Return water temperature problem to chillers

[saeedplc]

Dear Experts,

In a primary secondary chilled water system, we have problem of the chillers working when there is partial load on the secondary side,
and as you know loading chillers at low percent of load for long time can be harmful for the compressors so could you please let me know
how I can address this problem? any trick to be applied on the chillers or the secondary loop and etc
the system has been designed for LWT:10'C and RWT:15'C BUT in low load the return temperature reach to 12'c and for this reason the chillers
works on low capacity. all the valves on the secondary side are 2 way pressure independent contro valves

Regards,

 

[EnergyProfessional]

With existing chillers, you are limited to their ability. Better chillers can modulate and/or have multiple compressors or chillers. without knowing what chiller you have no one can tell. Maybe ask the manufacturer what options your chiller has.

 

[saeedplc]

thank you

How is the problem that I mentioned solved with other chillers?
could you please explain more.

 

[EnergyProfessional]

I think I did explain that. You can have one or a combination of these:
- multiple chillers staging
- chillers with multiple compressors staging
- modulating compressors
- some more chiller-specific methods like variable inlet vane geometry etc.

Options are dependent on size and type of compressor, controls and what the manufacturer deemed worthy. Assuming you want to use the existing chillers, you should review those. The setup and controls of the system also matter. An HVAC engineer should review this. You have climate, operation schedule, type of load, hydraulic system etc. to consider. A 24/7 server room in Florida will have different requirements from a 9-5 office in Alaska or an ice-system.

 

[saeedplc]

Thank you so much for the reply
It seems i have not mentioned the problem properly
The chiller is having two compressors that are controlled in such a way that if first compressor can not bring the temp to the desired value even in high capacity then the second compressor come into effect.
I meant the second compressor rarely is started because the return temperature is not high
And most of time the first compressor's load is lower than 20% that is harmful for the chiller.

 

[moideen]

this is centrifugal or screw? the screw compressor can generally modulate to 10%. what do you mean harmful to chiller...

The problem with the world is that intelligent people are full of doubts, while stupid ones are full of confidence.
-Charles Bukowski-

 

[EnergyProfessional]

If the first compressor runs continually, it shouldn't hurt too much besides efficiency. Frequent start/stop will hurt (any equipment).

Check if the compressors have different capacity, or if they are 50:50. If they have different size, you could change controls to use the smaller one under low load (limited by control options).

Positive displacement compressors could modulate speed, but depends on manufacturer. Centrifugal compressors can modulate speed only minimally. What YOUR SPECIFIC chiller can do really depends on if this is a 50 year old chiller, or the latest and fanciest 2022 model.

 

[saeedplc]

The compressors are the same capacity and screw type from bitzer company.
The chiller is almost new and from Geoclima
I have heard not only too much start stopping can hurt the compressors but also running at low capacity for long time can cause some problems for thd compresors because of not enough lubricating.

 

[BronYrAur]

So it sounds like you have a constant volume primary loop and a variable volume secondary loop, correct? In other words, your secondary loop has VFDs on the pumps to slow them down as the load in the building changes, right? However, the chillers do not have VFDs on the chilled water pumps I am assuming. So your only means of primary volume control is to stage the chillers/pumps on and off, correct?

Need to know more about the flows and sequence.

 

[EnergyProfessional]

since it is screw and brand new, are you sure it isn't modulating? My guess here is the chiller modulates and stages the screws to meet the setpoint. Obviously they only modulate down to a certain %. Maybe check the chiller is programmed correctly and takes advantage of everything it can do.

As for low load, if your load is lower than the lowest the chiller can modulate, of course, it will cycle. there is no other way unless you install electric pipe heaters :0)

I'm not chiller manufacturer. But if low speed hurts lubrication, I would program it to every once a while to speed up or re-design the lubrication system. Half of the chiller design challenge (or for any lubricated machine) is lubrication under all conditions.

How are you determining the 20% load? Maybe that is the screw speed?

And yes, a constant speed primacy loop will give you low dT under low load. That's why we don't really do that anymore, at least we shouldn't.

You said you "have heard". Did you "hear that from the actual manufacturer, or just as a general statement for a totally different chiller from 50 years ago?

 

[saeedplc]

@brunYrAur and EnergyProfessional yes the primary loop is constant speed and secondary is variable speed. Of course the primary pump is equipped with the VFD but at the time being it has not been programmed to be varible.
Chillers compressors have vfd drive and their speed chnge with the return water temperature.
If differencd between suplly and return water temperatures is low then the vompressore speed is too low and if the compressor works in this situation fo a while it will be faulty.
What i heard is right because the bitzer representative told low load on the compressor is cause of the problem.
Now please let me know if i force the contractor to proceed for converting the primary loop pump to a variable speed it can improve the current status?
When the load on secondary is low thevreturn water is mixed with the primary and it's temperature become lower more and it aggrevate the situation.
I want to find a way to load the compressors more.

 

[EnergyProfessional]

the speed of the primary pump has no impact on the load. Yes. lower speed will increase dT, but it also means less flow.

there are pump savings from reducing speed, but I would check with chiller manufacturer if the chiller has controls to regulate primary pump speed. You don't just want to do that and then have too little flow. I don't know your chiller, i know some boilers have primary pump speed controllers so that primary flow is what manufacturer wants.

i assume you are not the designer of the system. why not talk to the HVAC designer about changing what they designed? WE don't know your system, your equipment, or the design intent.

 

[saeedplc]

Unfortunately the system designer has passed away by an accident and the substitute person is not dominant on the system
and I want to cooperate with him to find a logical solution so could you please let me know what is standard way of controlling
primary pump of the system? by flow by pressure and it's algorithm
I know the vfd of the secondary pump should be controlled by the differential pressure sensor as constant setpoint or setpoint reset method according to the system design but dont know anything regarding the control of the primary pump.
As far as I know primary flow should be the same amount as the secondary side and any difference has it own problem so if the both flow can
be controlled in such a way that whatever amount the secondary pump demand that amount is provided by the primary the situation would be improved in aspect of the
temperature. i say this because if the primary flow be higher than the secondary they will mix together at the return side and cause low temperature to the chiller.
but controlling the primary pump is very important.

 

[EnergyProfessional]

Even sophisticated primary pump control will be lacking and that is why modern systems use variable-primary flow. The primary flow needs to be at or slightly above the secondary flow, AND needs to be above the chiller minimum flow. That minimum flow will depend on the chiller control and modulation options, and fluid temperatures. The fear is, you can freeze water.

To help you and not just let you break something, someone knowledgeable needs to:
- understand the system and design intents
- know the chiller and limitations and that also will require talking to the manufacturer
- understand the control system capabilities
- re-design the system and controls

For that you will need to hire someone. It may work in a semi-remote way. But it isn't something someone on a forum can answer without taking some time to figure out all the above. Each system is unique and if you do it wrong, you can damage or at least disable the chiller. Unfortunately there isn't one single "rule of thumb" that can be applied to every chiller system. And whatever you do, it has to be tested to work under all conditions.

Good luck!

 

[BronYrAur]

As EnergyProfessional said, there are many things to consider here. ideally, you would like the primary and secondary to have the same flow or the primary to be slightly more flow, but reality is sometimes different than on paper. I had a system consisting of 10 chillers in a primary-secondary arrangement. There was not adequate mixing in the bridge, so when running primary-strong, one bank of 5 chillers would get a much colder return water temp than the other bank of 5. Those chillers would sometimes get knocked off line.

Even when primary and secondary flows were perfectly balanced, I was getting bi-directional flow through the bridge pipe because one bank of chiller returns was colder than the other. The bridge piping was 36" pipe, and there wasn't room to make changes.

My remedy was to actually run a slightly secondary-strong system and have the chillers make water 3 deg F colder than I needed. When blended with the secondary-strong return, my desired chilled water supply left the plant. This cost me the penalty of making colder water, but it completely eliminated any supply water going back to the primary return. It was a fix to a design/installation flaw that was not cause be me. It was not ideal, but it allowed the chillers to be more fully loaded as my primary delta-T was always 3 deg F larger than my secondary delta-T. it was also an relatively inexpensive fix.

In your situation, hopefully your primary system will allow for flow to be more balanced, but there are many things to consider. There is only so much help that we can provide. An engineer needs to look at your system.

 

[saeedplc]

@brunYrAur and EnergyProfessional Thank you so much for all your valuable replies.