Question on chilled water pipe sizing

[Jcqa]

Hello, appreciate some insight on this.

For sizing chilled water pipes serving fan coil units, is there a minimum water velocity that the chilled water must have based on the flow rate and pipe size? I have searched online and I only seem to see maximum water velocity requirement instead of minimum. Is it important to ensure chilled water velocity does not drop below a certain value and are there any negative effects of a low water velocity?

 

[LittleInch]

Costs more.

Also time duration increases heat into the pipe ( long runs, really slow)

Realy slow you get laminar flow which isn't normally great, but not deadly serious.

Most folks go for 4 -6 fps, 1.5 to 2 m/sec as a good balance between low size/low cost per m and pressure drop (larger pump)

There no real "must have" velocity, a bit like there is no real "mustn't exceed. Lots of design guides and experience, but very few hard and fast "rules".

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[willard3]

You can ask a senior Engineer where you work for help with this.

 

[EnergyProfessional]

rather than a velocity, you design for a friction rate (i.e. under 3ftH2O/100ft etc.). That way you take into account the viscosity of chilled water, glycol etc. However, you don't want to drop too much below 2ft/s at some point to get air out better.
For coil sizing, the manufacturer has software for that.

 

[Jcqa]

If we oversize the chilled water pipe, would a low chilled water velocity have an impact on the cooling performance of the FCU/AHU? Would it have an impact on the overall indoor air quality? A lower friction rate would also be indicative of low water velocity right? Thanks for the insight so far everyone!

 

[EnergyProfessional]

Oversize pipes would not have impact on coil performance. The control valve will throttle the flow to what is needed. The only real downside to larger pipes is added cost and possibly space requirement. Valves ad other devices also get much more expensive for larger pipes.
Note the control valve size is based on flowrate, not the pipe connected. so if your pipes are very large, you will transition to a relatively small valve. Also the coil will have a pipe size that is at least in the ballpark of what you should be at. So if you have 4" pipes, and your control valve is 1/2" and the coil is 3/4", you at least know you should double-check.

 

[LittleInch]

EP makes a good point about velocity to drive air out. I would go a little higher myself (1m/sec / 3 fps) but too slow and you end up with air vents at EVERY high point.

But agree no impact on FCU performance.

Is this a new system or are you trying to trouble shoot an existing one?

Yes a lower friction rate is less velocity for the same size pipe.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Jcqa]

Thanks EP and LittleInch! Would the low velocity have any corrosion/wear effect on the pipes though? I also thought with a lower water velocity, the heat transfer effect would be lower too? @LittleInch what do you mean by air vents at every high point?

I'm actually asking about this because I briefly worked with a HVAC manufacturer at my previous job but I can't remember if I missed out on converting the chilled water flow rate for some FCU units from m3/h to l/s in the equipment schedule/summary sheet that I sent to a client. So I'm a bit worried that if I really did miss out on the conversion, would the person in-charge of sizing the chilled water pipes be able to realize the mistake and size it correctly. And if he didn't and used the flow rates in the equipment schedule instead of checking the actual manufacturing spec sheets, would that affect the IAQ, comfort level of the space the FCU is serving.

 

[EnergyProfessional]

If velocity is too high, water can be abrasive. But normally the pump is not able to just triple velocity due to square relationship of velocity and pressure. I have seen domestic water circulations with oversized pump where elbows started leaking after a year.

If your conversion is wrong by a factor of 3.6, I'd hope someone would question that. But it could get unnoticed. The person sizing the pipe may not be the person sizing the coil.

 

[danschwind]

Just to quickly add to LI and EP answers, a parameter known as the Froude Number relates the ability of a given flow velocity to eliminate air bubbles.

A Froude Number of at least 0,64 is needed according to some literature (Gilles Corcos). The higher the Froude Number, the fastest the bubbles will be removed.

I suggest you get a copy of Gilles Corcos's manual both because it's free and because it is an amazing resource:

https://aplv.org/technical_resources

Daniel
Rio de Janeiro - Brazil

 

[Jcqa]

THe flow rate I might have put in would be 3.6 higher than the supposed one in l/s, because the specs sheet had it in m3/h and my column for flow rate in the equipment schedule was in l/s. So the real flow rate might actually be 3.6x less than the value I put in the schedule. By that, I figured that if the person sizing the pipes didn't notice, the velocity he might have been going for would be lower than what he intended right? I was not a full time staff there but from what I understand, the FCU's cooling coil is already sized by my company, which supplies the FCUs. Not sure if performance and sustainability of the system will be affected?

 

[LittleInch]

I think you're worrying too much.

Too big a pipe / too small a flow velocity won't do the FCU any harm and might make it better.

What I meant was that if your velocity is too low and you don't sweep out the air bubbles, then you might need to put air bleeds at each high point in the pipework. But as you're not designing the pipework, don't worry.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Jcqa]

What happens if the air bubbles don't get swept out? The FCUs in question are wall-mounted, would the space the FCUs are serving be cooled and dehumidified appropriately? I understand there is testing and commisionning done after installation too, will a typical testing and commisionning process be able to catch the (potential) mistake and rectify accordingly? Thanks for the answers so far, I am very grateful

 

[LittleInch]

Well air gathers in the high points and if this is the AHU, then air doesn't cool as well as water. You can get air locks and flow issues.

Commissioning should be able to fill a system and bleed all the air out, but air is a tricky substance and can gather over time.

If the AHU has an auto breather on it you'll be fine.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[Jcqa]

Sorry could you clarify this a little, I'm a bit confused: if there are flow issues caused by the air bubbles, then wouldn't the FCU performance be affected since cooling capacity is influenced by water flow rate? Also, Wall-mounted FCUs will probably face the same issues as the AHU one you mentioned right? Also for commissioning, is the system normally tested at the load conditions it's designed for to see if it performs as expected? Thanks LittleInch, really appreciate your help!

 

[nglty]

Water velocity in pipes should not be less that 1.5 FPS. Less than 1.5 FPS, the water will will not carry air with it, air will not be picked up at highest point or at air separator, which will lead to premature pipe corrosion.

Ideal water velocities inside coils so=hold be between 3 and 5 FPS to insure sufficient turbulent flow.

Willard3: The Op has a legitimate question, and he may nit not have a senior guy where he works.