Question Relating to TXV

[LeedsAlex]

Initially, apologies if this has been started in the wrong area of the forum; I had a quick look through for suitable places to post and this seemed most applicable.

The company I am currently working for have asked me to look at replacing their current chiller unit another unit they have on site that is currently unused. The current system provides chilled water for various uses. Now, the capacity of the new chiller far exceeds that of the old one, around 90kW as a pose to 40kW, and so theoretically should easily be able to cope with the cooling load. However I am struggling with my selection of Thermal Expansion Valve.

As I understand if the TXV will adjust the flow of refrigerant dependant on the required cooling load; preserving superheat and preventing damage to the compressor. At low cooling load however the evaporation pressure may be low enough that the evaporation temperature will be below the freezing point of water. I forsee this causing problems because the water is continually pumped around the system; meaning it will tend to the evaporation temperature of the refrigerant. At low cooling load requirements I'm worried this would cause the water to freeze.

Though I come from an engineering background it is in a different discipline and I've never designed a refrigeration system before, so am unsure how to solve this problem. Can anyone offer any help, advice, comments on this?

[US Iceman]

I'm confused....

Are you replacing the entire chiller package (compressors, etc) or only changing the evaporators?

The second issue is the replacement chiller is over twice the capacity of the existing one. Does the larger one have multiple individual refrigerant circuits (one compressor per circuit), or, does it have a single circuit with multiple compressors?

I think these questions and more need to be discussed before we worry about selecting TXV's.

[NoNickName]

US Iceman has got the right questions for you.

[chemi-cool]

From your post, it seems to me that you are not a refrigeration engineer.
Most of your questions are irrelevant as a new chiller comes as a complete unit with all the necessary protections. You don't change TEV in a Chiller unless it is faulty.

What do worry me in your story is that if you double the capacity, you have to do something with water flow and pumps, check if the lines are suitable for the bigger chiller.

[NoNickName]

From your post, it seems to me that you are not a refrigeration engineer.

Brilliant deduction, Watson. That's what he admitted first.



Most of your questions are irrelevant as a new chiller comes as a complete unit with all the necessary protections. You don't change TEV in a Chiller unless it is faulty.


This is not the point. He wants to revamp an old appliance, which unfortunately is bigger than needed. Everybody knows a chiller comes with a TEV, of course... :o

[Peter_1]

LeedsAlex, all your components must match with each other: compressor with the evaporator, evaporator with the water flow, TXV with the load, the load with the compressor, the evaporator with your load, the condenser with your load and compressor.

Yeah, I know, refrigeration seems a little bit magic, a little bit math, sometimes resulting in something that really works, sometimes resulting in a real nightmare.

The nightmare mostly starts for those who put together all these components without the proper basic knowledge.
You came here for some questions- thats why this forum is for - but you didn't gave us enough information to help you.

If you double your refrigeration capacity, then make that you have some sort of buffer tank.
We need type of every component involved in this.

[nike123]

As Peter_1 said, you need primary/secondary buffer tank.

Configuration is something like this:http://www.cemline.com/products/images/steeltank/image003.jpg

Just imagine that instead of two boilers you have one over sized chiller, and instead of two boiler pumps, you have one pump matching chiller water flow. It is better that volume of buffer tank is high as practically possible and that is properly insulated.

[LeedsAlex]

Sorry everyone if I've not been clear about what was going on, I didn't want to put too much information in the original post. Yes the chiller system was originally used for a different application and has subsequently been partially dismantled; hence why I am talking about finding a TXV (and also an evaporator) for the system. I am supposed to be looking at the possibility of rebuilding the system for a different application.

Anyway, technical details. The 'chiller unit' I am talking about currently consists of two compressors and the relevant condeinsing circuit for both, so yes it is two different cooling circuits. The compressor model is a Performer SZ185 54RA, with the following Technical Spec:

Nominal Cooling Capacity:
43100kW / 147100 BTU/hr

Swept Volume:
249.9 cmcubed/rev

Displacement:
43.48 meterscubed/hr

At these rating conditions

Frequency:
50Hz

Refrigerant:
R407C

Evaporating Temperature:
7.2 degress celcius (dew point)

Condensing Temperature:
54.4 degrees celcius (dew point)

Sub-Cooling:
8.3K

Superheating:
11.1K


So I've been looking at trying to find a TXV and Evaporator. The problems begin not only because of the mismatched cooling load but also because the cooling load will not be constant. There are three systems which will be using the chilled water one (bizzarely) is actually an air conditioning system, which will generally be cooling in the summer and heating in the winter, so there's seasonal loads there. Another system that uses it is a vacuum pump which will be used intermittently; as will the third system which is jacket cooling for mixer vessels. The chilled water system will also have two 15 tonne tanks to store the chilled water.

In response to Peter_1 and nike132, I'm assuming the buffer tank you mentioned will need to be for the refrigerant? In which case if it is designed to cope with excess cooling capacity will it help with this problem of variable cooling load?

Thanks a lot to everyone trying to help me out, I really aprreciate it.

[NoNickName]

No, the buffer tank is to store chilled water. 2x15 tons tank is more than anyone would expect or recommend.
As for the TXV and evaporator, just ask for spares to the original supplier or manufacturer, but please note that if the circuit was left unattended and open to air for long time, now you are staring at a wreck.

[LeedsAlex]

No, it wasn't left open to air. They sealed it all up and filled it with something inert, Nitrogen I think. Anyway they got all that done professionally so feasibly it should be fine. I'm still trying to get my head round this TXV though, at low cooling loads is there a risk of the water freezing as I described, or have I completely misunderstood?

[nike123]

In response to Peter_1 and nike132, I'm assuming the buffer tank you mentioned will need to be for the refrigerant? In which case if it is designed to cope with excess cooling capacity will it help with this problem of variable cooling load?



No, tanks are for chilled water. If I correctly understood your text, you already have tanks (2x15000L). They only need to have 4 inlets (or more if you want to combine more loads) to be primary/secondary buffers.
You see, water chillers don't have any problems with varying loads until their compressors don't need to be started more than 6 times per hour and load don't exceed their maximum capacity .
Since you have 2 compressors, 2 refrigerant circuits and buffer tanks, you could work with widely varying loads without any problem.
Just fix that chiller with original parts and ensure that your buffer tanks are primary/secondary type.

Chiller will work with both circuits until buffer tank is at set temperature+differential of second circuit, then it will stop second circuit, and when set point is reached it will stop first circuit. Now you have lot of stored energy in buffer tanks and chiller doesn't need to start cooling immediately at change of load.
That is like accumulation lake with controlled filling.
You use as much as you need and lake is refiled to maintain level from set point to set point-differential.

[NoNickName]

No, it wasn't left open to air. They sealed it all up and filled it with something inert, Nitrogen I think. Anyway they got all that done professionally so feasibly it should be fine. I'm still trying to get my head round this TXV though, at low cooling loads is there a risk of the water freezing as I described, or have I completely misunderstood?

No, the compressors will stop at setpoint whatever the amount of water or heat load.
The problem I see is low superheat or liquid refrigerant return, but this is a different story altogether.

[LeedsAlex]

Brilliant! That makes it a lot easier. When you say the tanks need 4 inlets, it that 4 each or 4 in total? Is it likely the chiller will have built in control to allow it to trigger at the set points? I'm sorry I know I'm asking a lot of questions.

[NoNickName]

Is it likely the chiller will have built in control to allow it to trigger at the set points? I'm sorry I know I'm asking a lot of questions.

99.9% of chillers have it. Make sure yours has.
Some of those also have built-in double or triple setpoints for different time shifts of the day.

[nike123]

Brilliant! That makes it a lot easier. When you say the tanks need 4 inlets, it that 4 each or 4 in total?
That depends on system configuration and how much of buffer tanks is needed/used.
As a rule 2 for chiller (feed and return) and 2 for each load. Positioning of inlets is also important to ensure proper water mixing.
That is job for someone with engineering degree and experience in plant construction.



Is it likely the chiller will have built in control to allow it to trigger at the set points? That is right!

[icecube51]

i think wat you need is a worked out plan of wat is needed to be child and how you gone get it there and so on.then you need to specify the tube's length and diam. then calculate the pomps,by that time you will now the amount of child water is needed to specify the tank capacity, and now if the chiller is big enough, or to big. than perhaps you can look if your system can be big enough for "cool thermal energy storage" which in that case whit the savings you have made for your boss, its time to ask for a raise;)

just an idea,

Ice