TH6 water flow temp less than 5 deg C stop on 2134 fault when this happens twice in 30 mins.
The most common problem with shopping centre water cooled systems is that some 'service engineer' who you would not trust to change a light bulb gets set loose in the centre plant room & switches off the ring main pumps.
Of course the shopping centre will not admit that.
I would aim to adjust the water flow for 5 to 10 deg water temp diff at flat out cooling & then you will see temp diff reduce as the compressor ramps down. Then the system should work well with any water on coil temp between 20 & 30 deg C for cooling or heating.
But you are always dependant on the shopping centre providing the correct water temp.

John MacK
14-01-2008, 07:34 PM
Is it Braehead Centre?

I had problems there a few years ago, with Hushon units.....
Part of the problem was from the centre's side of things, valves being closed, pumps going off at wrong times on time-clocks etc.

puddleboy3
15-01-2008, 10:30 AM
Thanks for the replies guys! I understand what thermatech is saying but i am still struggling with the thinking behind this type of layout. I cant see how it would be possible to prevent short cycling as there is now way of reducing the flow between the equal tees. The only commissioning valve is on the flow.

And the system is not in braehead shopping centre.

John MacK
15-01-2008, 12:00 PM
From your diagram, the flow and return appear to be opposite from what they should be.

The system works, not in a conventional way, but because there is a pump etc on the leg to your unit then it negates the need for a control between the two T's. The pump will come on, on-demand, presumably by a time-clock or load.

Think of the centre water circuit as a reservoir that you are drawing your flow from and putting your return back into. It is not getting pumped through your unit.

grump
18-01-2008, 12:18 PM
Hi I cant open your pipework drawing.
The centre should be supplying your job with a constant condensing water flow and temperature don,t rely on it.You may also have a modulating valve in the pipework circuit to help with head pressure control this could also provide bi-pass.(Three gate valves should be in circuit).
In the meantime where there is a filter in the condensing pipe work ALWAYS check that it is clean,basics clean all filters,coils,blower fans,check drive belts if fitted,refrigerant charge.Remember some finger poking prat may have been on the loose in the main plant room or on the said job what you are now responsible for.
Grump:D:D:D PS I have a story to tell re blocked filters.

grump
18-01-2008, 12:31 PM
Sorry strainer in the pipe work is the correct terminology in the meantime please get back to us , re the solution to the difficulties you are are experiencing
Grump:D:D:D

grump
18-01-2008, 12:49 PM
Sorry always read the question properly.The pipework layout ,John Mack appears to have an understanding of system .In the meantime check the strainer,let me know what you find.Grump:D:D:D

Gary
18-01-2008, 08:49 PM
Considering the system is working, I would assume there is a blank plate installed with the gasket between the two tees. If there is a temperature difference between the two tees, then it must be so.

puddleboy3
01-05-2008, 10:27 PM
Just a little update! The system has been shutting down on 2134 fault code due to water temperature. Im sure this is down to the flow and return mixing. Ive been digging about on the internet to find out information on this type of setup and it seems to be common practice in the likes of shopping centres. My gut feeling is that the system that i am now looking after has not been designed correctlly. The only thing I can add to my pipe work layout is that the flow and return pipe work is the same size as the equal tees 65mm.

Any suggestions would be much appreciated.

John MacK
08-05-2008, 12:31 PM
Is the strainer clean?

Are the flow and return the correct way round, as I said before going by your sketch and the info given, in my opinion, they are wrong.

The strainer should be before the pump, either in the flow or return.