High hydraulic separator inlet velocities

[sider]

Hi all,

would need some advice on the low-loss header application. Currently, I am designing a chilled water test ramp with rented equipment. The chilled water system will be with the primary-secondary pumping arrangement. When the load comes online, I have sufficient liters of water as per the chiller manufacturer's recommendation.

I would like to use a 4-pipe buffer tank that I have on my hands (connections DN125, 2000 L, vertical, Height 2,4 m, D approx 1,4 m) as a low-loss header between primary and secondary circuits, while my piping around it is DN200 (velocity of water around 1 m/s).
Can I have higher inlet velocities on my buffer tank (low-loss header)inlets? I believe the velocity is important when low-loss headers with smaller volumes are used, but could that be in my case?
I am afraid to mess up with the stratification inside the tank, but I believe when I match loads on primary and secondary circuits that shouldn't be a problem?

 

[HVAC-Novice]

Review the design manual and ask the manufacturer. If they give a maximum flow/velocity, it has a reason. I bet they have larger sizes of those devices for the flowrate you need. Don't install undersized equipment and expect good results.

Looks like a hydraulic separator with extra large volume. Also looks like your connections are wrong. At least how most hydraulic separators work. Is this what the manufacturer recommends? I would start out with their design guidelines. Do they have internal baffles?

you also should add a de-aerator on top of the separator.

I only know 2-pipe buffertanks and they have baffles.

More information needed and share what this actually is.

 

[sider]

Dont have a design manual but will try to get it. The manufacturer of the equipment is not listed.

The thing is...this is not a typical design situation...we are installing this chilled water ramp only for a 3-day test and then we disassemble it...so we must work with rental equipment only, which as you may guess is not something which can easily be pulled from the sleeve because such equipment is not rented frequently.

The largest size of the buffer tank available currently is the one mentioned above. An alternative to this is just connecting the primary and secondary circuits with a bypass of the same size (DN200, which will act as a hydraulic separator) and installing this 2000 L buffer in return or supply of the primary circuit (with 2 openings closed with valves.

My concern regarding using a buffer tank as a hydraulic separator is only during the starting conditions, when there would be a large bypass water amount circulated in the primary (fixed speed pump), compared to the secondary(variable speed, which will take some time to get cooling units to design flow to match flow in primary), because of potential stratification issue. But, when the loads are matched, an inlet of cold water into buffer should be similar to the outlet of cold water to cooling units (same for return), and I think stratification in this case should be ok.

 

[BronYrAur]

I think one of your flow arrows is the wrong direction. otherwise, it makes sense to me. The tank should serve better than just a piece of pipe as a bridge. Do you have enough volume not to short-cycle the chiller?

 

[HVAC-Novice]

Not knowing what it is and not having the product data, makes this a hard no. We engineers don't just connect a black box to 4 pipes and see what happens. If it is a buffertank, it likely has baffles. If it is a separator, it likely doesn't. Makes a big difference how to use it.

If this is a separator, yes, under different conditions in the secondary loop sometimes flows will match, sometimes don't. that's the whole point. And as BonYrAur pointed out, your pipe connections are wrong (if this is a separator). Look at some hydraulic separator manuals how they usually are piped.

 

[sider]

Hi, I agree on product data. I am sure this is not a separator as it is, but my idea is to use it as one. I believe the manufacturer didn't state in its product data such an application.

However, dont see a reason why 4 pipe buffer tank needs baffles, maybe if it has 2 connections then it would make sense.
Sorry, I wrongly put arrows, but as color coding states, colder pipes are blue on bottom, and return is on top.

In any case, alternative is to use 2 close coupled tees of same size.

 

[HVAC-Novice]

I don't know why a buffertank would have 4 connections except to give you some choice. At any time you only would connect 2 pipes.
I can't speak for all buffertanks, but the point is to not mix the water and create a very long plug-flow vessel. So you need baffles depending on where pipes connect.

A hydraulic separator typically doesn't have baffles and in theory it could be a pipe (a tank is just a fat pipe after all).

Only reason I use a hydraulic separator (for the very few applications that are not variable-primary) is that the ones I use also are a de-aerator and dirt collector. And they do increase the volume of the primary loop a bit.

You have to make your own decision here. But if I had to design a system with a "device", I would want to know what it is and does. How come there is no information? Does it not have a plate?

 

[sider]

It does not have a plate. It has perforated pipes on top and bottom.
Snip attached.

As you mentioned, this buffer would behave as a big pipe.

 

[HVAC-Novice]

why are you calling this vessel a buffer tank? is that what he manufacturer calls it? That devices doesn't look like it will do a good job buffering. Maybe a little bit if you have inlet/outlet diagonally across each other. but a real buffer tank would have baffles or a more slender shape.

that device doesn't work better as a buffertank just because you call it that. and if inlet velocity is high, it sure isn't the correct size.

 

[sider]

I am calling it a buffer tank because it is a volumizer in the system.

Based on the inlet/outlet locations (one across each other), there will be a certain level of stratification in the system.

Why would you have diagonally located inlet/outlet?

https://www.fiorini-industries.com/en/vkd/

Link for a similar product which is used for decoupling of primary-secondary circuits.

 

[BronYrAur]

I would be concerned that you truly have hydraulic separation with this. It would seem to me that the path of flow straight through the perforated pipe may have less resistance than existing through the perforations. Hard to say without knowing more. When you want considerably less flow in your secondary system, will the primary recirculate through the perforations and still maintain hydraulic separation? I have never seen a tank like this before. I am trying to figure out if it is stupid or clever.....sometimes there is a fine line between the two.

 

[HVAC-Novice]

Water flows along the path of least resistance. What you linked has perforated pipes built in (small holes to create a resistance). That will work different than the empty vessel you are attempting to use.

The purpose of a buffer tank is to increase the effective loop volume. If water short-circuits, you can't count the entire volume of the tank.

In general a buffer tank works best on the return to a chiller/ or boiler. what yo are attempting to do is combine the function of buffer tank and hydronic separator. This will have less ideal results.