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tonikh
17-02-2013, 01:39 AM
Hi all,

I would really appreciate your help with the following matter:

I have an ammonia refrigeration system made up of three circuits with evaporation temperatures of -10oC, -35oC and -45oC

Now what I really want to do is find out the refrigeration thermal load for the system, all I have is the compressors electrical consumptions meter readings in kWh measured on a weekly basis and a drawing showing for each of the 16 compressors in the system the Capacity (kW) and P. Inst. (kW). That is literally all that I have.

A bit as to what I am trying to achieve, I am not a refrigeration engineer and I don't need to be spot on all I want is an approximation of the amount of waste heat going through the cooling towers for the exercise that I am doing.

is there anything that you can recommend ? such as an overall COP I can assume ?
and is dividing the Capacity (kW) for each compressor by the P.Inst. (kW) would give me the COP for each of the 16 compressors ?

As I was thinking if I can manage to assume a COP supported by some argument or refenrece then I can multiply it by the electricity consumption of the ammonia compressors meter readings that I have in order to obtain my thermal load, am I right to think that ?

Thanks in advance !

The Viking
17-02-2013, 02:07 AM
So you want to see how much heat is rejected by your cooling towers?

So why start at the other end???

The easier, and more accurate, way would be to look at the water flow and temperature differential at your cooling towers.

:cool:

.

tonikh
17-02-2013, 03:11 PM
So you want to see how much heat is rejected by your cooling towers?

So why start at the other end???

The easier, and more accurate, way would be to look at the water flow and temperature differential at your cooling towers.

:cool:

.

Hey Viking,

Thank you for your post, however I don't have that data. Let me brief further, I have evaporative condensers on the drawings where the NH3 refrigerant enters in gas form and leaves in liquid form. I also have the total capcity installed for the evaporative condensers.

I managed to add up the evaporator load and compressors load in order to calculate the total heat rejection. Furthremore, I managed using manufacturers table to figure out the capacity factor for my condensing temperature and WB temperature. Then I multiplyed the capacity factor with my total heat rejection I previously calculated and I end up with a capacity for the evaporative condensers as assigned on the drawings. So, I know now I am on the right track and have an idea of my total heat rejection if everything was running at peak.

However, my compressors meter readings are showing that they are running at 30% of their peak load, is it fair to assume then that my evaporator load is also at 30% of the peak ? As this would make my life easy and would allow me to assume 30% of the total heat rejection. The problem the system is a cascade one and I am not sure if this logic applies.

Looking forward for your reply...

Nico
18-02-2013, 09:19 AM
It won work like that Tonikh, just a general COP, unless you have recip compressors.
Recips have relative constant COP down to 50% capacity Screw compressors screw up when not running 100% unless you have the new Vilter or VFD capacity control.
Furthermore you need to take into consideration the actual condensing temperature which is a combination of klimate and condenser control strategy. To our shame I have to admit that you should not be surpriced if your system uses like 30-50% more energy than strict necesary for today's state of technology. I guess that is about the same as 50 years ago when the operator was controlling capacity and condensers by checking the apm meters:-)

The Viking has the best approach for measuring the heat that you reject: read your water meter for main numbers.
One kg water that evaporates represents 2260kJ heat. Note that normally like up to 50% of the water supply to the evaporative condenser is drained to avoid too high level of mineral concentration.

Concluding, you need to do deep investigation on the actual running conditions and muliply that with the COP of the compressors.
You can find COP tables of any compressor at the compressor manufacturer's site.

May we know the reason behind your question? What purpose would that knowledge serve.

tonikh
18-02-2013, 02:12 PM
It won work like that Tonikh, just a general COP, unless you have recip compressors.
Recips have relative constant COP down to 50% capacity Screw compressors screw up when not running 100% unless you have the new Vilter or VFD capacity control.
Furthermore you need to take into consideration the actual condensing temperature which is a combination of klimate and condenser control strategy. To our shame I have to admit that you should not be surpriced if your system uses like 30-50% more energy than strict necesary for today's state of technology. I guess that is about the same as 50 years ago when the operator was controlling capacity and condensers by checking the apm meters:-)

The Viking has the best approach for measuring the heat that you reject: read your water meter for main numbers.
One kg water that evaporates represents 2260kJ heat. Note that normally like up to 50% of the water supply to the evaporative condenser is drained to avoid too high level of mineral concentration.

Concluding, you need to do deep investigation on the actual running conditions and muliply that with the COP of the compressors.
You can find COP tables of any compressor at the compressor manufacturer's site.

May we know the reason behind your question? What purpose would that knowledge serve.

Hi Nico,

Thank you for your reply, maybe if I clarify few things it would help you guys to help me out further. I am not a refrigeration engineer. I am a Sustainability Energy consultant and I work a lot with reducing energy consumption through waste heat recovery strategies in commercial projects. I recently started a research study and I am interested in the waste heat produced by industrial refrigeration systems.

The project I am talking about is part of my research and the system is for a factory, I am not going to do any refrigeration recommendation on it, it is simply not my field. I was provided some drawings and a list of meter readings, all I was hoping to achieve from that data is to estimate the amount of avialable waste heat from the refrigeration system. Once I can quantify the amount of waste heat, I can start my analysis as all I want to do is export this waste heat for other uses rather than having it rejected into the atmosphere.

Now, the system is too complicated for me and unfortunately I don't have any access to any refrigeration personel for this case study.

If I brief it to you guys maybe you can tell me in simple terms what is going on:
The system has three NH3 separators each at different evaporating temperatures.
Separator #1 which has the lowest evaporating temperature is receiving the suction line from the evaporator and then liquid NH3 is coming out of it and going back to the evaporator and also there is a suction line coming out of it and going to a series of compressors, then the NH3 gas from those compressors is going to sperator #3 which has the highest evaporating temperature.
Then seperator #2 which has the second lowest evaporating temperature has similar connections to seperator #3and the compressors it is connected to are sending the NH3 gas to seperator #3.
Now seperator #3 has suction line connected to it from evaporators, it is receiving NH3 gas from the compressors mentioned previously, it is sending liquid NH3 to the evaporators and it has a suction line connection going to a series of compressors, now half those compressors are connected to a cooling tower and all the compressors connected to this separator are sending the NH3 gas to evaporative condensers.

Does this system makes any sense to anyone ? There is also a NH3 reservoir.

So I think all what I am after is the rejected heat in the cooling tower and the evaporative condensers, right ? As well, if I want to divert the waste heat I obviously need for the NH3 gas entering the evaporative condensers its pressure, condensing temperature and flow rate, right ? and for the cooling tower I am assuming I need the water flow rate, the entering water temperature and leaving water temperaute, right ? This is all data that I never received.

Now at this stage I don't need to be accurate I am building the argument for my research proposal,I have found in the meter readings I have the flow rates going to the evaporative condensers and they fluctuate over the year between 70 l/s and 95 l/s. I have a book saying that a nominal value is that in evaporative condensers spray water circulating rate is 0.018 l/s per kW of heat rejection, can I do something with this for the time being ? the meter reading I have is before the water reaches the evaporative condensers whereas the book is referring to spray water, so can I divide then 70 l/s by 0.018 l/s to get an estimation or not really ?

Looking forward for your reply...

Thanks in advance for everyone's help

Segei
18-02-2013, 10:10 PM
The Viking mentioned about cooling tower. As far as I understand you have evaporative condensers. It is more complicated. You should measure air flow and air enthalpy in and out. it is not easy to do.
On refrigeration side. heat rejection= refrigeration capacity+compressor energy use. You can measure compressor energy use. For refrigeration capacity, you should measure refrigerant flow for each vessel(separator). It is not easy. Roughly you can estimate capacity based on compressors capacity. However, it is not easy as well. When capacity potentiometer snows 30%, it does not mean real capacity is 30%.