Can somebody explain how we can select the pipe diamension for chilled water lines(central air conditioning)

This is a fluid mechanics problem. Usually there are software programs to make the job much easier.

Once you havs the flow rate known, you can begin the problem.

There are frictional losses for pipe
There are frictional losses for fittings.
These have to be taken into account.

The Head (weight of water) has to be taken account for sizing the pump.

To better explain it. Each design has target values of pressure drop (feet of water or lbs/sqin per 100' of pipe) and velocity.

The pressure drop values usually are 3 to 4 ft/100' and 6 to 8'/s and as low as 2'/s in larger pipe.

Pressure drop limits governs in small pipe and velocity governs in large pipe.

Larger pipe typcally has a lower operating cost.

To better explain it. Each design has target values of pressure drop (feet of water or lbs/sqin per 100' of pipe) and velocity.

The pressure drop values usually are 3 to 4 ft/100' and 6 to 8'/s and as low as 2'/s in larger pipe.

Pressure drop limits governs in small pipe and velocity governs in large pipe.

Larger pipe typcally has a lower operating cost.

Sir, I will consider preassure drop for 100 feet(pipe MS Bclass) as 3 and 2500 usgpm ,how can I find out the diamension of the pipe and which are the tables that ican use...

Sir,can you please explain the direct return and reverse return piping(chilled water s/m) with some diagrams... and how we can select which is suitable for a particular situvation... which is cost effective and reduce complxity...

I don't know the direct/reverse return terminology.

Now because of the advances in variable speed pumps a multiple chiller plant, floating bypass and secondary pumping systems are the most popular.

You can stage the chillers and have an optional "free cooling" heat exchanger in the system which uses outside air.

The chillers can have different capacities. Chiller pumps are generally fixed flow. The chiller piping system can also be arranged that any combination of chillers and pumps can be used which provides some redundancy.

You don't really want to lose your cooling loop. At a minimum two pumps can be used that cycle. When one breaks, it's taken out of service and rebuilt.

The secondary loop consists of pumps with variable speed controls and the flow volume stages the chillers. Pumps have two isolation and a check valve. The chillers might also have a way to modulate flow based on what the manufacturer say is the min and max limits.

Furthermore, each AHU has a modulating valve on it, a chack valve and two isolation valves.

For pipe sizing and system design, you should be able to find everything you need in the ASHRAE Handbook.

Sir,can you please explain the direct return and reverse return piping(chilled water s/m) with some diagrams.... and how we can select which is suitable for a particular situvation... which is cost effective and reduce complxity...

All water seeks to return to the source (or chiller) at the lowest possible pressure drop. In a direct return system the water is permitted to return to the chiller directly after the connection to the coil. In these systems the balancing of the flow relies heavily on circuit balancing valves to introduce a false pressure at the take-offs closest to the pump.

In a reverse return system each gallon of water is forced to travel an equal distance thereby equalizing the pressure drop through the system.

Look at my sketch. Trace the line along the pipe with a pencil imagining it is a gallon of water. Take off the main line through any given coil and return to the pump. Note the distance.

In the first example the shortest distance to return to the pump is through the first coil. In the next examples the distance is relatively equal for the water to return to the pump. Therefore the pressure drop is equal.

This is a pretty simple explanation and there is more to it but hopefully it helps one understand the differences.

Sir, i will consider preassure drop for 100 feet(pipe MS Bclass) as 3 and 2500 usgpm ,how can i find out the diamension of the pipe and which are the tables that ican use..........


Engineering toolbox has a few examples as to how to size pipe. I have simplified it for estimating projects as:


3/4" = 2.5 GPM
1" = 5.5 GPM
1-1/2" = 15 GPM
2" = 31 GPM
3" = 90 GPM
4" = 190 GPM
6" = 540 GPM

These are based on fairly rough iron pipe at a pressure drop of 1.5 PSI/100 feet.

HTH

Your right, the reverse return system is easier to balance, but they both are very bad at energy efficiency and energy efficiency or "green building" is the game right now.

Thus I agree with what I read, where two loops are used. One is essentially fixed flow, almost infineately variable capacity and the other is variable flow.

Chillers can be throttled only within certain ranges and capacity can be increased in a step-wise fashion by adding and subtracting chillers of variable capacity resulting in huge energy savings.

The individual coils can be throttled infinately without losing a lot of energy to the surroundings.

direct/reverse return may only be appropriate for a large space like a gym or warehouse. The reverse return is easier to balance.

But like all spaces, there may likely be offices or areas that require different cooling loads, so we are back to the two loop system with a bypass and variable speed drive pumps.

One could conceiveably argue which one is simpler and one can easily argue which one is more complex. You can argue which one is cheaper initially and which one would be cheaper in the long run. You can also argue which one is more energy efficient.

Creature comfort with minimal energy usage and high reliability is required for the long haul.

I liked your diagram.

Here are the answers to your questions

For pipes that are less than 2" use water velocity inside pipe between 2 & 4 feet per second

for pipes larger than 2", size the pipes in accordance to friction rate of 4 ft per 100 ft with preferable maximum velocity of 8 feet per second.

In all pipe sizes the velocity inside pipe should not be less than 1.5 feet per second.



As for reverse return vs direct return, here is the difference

1- The idea of reverse return system is that the length of runs between any two points in the system (such as for the nearest and furthest point in the circuit) is almost equal. Thus the reverse reurn is thought to be selve balancing or requires less balancing valves and less balancing effort. However, due to the advancement of control valves, and that becoming reasonably priced, direct return is becoming easier and more accuratly balanced.

2- Reverse return is usually more expensive system due to the use of more piping than direct return. Reverse return have supply pipe, Return pipe, and reverse return pipe while direct return has only supply pipe and return pipe.

3- Reverse return is an older system and is not used much in newly design building. Due to above reasons.

4- Reverse return system require more space in the shaft and false ceiling.



Hope this help

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Here are the answers to your questions

for pipes that are less than 2" use water velocity inside pipe between 2 & 4 feet per second

for pipes larger than 2", size the pipes in accordance to friction rate of 4 ft per 100 ft with preferable maximum velocity of 8 feet per second.

In all pipe sizes the velocity inside pipe should not be less than 1.5 feet per second.



As for reverse return vs direct return, here is the difference

1- The idea of reverse return system is that the length of runs between any two points in the system (such as for the nearest and furthest point in the circuit) is almost equal. Thus the reverse reurn is thought to be selve balancing or requires less balancing valves and less balancing effort. However, due to the advancement of control valves, and that becoming reasonably priced, direct return is becoming easier and more accuratly balanced.

2- Reverse return is usually more expensive system due to the use of more piping than direct return. Reverse return have supply pipe, Return pipe, and reverse return pipe while direct return has only supply pipe and return pipe.

3- Reverse return is an older system and is not used much in newly design building. due to above reasons.

4- Reverse return system require more space in the shaft and false ceiling.


Oh contraire’:
1. It is not the advancement in control valves that makes a direct return system more viable, it is the increased use of circuit balancing valves. In a direct retrun systems without balancing valves the flow of water will satisfy the coils closest to the pump first. Then as those valves close the water will be forced to the more distant valves.

2. Reverse return CAN be more costly to install but in the third example of my sketch it is actually a significant cost savings. I have done many buildings where perimieter radiation is used to take the heating load of the exterior walls and looping the building with a RR is the least expensive most efficient way to go.
You still only have a supply and a return pipe, it's just a matter of how you pipe it.

3. Reverse return is still a very viable solution to piping in new buildings and it is used in all types. From grocery stores to warehouses, to high rises to semiconductor facilities.

4. Reverse return requires more space only when used improperly. There is a design solution for every problem and the cool thing about it is there may be multiple right answers.

:)

Hello Rodbutler

I like technical discussions and exchange of thoughts

Here what I think

1- I know for certain that there was improvement in the functioning of balancing valves, I remmeber how inadequate the balancing valve graph 25 years ago and how it is becoming now. It made it possible to accuratly balance the flow in branches. It made a big difference and reduced balancing headaches.

2- It would exceptional case where the reverse return will not be costlier, especially with the drop in prices for the balancing valves.

3- It will require more space. More piping results in more space requirement, seems logical to me.

Having said all this, I can say that there is no one packaged method that is right for every thing or right all the time and there can more than one right way to do a design. Two engineer with different style can both be correct.

Q