CHW Demand Reduction Sequence 1

[cdxx139]

Hello,
I have CHW campus loop (4 chillers), that would like to shutdown one chiller during the highest power demand load (design days, when everyone has an AC going and the grid needs as much power as possible). Since my client is a big power client as well, they are trying to do their part, by killing a chiller.

Their old method is by closing each building CHW control valve by about 30% and having each building fend for itself. I disagree with this system, because the closest building may get the flow required, while the furthest building will be more than starved. I prefer to control the flow, and have everyone a little starved.

They do not have a tiertiery system, but rather CHW control valve in each buildign, which i disagree with, but is another post for another time.

My solution is to implement a CHW Demand Reduction Sequence, which basically is a two step process. One is increase the AHU DAT from 55 up to low 60's, while keeping a limit on thje dewpoint for humidity controls. This basically will trade fan energy for chiller energy. I havent done an energy analysis (and not sure how) but everything I read says you will get the most savings with the chiller.

The second step is to increase the zone setpoints from 1-5 degF, depending on the type of space. We expect the occupants to be slightly uncomfortable but not too uncomfortable for the one or two days.

Wondering if there were any other ideas out there? or if there are any issues you may have with my proposed solutions.

Thanks.



when the grid is being would be when the AC is required the most)

knowledge is power

 

[EngAzat]

Hi cdxx139,
Increasing the zone set-points is part of the solution plus reducing the fresh air quantity a little bit will also do.
I would like to draw your attention that you can get more of your AC system by using the recovery technique(please,see the attachment).
Good luck

 

[EngAzat]

This is the second attachment.

 

[willard3]

Heat recovery will decrease your electric loads considerably and you can leave the CW plant alone for starters.

 

[SAK9]

It is worth doing a pie chart for the HVAC energy consumption.Raising chilled water temperature together with room set points may reduce chiller and fan input kw .Looking at the pie will give some idea as to where to focus.


A few capital intensive options:

1.Chilled water /ice storage (produced during offpeak).
2.Onsite power generation with an absorption chiller.

 

[cdxx139]

Thank you all for your input.

EngAzat - I would like to reduce OA, but am not sure how to and keep with 62.1

Eng & Willard - As for the energy recovery, these are all existing buildings, so it would be a big venture. On the one building I did design, we had 30% OA and found energy recovery did not have a good enough ROI.

Sak - Ice storage would also be a big investment, and absorbtion chillers would require a steam source, which they no longer have.

I guess we are going to start with easy controls strategies, that will allow minimal occupant discomfort. This peak demand period only happens a few times a year.

My next step will be to review CO2 control. Right now we measure CO2 override as a differential from the OA CO2. I want to see if I can increase that differential and stay within code.

Currently 62.1-2010 App C suggests 700 ppm above OA CO2 will be satisfactory to a majority of people.




knowledge is power

 

[ChasBean1]

cdxx139, I like your idea better than the 30% valve reduction but one concern: as soon as you go to low 60s on DATs your dew points will be high. The demand limitation concern is only applicable during high dew points; putting in dew point limits will have a sequence that invites trouble, mitigates trouble, then re-invites it. This could result in intermittent dew point cycling and could lead to mold…

Here’s an alternative to try: don’t increase your supply temperature but instead reduce all zone minimum VAV values by half whenever outdoor air dew point is above 58°F. Also, increase the amount of time to engage reheat after a zone has achieved minimum airflow and has not provided enough heat. Interior VAV zones might also reduce their MAXIMUM values above that outdoor dew point. It’s a bit of hairy programming but might help reduce demand…

 

[willard3]

ROI from heat recovery will be better than anything you can do in present markets.

Short term thinking leads to short term solutions....since when is the short term the object of Engineering design?

 

[ChasBean1]

Totally agree willard3 - just thinking about what's possible with what exists presently

 

[cdxx139]

thank you for your input.

Chas - The purpose for putting dewpoint limitation on the DAT is to prevent humidity and mold issues. Not sure why you feel this may CAUSE mold issues?

And Im also confused on reducing the VAV min will do. This sequence will go in affect when they need to shut a chiller to help out the grid. This will be an above design day, so I wouldnt expect any space to require minimum flow. I think your hairy programming confused even me.

Willard, we can only present the information to the client, if they dont want to pay for energy recovery, then you design the best we can with what we are given.

With a 30% OA unit, even energy recovery cant do much. On the winter end, the MA almost reached my 55 degF DAT, so not much savings there. On the summer end (30% at 90degF + 70% at 75 degF) comes to a mixed air of 80 degF. The wheel wouldn't do much more.

Now add the cost of energy recovery to 2x35k cfm units, then I agree that it just isnt worth it.




knowledge is power

 

[Waramanga]

from my experience ROI on outside air is definitely not always guaranteed as willard and chas seem to indicate. Depends on the climate, how much economy cycle you run, how your electricity use is billed.....

 

[ChasBean1]

cdxx, the discharge dew point limit will be based on a humidity sensor. These don't do well above ~90% approaching the condensing region.

You said: "One is increase the AHU DAT from 55 up to low 60's, while keeping a limit on the dewpoint for humidity controls."

If your outdoor dew point is high, you already know the conditions that will occur in the space, but you propose using discharge sensors as reactive devices versus outdoor sensors as proactive devices. Base controls on the outdoor values. Don't base it on your discharge, which might not read well at these values.

Reducing minimum VAV airflows and extending time until reheat is needed might reduce net airflow, reheat, and improve energy savings.

 

[cry22]

the OA reduction, AHU SAT increase to about 58F, raising space temperature to 78F, raising chilled water temperature to keep flow to all AHU's are all good approaches.

I would reduce OA to the minimum possible, i.e make-up air for exhaust and 5% for building pressurization - no monitoring of CO2 during emergency - No one should worry about ASHRAE 62 in an emergency situation.

 

[cdxx139]

I guess I always assume the sensors will do what they are supposed to do. And it looks like we got lucky, as the sensors we used are good up to 100% rh with a +/- 2%. I am going to believe the cut sheets (see attached). (Not sure if that makes me gullable or not).

Sill not sure how to control off of OA? IF OA IS GREATER THAN ?? CHW VALVE SHALL MODULATE TO ??. At least with discharge control I know what I am serving to the space. We generally do not control on MAT (mixed air temp) as that can get us in trouble with ASHRAE 62.1

I do plan to reduce the VAV box min airflow during this period, thank you.

Not sure if this is a legit "emergency", more of a PR stunt with the local electric co, and they make some money at the same time. I sort of agree that not to control to CO2 during this brief period, but am too scared to implement it, so instead will increase the differential from 700 ppm to 1000 ppm. (So 1000 ppm + 400 ppm ambient, would be 1400 ppm in space. Dont see any code requirement to keep ppm low)

Thanks

knowledge is power

 

[317069]

There is a plant with 4 chillers, on the peak load day they want to shut down one chiller, means about 25% of total capacity.
- How long did they use the old solution and what they are complaining about now, I mean why they want to change their procedure?
- If they want to shut down one chiller on peak load time that mean the system will use three chillers all the times. It is more than control issue, whole original design may be need to be reviewed.
- You are trying to reduce the coil load by working on air side of the system, but you didn’t mention about water side such as water flow rate, will you keep it as is or reduced too.
- The old solution looks easier and less work, as an example let say we have a chilled water coil has a 100 GPM water flow and 10 F delta temperature, that mean the load is about 500000 Btu/hr, for the same load with 75 GPM the coil delta would be about 13.3 F, and with primary secondary piping we could keep 13 delta on coil and 10 delta on chiller ( 13 F delta may be better for chiller too)
- And if the piping system is already balanced and well designed it would stand flow reduction and can make fair water distribution between buildings or need minimum balance work.

- Allowing people discomfort will result decreasing people productivity and it also a loss for the business. the engineer role is not only to satisfy owner request (business owners want everything for free if they could have it, what if your boss told you to work in summer with room temperature of 80F for few days), engineer has to protect the public first, take it this way: an owner wants to open a business, that means he want to provided a service to a public, the engineer’s role is to make sure this service is a safe for public and comply with law and local rules.

 

[317069]

focuse on why they want to change thier old procedure.
may be there is a hidden legal issue behind this job.
if thier old solution was working and they were able to shut down one chiller, why they pay for an engineering firm to change preocedure to get the same what they are already getting?
it is just an idea.
if an employee got some problems related to discomfort, he migh start legal issue against onwner, in this case owner might need professional aprroval for his old solution to justify chiller shutdown and drop responsibilty on engineering firm sholder.

 

[ChasBean1]

The code value of 20 cfm per person correlates with about 850 ppm CO2. If you want to be safe, probably shouldn’t go below this (this is a differential of about 450 ppm from the OA value). A minimum OA that makes up for exhaust so as to pressurize the building often handles this. A 1400 ppm target is high.

For dew point control, if OA dew point is greater than 58°F, CHW valve modulates to maintain a cooling coil discharge temperature of 52-55F while reheat modulates to maintain X (space temperature or DAT based on space conditions).

When OA dew point drops below 56°F, resume normal temperature control. The 56-58°F band can reduce cycling between modes.

 

[EngAzat]

Hi,
(1000 ppm Return + 400 ppm ambient, will not be 1400 ppm supply).
It can be calculated as follows:
(CFM return/ppm return)+(CFM OA/ppm ambient)=(CFM supply/ppm supply)
So,
ppm supply=CFM supply/[(CFM return/ppm return)+(CFM OA/ppm ambient)]
Sure it will be less than 1000ppm.