How do Chiller Plants and Chillers unload? - The practical guide

How does chillers unload? How do chiller plants unload?
by: Pedro Guererro

After years of talking to people from different trades of the HVAC Industry in the Philippines, I have come to a conclusion that the level of knowledge on this topic is not yet so matured. Thus, as one of my goals to help the industry in our country to some extent move forward with what they know, I have decided to put some time and effort to simplify this seemingly complex topic.

I know I may not be a decorated person and what I say here might not appeal to you that much because of this but I assure you, I am well capable of explaining the science of this and what I share to you is not by my own invention or whatever but it is what I learned through my experience, education, and backed up by over 100 years of knowledge from the company that I work with.

Should I have not addressed the necessary information for you to understand then please do let me know so that we could meet up if ever and discuss it personally and or perhaps I will try harder so that the topic would be fully grasped by you and or your group. Please also feel free to leave any questions, comments and or suggestions so that the information sharing would be mutual and so that I could attend to any of your concerns if ever there are.

And now, from the wonderful words of the guys from UFC:

Lets get it on!!!!!

HOW DO CHILLER PLANTS UNLOAD????

To put simply the answer onto a single statement then it would be "It depends".

It depends on the following:

1. What type of system do you use?

a) Decoupled (primary-secondary)?

b) Primary flow?

c) Variable Primary flow?

d) Do you operate your plant manually or is your operation automatic?

2. How is your piping/pumping system configured?

a) Do you preferentially load chillers?

b) Do you have a bypass pipe?

c) Do you have valves on your bypass pipe?

d) What type of valves do you use on bypass pipe if ever?

e) How does your pump direct flow to your chiller?

f) Are your pumps manifolded or dedicated?

g) Are your chillers in parallel or series?

3. How is your distribution system (air-side) configured?

a) Do you have constant/variable flow of chilled water on your air-side equipment?

b) What valves do you use? Three way? two way? Motorized? Manual? Etc.

Now, since my goal here is not to waste your time and I know that you're a very busy person; let us simplify things.

Indeed all of those stated above is important but let us just deal with the things that make a difference.

Since the main component of a Chiller plant is the chiller itself then let us discuss first how the Chiller unloads.

Below here’s a schematic diagram that shows a water cooled Chiller:

You may notice that out of all the arrows above only one has an indicated temperature (44F). The reason for this is because the main job of a chiller is to produce cold water; typically 44F depending on the design value. In reality, it is the only thing we have control over since everything else is dynamic. Now, if the chiller/s have/has been designed to match the load correctly then whether it is full or part-load, it will produce 44 F (if that‘s the set point) regardless of the other 3 in simple terms. Of course there are set limits to what the other 3 could have since the chiller may not be able to handle the temperature going back to it but for now lets just make it simple and focus on how it unloads.

Depending on what type of chiller you have since different types unload differently in terms of mechanical unloading then let’s just put a peg on what makes it unload to make it more general… The answer is very simple, since the chillers most important job is to produce the cold water then the parameter that makes is unload is the temperature of the water coming back to it.

IT IS THE RETURN TEMPERATURE THAT MAKES IT UNLOAD!

Now you may ask which of it since there are two (2) arrows going back to the chiller? Well, it’s both actually. It’s the temperature of the water coming back from the cooling tower and the one coming back from the load. In super technical geeky terms : The return chilled water temperature controls how much refrigerant will evaporate in the evaporator and the return condenser water temperature controls how much the compressor needs to elevate the refrigerant pressure onto the condensing pressure of it in the condenser that will match the return condenser temperature.

Now did that make sense? I guess not… But if you look at it another way then the light bulb in your head might shine brighter with this example:

At full load (say 1000 tons) you will turn 1000 kilos of refrigerant liquid into gas.

At half load then only half of the refrigerant will evaporate (500 kilos).

Wait…. What about the pressure thing and whatnot?

Well, think of it this way, the chiller will unload when the temperatures of the water coming back to it is cooler than at full load. Actually, since there are two (2) temperatures coming back to the chiller then logic will follow that there are two (2) types of what makes the chiller unload:

Note: We follow the example above to maintain consistency.

1. Load unloading – This is directly related to the load – If the load is half then the water coming back to the Chiller will be cooler than at Full Load and the chiller will match that load. But this is not entirely true since in a variable flow system the temperature differential is the same because the flow is variable.

2. Condenser relief - This is not directly related to the building load since this is based on what the cooling tower (or air , if its air-cooled) is making.

Now to move on and go back to the list above and highlight “what makes a difference”, then let’s highlight them so that we could discuss how the chiller plant unloads:

1. What type of system do you use?

a) Decoupled (primary-secondary)?

b)

c) Variable Primary flow?

d)

2. How is your piping/pumping system configured?

a) Do you preferentially load chillers?

b)

c)

d)

e)

f)

g) Are your chillers in parallel or series?

3. How is your distribution system (air-side) configured?

a)

b)

THE MAIN EVENT!!

To make this simple then our discussion will focus on the following example:

1. The Building has four floors

2. The building is being cooled by four (4) 1000 ton chillers

3. Each floor of the building has a single AHU (1000 tons each)

4. The system that we will use in this example is decoupled (Primary- Secondary) since it is the most popular system in the Philippines.

5. The chillers are in parallel and the pumps are dedicated (one chiller=one pump).

6. There are four (4) secondary chilled water pumps to distribute the cold water to the building

7. We assume that this is a perfect system and there are no losses whatsoever and that the system is fully automatic.

OPERATION AT FULL LOAD (4000tons):

1. All four floors of the building are operational.

2. All four (1000) ton chillers are running at full load each

3. All four (1000) ton AHU’s are running at full load each

4. All pumps are running to serve the chiller and the building load.

OPERATION AT HALF LOAD (2000 tons):

1. Only two (2) floors of the building is operational.

2. Only two (2) chillers are running at full load each

3. Only two (2) AHU’s are running @ full load each

4. 4 pumps in total are running, two to serve the chillers and two to serve the building

OPERATION AT QUARTER LOAD (1000 tons):

1. Only one (1) floor of the building is operational.

2. Only one (1) chiller is running at full load

3. Only one (1) AHU is running at full load

4. Two pumps in total are running, one to serve the chiller and one to serve the building.

Wait, you may now ask yourself now…… What???? I’ve heard somewhere that full load for the chiller only occurs 1% of the time! Then what is this?

Well, indeed there are some crafty “magicians” in the industry that might have mentioned that to you and the statement might be true in some remote case (if you have only 1 chiller in your building) but now that you’ve taken some time to read this then you are now well informed what the trick is all about.

So then the next question would be…. When does the chiller unload????? Really????

OPERATION THAT WILL MAKE THE CHILLER GO INTO PART-LOAD

(3200 tons (80% of the total)

1. Four floors of the building is operational

2. Four chillers are running – Each running at 80% (800 tons/chiller)

3. Four AHU’s are running but some are at part load.

4. All pumps are running to serve the chillers and the building load

OPERATION THAT WILL MAKE THE CHILLER GO INTO PART-LOAD

(2400 tons (60% of the total)

1. Three floors of the building is operational

2. Three chillers are running – Each running at 80%(800 tons/chiller)

3. Three AHU’s are running but some are at part load.

4. 6 pumps in total are running to serve the chillers and the AHU’s.

NOW, WHAT ABOUT THE OTHER LOAD POINTS???

This might seem odd to you at first but later I will show you the calculation based on thermodynamics and whatever to further strengthen the explanation. For now, please allow me to explain why this is so. Since as what we’ve set earlier is that the chillers are configured in parallel then that means that the piping has a common header. What this means is that the temperatures returning from the building that mixes with the bypass and goes to the chillers are the same. Thus, the running chillers will share the load equally. This is not true if the piping system is configured to preferentially load chillers and or if the chillers are configured to be in series and that is why I highlighted items 2a. and 2g. to be ones that “make a difference”.

To make a long journey simpler, it’s wise to have a map around. Thus, in order for us to tackle this topic, we draw the sketch and this will guide us through the process.

Since this is a decoupled system then typically the design is that the secondary pumps have VFD’s.

Because we have a decoupled system then the design of the primary pumps should be constant flow and thus at any load point, the flow would be the same.

We then calculate for the gpm requirement of the building so that we could look for the mixing temperature going back to the chillers.

After doing the math and crunching the numbers we figure out that the mixing temperature is 52F. Each chiller will then receive 2400gpm water flow @ 52F and it will cool it down to 44F. To check the claim that the chillers will load equally, we check using the capacity equation and inevitably we will get the answer to be 800 tons.

So, that’s it! The chillers will each be operating at 80% even if the building load is at 40%. But please be warned, don’t use this calculation steps if ever your system is different from this. Remember the points that I told you to be items that “makes a difference” since it will indeed do and it might lead you to make a faulty design/analysis.

If ever the chillers are not equally sized, lets say 1 x 1000 tons and 1 x 500 tons, load sharing would still be the same. For example: The load is 1250 - the bigger machine wouldn’t run at 1000 and the smaller at 250 but it will again “share the load”. Both will run at 1250/1500=83.33% of the load (833.33 tons and 416.6 tons).

Well, I hope that this blog entry has shed some light on this topic and perhaps have helped you understand more on it. Should you have any questions/clarifications, please email me at coolmoako@gmail.com.

I would also like to thank the one of the “powerpuff” ladies of Trane (Jane) for helping me on the drawing and calculation sheet. My penmanship is too sloppy even I can’t understand some of it. :DDDDD

Hehehehe