Low Temperature Derate

[jpflyer]

Hello Everyone,

I'm a new member and not particularly technical, but I'm looking for some overview on a question.

I understand that a liquid chiller losses capacity as the leaving water temperature is decreased below the design output temperature. My questions are:

What is the mechanism that causes the reduction in capacity?

If the package is derated by 50% due to low temperature leaving water, does the system use 1/2 the energy?

What components in the chiller package would need to be changed to restore design capacity?

Thanks for your help!

[weeksy]

Hi,

As you mentioned you're not particularly technical, I thought I'd help by giving you an image.....this is how we explain stuff to salesmen

If a Ferrari has 500bhp and can travel at 180MPH, you wouldn't expect it to be able to reach 180MPH if it was going up a steep hill. If it only reached 90MPH up that hill, you wouldn't expect it to have done so using any less fuel either would you........same theory

The packaged chiller is designed to remove a certain amount of energy from an application. The key word here being energy, not temperature.

You may be able to modify the system slightly to improve capacity & efficiency, but you will not be able to make any truly significant increases to its capacity. To achieve this, you will probably have to up-size the full C/U assembly & fluid pump....which is near as damn it building a new chiller.

What temperature are you chilling the water down to BTW?

regards

weeksy.

weekstechnic.com

[jpflyer]

Thanks for the reply. We are cooling to 18F. As to the car example, I would expect the engine to expend 500 bhp in either case and use the same fuel. In addition, I would expect the engine to be doing similar work either way.

I'm not asking the chiller to remove any more or less energy, I just asking it to do the same amount of work at a lower temperature. Why can't the chiller remove the same amount of heat energy when that energy is at a lower temperature?

[frank]

You must also consider the flow rate of the cooled fluid. If your chiller was designed to cool say, 5kg/s of glycol/water mix (20/80%) from 30F to 18F (dt = 12F) then you could in theory increase the dt but at the expense of the flow rate.

It all depends on what the original calculations were for the chiller.

Remember, energy cannot be created nor destroyed.

[weeksy]

In order to achieve a fluid temperature of 18F (-7°C), the chiller has to use some of its cooling capacity, therefore it has less capacity remaining to remove heat energy at this lower recirculating temperature.

It has to continually remove heat energy to maintain the fluid at a temperature considerably lower than your ambient, so even without any heat-load from an application, the chiller is working quite hard already.

The higher the volume of fluid in the loop & the colder you go, the less "spare" capacity is available.

I assume you have a suitable "anti-freeze" in your fluid btw? The evaporator temperature will be considerably colder (dependant on gas type etc.) than your fluid temperture of -7°C.

I hope this helps.....if not, please provide all the relevant figures & I'll do the maths

regards

weeksy.

weekstechnic.com

[weeksy]

Frank said:

Remember, energy cannot be created nor destroyed.

answer in a nut-shell.....what he said




regards

weeksy

[jpflyer]

The question would be why a given chiller can (for example) move 1 ton of heat energy at 45F leaving water but only 1/2 ton of heat energy at 18F leaving water. Why can a given chiller cool a given quantity of fluid from 55 to 45 over some time, but not have capacity to cool the same quantity from 25 to 15 in the same time?

[US Iceman]

In an effort to try to keep this simple, let's just look at the compressors. The compressors are constant volume devices at full load and full speed. They pump the volume of gas no matter what.

The amount of refrigerant circulated (mass flow) by the compressors is based on the total compressor volume pumped and the gas density.

I am of course omitting some of the more complex arguments that are required to substantiate this.

At the original design point for water cooling the refrigerant gas density is "X". As the evaporating temperature is decreased (for the lower supply liquid temp) the gas density decreases to something less than "X".

As the evaporating temperature decreases, the mass flow also decreases. When the mass flow decreases, the capacity is also decreased. Conversely, the opposite is true if the gas density increases.

Any change in mass flow also causes subsequent changes in the performance of the other system components (chiller, condenser, expansions valves, etc.)

This is also compounded by the fact this is a chiller. The addition of a freeze point depressant (anti-freeze) also causes some other changes which can reduce the capacity due property changes (lower specific heat, conductivity and higher liquid viscosity - what you are pumping).

You might be able to get some approximations of the chiller performance from the manufacturer, but don't expect to get what I believe you are looking for.

[jpflyer]

OK, I'm starting to get it. If the pressure is lower, than the mass flow is less, resulting in less compressor capacity. With the exception of reductions due to the use of anti freeze, would the substitution of a larger volume compressor restore the nominal tonage? Are there delta issues related to the lower leaving temperatures in the evaporator that would need to be addressed with resized components?

[US Iceman]

Well, to be perfectly frank with you the whole system should be re-engineered to verify the performance of all components under the revised operating conditions.

What I mean by re-engineered is; the individual components can be reviewed for performance under the new conditions. Each of these should then be checked for some reasonable balance of operation when combined.

Otherwise all of the efforts are trial and error, which can be costly and frustrating and potentially embarrassing.

When you change one part (compressors as you stated), it can affect the operation of the other parts. If the changes are drastic enough (and it sounds like yours would be) you can create a real mess to fix.

If this was a commercially produced chiller, my suggestion is to call the original manufacturer to get their input before you attempt this. They should be able to provide you with the de-rated performance you are looking for with the existing equipment.

But as I said before, don't be surprised if you don't get the answer you expect. You could easily loose 30-50% of the nominal air conditioning capacity.

[frank]

Have a look at any compressor manufacturers selection data and note the capacity as the evaporating temperature decreases