8 replies to this topic

[ShaunHill]

To clarify:
I'm not sure about how you've expressed the chiller efficiency in kW/ton. kW are units for flow of energy (power). Ton (or tonne in metric) is a unit of mass. I'm assuming that what you want to look at is either the energy extracted for a specific mass (kJ/tonne) or the flow of energy associated with a particular flow of mass (kW/tonne/day or kW/tone/hr).

Having said that, in the rest of my response I will assume that you have your units correct.

The value that you give as an efficiency looks like a design value, and it doesn't mean that higher efficiencies aren't attainable. The only thing to check to make sure your values are resonable is to perform an energy balance around your heat exchanger. The fact that you use a methanol/ water mixture (obviously since water will freeze at the temperatures you've given) just means that you need to consider a different heat capacity than for pure water.

As to what is "optimal" efficiency, I would think you would want to get as much chilling duty as possible. Whatever duty you take out of the chilling fluid, you will have to put back in later. The adavntage to getting more duty per mass is that you can have lower pumping rates and save energy there. There are only two limits I can think of, which are the temperature approach on the heat exchanger and the velocities required to maintain effective heat transfer.

[Art Montemayor]

Mike:

I believe your confusion stems from your use of terminology that is not used in a clear or accurate manner. This causes confusion on your part as well as that of others you communicate with.

Efficiency is a measurement of a consumption (usually energy) relative to an accepted (& identified) base. For example, you can measure the amount of total energy your refrigeration system consumes and compare it to the theoretical, perfect thermodynamic consumption required by a Carnot cycle. You can use any base you desire, as long as you identify it.

What you are stating are unit consumptions of energy, not efficiencies. This is what tends to be confusing. You also fail to identity what "ton" you are referring to -- a mass ton or a "refrigerating ton"? This additionally causes confusion. I believe you mean a refrigeration ton. Ben Thayer is correct in identifying the normal use of a refrigerating ton. This is quite conventional and usual in the refrigeration industry - either applied in chemical plants, phamaceutical, or food processing -- at least in the USA. It is not used in SI unit countries or environments.

Shaun is correct in pointing to the use of unit consumptions as not being representative of "efficiencies". To be "efficient", a machine must be related to a base or a standard measurement. Efficiency is inherently relative - not absolute, like unit consumptions.

I hope this helps clear up what you presented.

[Art Montemayor]

Selvan:

I'm afraid I have to contradict your rankings on the relative efficiency of the three type of compressors you have listed. I have used all 3 types, in various applications and sizes and can attest to the following:

1) The most energy efficient refrigeration compressor continues to be the positive displacement reciprocating compressor - specifically, the 2- stage models with an interstage economizer. No other type of compressor comes close to the mechanical efficiency and low unit energy consumption (KWH consumed / ton of refrigeration output)

2) Right behind the reciprocating compressor ranks the oil-flooded screw compressor. Here, the energy required to pump, cool, and separate the required oil needed to "flood" the screw must be taken into consideration because without it, the machine is very inefficient and can't compete.

3) The centrifugal compressor is the least efficient in converting electrical energy into mechanical/refrigeration energy. This is well documented and can easily be found in the literature. Besides, it is to be expected. It is the tradeoff that one must pay for gaining simplicity and less maintenance costs.

I didn't want anyone to get the wrong idea about the basic characteristics behind each of these machines when employed in a refrigeration application. There are good, engineering reasons for picking any one of the three for a certain application. But an engineer must take into consideration each of the vital and important tradeoffs as well as the advantages that each machine brings with it. Volumes of good books have been written on the subject and I highly recommend all Chemical Engineers to strive to familiarize themselves with each one of these machines and their mechanical features and drawbacks.

I hope the above helps in explaining what I have tried to point out.

Art Montemayor
Spring, TX