Chilled water system fault finding

[mechanicaldup]

Hi

What is the signs to look for if you want to determine whether a chiller is sufeciently size to handle its load?

What is the other signs to look for to determine an exist chiller's functionality?

 

[Ashereng]

Very generically:

1) What are the inlet temperature and outlet temperature like as a start. Is the chiller working for you?
2) Measure the flow. Is it sufficient for your needs?
3) Look up the original datasheet.
- is the original design flow rate still valid?
- is the process and chilling process fluids still the same?
4) Is this a case of fouling? The chiller used to work, but is now not so good?

Is this a design problem, or a not working in the field problem?

 

[mechanicaldup]

Thks Ashereng
for instance if your design return temperatur is 12°C, 13°C is sign that something is wrong?

 

[Drazen]

One very simple check of proper sizing of chiller aggregate is to watch at motor working cycles.

In chiller documentation it will often be shown what is considered normal motor cycle (how many minutes in hour it is working, how much of switching on/off in an hour). If there are big deviations from this in actual run, that will indicate oversized or undersized chiller.

 

[tombmech]

Most chillers have a current draw reading on the control panel. A chiller is sized to provide a specified cooling tonnage capacity. If that capacity is not met, but the current draw is close to 100%, then your chiller is undersized.

Capacity is measured with flow and Delta T. A higher return temp by itself means nothing - that could be an indication that the end-user coils (AHU's?) are not sufficient. The chiller may still be able to compensate. It's the temperature difference that's important. Nevertheless, it still has to be combined with flow (and technically the fluid properties, too) to get an accurate idea of the load/capacity.

Q = Cp*m*dT, where Cp is the specific heat of the fluid, m is the mass flow rate, and dT is the temperature difference.

To check rated capacity, you will need a load that corresponds to the peak value, or at one of the certified test points performed at the factory when it was purchased.

Large centrifugals are usually run constantly during cooling cycles, and the load variance is absorbed by the chiller turn-down capability. If you get surging, the current draw is always at 100%, and you cannot supply water at the desired temperature, then the chiller is undersized. Surging does not happen as often in low load conditions (less than 10-20% rated load) because the chiller's internal safeties cycle the machine off. If you get a lot of on-off's (short-cycling), then the chiller is tremendously over-sized, because you're driving it into a regime below its minimum turndown.

Anything inbetween, and it gets a little harder to tell. All things being equal, the current draw and/or failure to meet supply temperature is usually the basic indicator for chiller performance issues (undersizing, refrigerant loss, or fouling).

 

[Ashereng]

if your design return temperatur is 12°C, 13°C is sign that something is wrong?

The short answer is yes, you have a problem.

The real question should be what is the problem? It may not be the chiller - or it may.

As tombmech mentions, a single temperature is not an indication of anything, other than you don't have the temperaure you need. It's the temperature difference that is more indicative. If the temperature difference is suppose to be 5 degrees, inlet of 17 degrees and outlet of 12 degrees, and you increase the inlet temperature to 18 degrees, then the outlet temperature will be higher. Or, if the flow rate changes, well, the temperature in the outlet will also rise.

Other causes of course include undersized, refrigerant loss, fouling, change of medium, etc.

So yes, the outlet temperature indication is an indication that something is wrong with your process. Now, you need to determine if anything is wrong with the chiller.

 

[HVAC68]

Are you using the chiller for comfort air-conditioning ? If yes, is the space being cooled ? or do you have complaints ? Inadequate chiller capacity could be a problem, but need not be always.

It's a very generic question to answer. You should do basically check the complete system before you can arrive at an answer.

HVAC68