COP clarification 1

[MineAutomat]

Hello all HVAC Engineers,

I have done some research on this site regarding COPs as I am trying to theoretically estimate the operating power consumption of our 6,000 BTU A/C units.

One particular discussion (thread403-136672) mentions that COP, defined as cooling effect/power input, varies from model to model and is variable depending on conditions.

Based on this, I found the manufacturer info for our units hoping to find the COP. I have 2 COP values: COP Dehumid. (pts/hr)and COP Dry Air (CFM). Would manufacturers normally publish the COP for cooling effect/power input or is this an optional criteria? Is the COP related by MintJulep in the abovementioned thread only viable for larger cooling units?

I sent an email to the manufacturer requesting more details on our models. Am still waiting.

I am trying to figure out if these units (we have more than 400 on site) could be replaced by one or a few central systems for the benefit of cost and electricity consumption reduction. I don't want to use the rated amps as I know the calculation will be way off current operating conditions.

However, I'd like to start with the "theoretical calculations" route before measuring actual power draw... and even if I have my power draw data for all 400 units, how will I be able to choose a new system; how can I know if it will use less electricity?

Any thoughts?

 

[gepman]

MineAutomat

I haven't heard of those two types of COP. Do you have some type of specialty A/C equipment (or is it a through the wall unit?).

The real simple answer is just to compare the EER of your unit to a new unit. I just recently analyzed a rooftop system with two 75 TR VAV units that had a 9.8 EER. Most small package units will have a larger EER which means it (supposedly) is more efficient but you need to understand the conditions at the rating point and compare them to your conditions and the nature of your operation. The complicated answer follows:

I thnk that you will be wasting your time trying to theoretically calculate the energy consumption of a small package unit. The reason is that the COP (usually these small units are rated in SEER or EER) is at a particular condition (usually specified by ARI for these small units). The COP for a mechanical refrigeration system depends on the suction pressure, discharge pressure, suction superheat, liquid subcooling, condenser fan operation, and supply fan operation. You will probably NEVER get this information out of the manufacturer. Then you need to match your indoor load to outdoor weather conditions (which affects condensing pressure). Your 6000 BTU units (are they really .5 ton?) are most likely CV (constant volume) units which are either on or off.

You will also find that the SEER or EER of small units tend to be better than larger units. This is partly due to the ARI rating conditions for the units and partly due to the fact that the manufacturers design for the best efficiency at the rating conditions and NOT at normal operating conditons (depending on what part of the country you are in). Normally you will not see a lot of small units on a large building unless it is large flat warehouse. Even then you won't find a lot of .5 ton units but maybe 25 to 75 ton units. What kind of application do you have anyway?

A larger system is usually easier to analyze since the manufactuer publishes more data. For a large system you will need to calculate efficiencies at part load.

You would be much better off taking a few typical representative loads and then datalogging the power usage along with whatever is causing the cooling load (people, weather?).

To summarize I think it would be better to datalog some power measurements (along with ambient temperature and perhaps occupancy [there can't be too many people in a room with a .5 ton air conditioner]). My guess is that it won't be worth the cost since if you have 400 units (and if they are separate spaces and not a common space) then you will have a lot of ducting (or piping if it is a chilled water system) to do.

 

[MineAutomat]

... yeah I think you are quite right. I was not aware of the amount of factors influencing the COP. The EER is 9.5. I think those units are old generation though.

The application is a mine camp located in tropical South America, less than 500-ft above sea level, very humid environment. There are more than 400 rooms, divided in 20 camps, all flat buildings. I want to reduce, as best as I can, the power usage of these units.

The rooms are quite small and most times, there can be 2 people in a room at any given time. The 0.5 Ton does the job quite well. I had not thought much so far on a large-scale unit. I am not familiar with HVAC design so I was trying to figure out our total A/C load for the camp and go from there. But I can see now that it would be quite the job to install one central unit... I'll keep this in mind.

I know it will be a tough battle and am trying to throw any ideas on the table, am also starting to investigate solar power but since we are at a mine, and life-expectancy is not long, it may be tough to justify the cost of initial investment...

Thanks for your post. Very informative. One star for you.

 

[gepman]

MineAutomat
I can tell you based upon your application that you have the best system. If you put in a central system besides ducting and/or piping you would have to put in controls for each room.

To minimize energy you should obtain the units with the best EER and put in some kind of occupancy sensor or setback thermostat so the units will not operate when the miners are out of their rooms. That will save you the most money. They probably leave them on whenever they leave so they can come back to a cool room.

There is a solar powered air conditioner that I have investigated but it will cost about 10X that of a regular through the wall unit.

 

[emanager]

You mentioned that you have 6000btu units. What is the amperage of the units when the unit is fully running ?
If your voltage is 120v and the amperage is, for example - 5 amps then power used is 5 x .120 = .60 kw for 6000btu or 1.2 kw per ton which is the same as an EER of 10.
Compare the EER of the units youre intending to purchase with the measured amperage. If your existing units arent producing as much cooling because they are old and possibly corroded your probably not even getting 6000 btu.

Maybe someone else can enlighten both of us as to how to calculate the actual BTU by measuring the refrigerant delta.

 

[gepman]

emanager
I am not an expert in the testing arena but ASHRAE has written required testing procedures. At least for small air conditioning units the way that they are measured is to build a well insulated well sealed test chamber and recirculate the air within the room and measure the amount of electricity that is used to have a heater maintain a constant temperature while the air conditioner is running at full load. Obviously this is impractical for the average user.

It is relatively easy to measure the suction and discharge conditions of the compressor (i.e. suction pressure, suction temperature, discharge pressure, condensed liquid temperature) so that the refrigeration effect can be measured. The hard part is measuring the refrigerant mass flow.

I have always tried to estimate the output of an air conditioner by measuring the enthalpy of the air before and after the cooling coil and measuring the airflow. On a small air conditioner like a through the wall unit you won't be too accurate since there is no straight duct but at least you can get an estimate and if the manufacturer's literature gives a real cfm rating then you are in better luck. After taking these measurements you can then use a psychrometric chart to calculate the net cooling affect.

It won't match up very well with the EER rating of the unit since you are almost never operating at the rating conditions.

 

[RossABQ]

The real question is not the COP of the refrigerant system; it is the energy of the HVAC system. I doubt in your climate there are many complaints of too-cold rooms. But the total energy costs of a central system needs to be the basis of your comparison, and as noted above that includes costs of fans, ductwork (with heavy insulation and vapor barrier), VAV's for control, etc. I can't believe that you could do significantly better than you've got. Keep in mind that with your present system, at most 2 people will be banging on your door if something breaks; you don't want to take out 100 people at a shot, do you?!

 

[MineAutomat]

All, sorry I was out of the office for the last week; thank you for your valuable input.

The occupancy sensor is a great idea, I was thinking more about timers but both working together would be a nice addition, good thinking. I will research those. Plus, I will ensure the insulation is adequate.