Chilled water Temperature Requirements 1

[HarrisQ]

Hi,

I have a unique situation on a hospital project where the medical equipment supplier for the LINAC bunker requires a temperature over 10 C as a supply temperature . My chilled water supply is 5.5 C with a return at 14.4 C .

Since I cannot supply using 5.5 C , i was wondering if i can use the return side and pipe it opposite or return to return .

Alternatively can we use a thermostatic mixing valve as a blending device and adjust the setting to supply the water at around 12 C , to ensure no condensation occurs.

 

[quark]

If 14.4C is ok then return to return is not a problem. In a 3way valve, mixing happens away from the equipment (as it is actually a bypass arrangement). So, I am not sure how you will do this. 8.9C dT is quite high unless it is process cooling. Hope you are sure on this.

 

[HarrisQ]

My only other concern is the flow on this leg. Once piped return to return how to ensure that there is proper flow.

The LINAC bunkers are also adjacent to each other with a common header , so a spike in the load ( 25kW is max load ) would sent the hotter water back to the main line which creates the risk of the neighboring LINAC receiving warmer water.

During idle time ( no treatment ) the machine is using 10% of its overall load.

That is why i was thinking of blending it with 66% return and 33% supply to reach a temperature of around 12 C. If return to return can work it would be great as its will be minimal disruption on site.

 

[BronYrAur]

You could have a pump that only serves your Linac equipment. See attached. Pull 10 deg water from the main and use a 3-way valve to blend it up to 12 deg with your Linac return water.

Not sure if your main system pumps will interfere from this flow arrangement without knowing more. You could install a "crossover bridge" between the main supply and return lines. Then this new arrangement with the pump and 3-way valve could connect to closely spaced tees within the bridge. That hydraulically separates, but now if your new little pump fails, you have no flow. So you would probably want a redundant pump. Crossover bridges also waste water and lead to low delta-T, so I'm just throwing it out as an idea.

If you can live with 14.7 deg, then connect the two pipes to the return main in a closely spaced tee arrangement. You should not need the 3-way valve in that case.

 

[HarrisQ]

Thank you for the response. The linac can live with 14.4 C easily. If i get your point correclty its better to put in duty standby pumps and push the return water through the linac loop and connect it back to the main line.
Regarding the system this is part of larger hospital that has 3 1000 ton chillers in a central plant. As compared to that the load we r talking about is only around 7 ton per linac for the 2 linacs.

I can upload my sketch tomorrow. I was worried about how far away i have to keep the connections on the return header.

 

[BronYrAur]

Some sources have strict guidelines on how close "closely-spaced-tees" have to be together. The point of this arrangement is to hydraulically separate the loops so that the pump/flow from one does not affect the pump/flow of the other. Some references will be very specific that the pipe centers need to be something like 12" apart or so many pipe diameters apart. To me, as long as they are reasonably close with no fittings or obstructions in between, there is essentially no pressure drop between the 2 connections. So, 12' is essentially the same as 12" in terms of significant pressure drop.

Will you always have significantly more flow in the main return line than in the Linac branch line?

 

[quark]

There are two ways to do that. First is to have a separate supply header from return line and feeding equipment and then connect returns to main return header or you can connect supply1, supply2 and then return1 and return2. If you have the first return after two supplies then hotter water from one equipment doesn't go to second equipment.

 

[HarrisQ]

Hi Guys,

I realized another issue is that the header is connecting all 4 bunkers , 2 of which will not even be live for the 1st phase , which means the return water is not going to be at 14.4 due to no load or minimal load. My airside is all AHU so there is no real load on the system on this zone. I dont think the return to return would work. I am now looking at either introduing a heater to warm up the 5.5 C water or put in a heat exhanger skid. The only other way would be to take a branch from the main plant room but thats quite far away. Refer to the attached layout.

 

[HarrisQ]

The Bunker details as well.

 

[BronYrAur]

It looks like you have fan coils and other Data Center type equipment on the same chilled water system. Don't those units require the full 10 degree water?

 

[HarrisQ]

Apart from the 4 bunkers everything else is a cooling coil that is sized for 8.9 degree C delta T .

Thats why i guess the best way is the split the cicruit via a plate type hx put a seperate loop for higher temp for the bunker loads .

 

[BronYrAur]

I would still favor a 3-way valve over a heat exchanger. Otherwise you need an expansion tank, air sep, etc. Perhaps you can make 2 connections (4 pipes) to the mains. One connection would be the typical supply and return that will feed the fan coils, data room units, etc. with 10 deg water. The other can be the closely-spaced tees in the return main that will only feed the bunkers with the pumps and 3-way valve.

 

[WilliamTz]

Hi

Best option is to use a plate heat exchanger to ensure the medical equipment side of things does not interfere with the main CHW system. Size the primary side of the HX on the main CHW F&R temp and the secondary side on the medical Equipment F&R temperature
Alternatively, if you can ensure the return water from the medical equipment is no higher than your main CHW return temp 14 degC then an injection circuit can be used. If the temp is higher but you feel the overall % of the main system is low then that is probaby ok.
See attached both option on a sketch. If your CHW system is a 3 port system use 3 port and a bypass in lieu of the 2 port on my sketch

Best of luck

 

[HarrisQ]

Thanks Guys . Finally went for a supplier selected unit , but its very close to what WillamTZ mentioned. I agree its the safest option. Refer attached what will finally go in.