I am looking to install mini splits throughout my entire home for air conditioning and supplemental heat for my steam boiler in fall/early spring. I have received quotes from multiple reputable Mitsubishi diamond installers but confused on the condenser sizing I&#;m being told I need. In total I am going to need 8 heads and his quote is split between two condensers that are 36k btu each (MXZ-4C36NA is the model) but the totals for the indoor units for each handler is 39k and 45k btus? Guy is assuring me this is ok? Said &#;I have a diversified load, so they'll never really ever be running at full power all at the same time? In summer I will definitely be running all of these at the same time.&#;
Any advice on if this sizing sounds right is appreciated. Want to be sure this is right before I move forward.
Thanks!
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residential water-cooled air conditioning
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(Mechanical)
(OP)
18 Oct 07 23:05I was hoping someone in the HVAC industry could tell me why the residential market is almost devoid of any type of water-cooled condensing technology to cool homes. Water-cooled condensing is used extensively in the industrial segment because it is so much more efficient than air-cooled condensers. Seems logical to apply water-cooled condensing technology in residential as well. Any thoughts?
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(Mechanical)
19 Oct 07 01:40Why not indeed. According to experimental results, the obtainable COP at 75% nominal capacity is 2.98 and the maximum achievable COP is 5. These figures are much higher than that of the conventional air-cooled air-conditioners, which at nominal cooling capacity are in the range of 2.2 to 2.4. The results confirm the needs to promote wider application of water-cooled units in the domestic sector for energy efficiency. Water consumption which is the main obstacle for wider use of water-cooled system has been investigated. The rate, amounts to 0.38% of the condenser water flow rate, can ease the mind of most policy makers who have concern about water usage. The result of this study provides useful information for future studies on large scale application of water-cooled air-conditioners in residential buildings. In fact this trend is taking hold in places like Las Vegas.
(Electrical)
19 Oct 07 12:08 congo40There is a recent thread on this issue titled "Air Cooled Chillers vs Water cooled Chillers". You should read that. I am not sure wheregot his figures (you should give us a link to this study) since it all depends on the refrigerant, suction pressure, suction superheat, discharge pressure, liquid subcooling, and compressor design. If you follow the refrigeration cycle on a mollier diagram then you know that it doesn't specify whether the system is air-cooled or water-cooled, only that the temperature at which the system rejects the heat. I AGREE THAT IN MOST HOT CLIMATES THAT THE TEMPERATURE OF THE WATER IS COOLER THAN THE AIR. We also must be clear by what we mean by water-cooled. I believe that bothandmean water cooled systems where the water is recirculated and cooled by evaporative cooling. There are also one pass through water-cooled systems (sometimes used where there is a large source of surface water such as a river, lake, or ocean).There is a lot more to achieving energy efficiency than lowering the temperature of the heat sink. Most (if not all) residential and commercial air conditioners are DX systems which use a thermostatic expansion valve. These TEVs require a minimum pressure drop across the valve in order for the valve to function properly. If the condensing pressure is too low then the unit will not function properly. In fact many air conditioning units artificially hold the condensing pressure high with features such as hot gas bypass (very energy inefficient), condenser flooding, fan cycling, or other methods. There is no use in lowering the heat sink temperature if they system cannot take advantage of the lower temperature. There are balanced port TEVs and electronic TEVs which can minimize this problem but they are not widely used.In some areas of the country there is not much difference between the wet-bulb temperature and the dry-bulb temperature so there is not much advantage to evaporative cooled water condensing units. The other big issue is the cost of treating the water and the cost of pumping the water (both in dollars and in energy). A ground source heat pump (which uses the ground as a heat sink and does not evaporate any water) at leasts eliminates the treatment of the water and the use of water. The water needs to be pumped. Water usage in an evaporative cooled condenser is about twice the amount evaporated (you need to bleed the same amount as has been evaporated in order to prevent buildup of dissolved solids in the system). I have never calculated it based upon the rate of water flow through the system.I don't believe that water usage is the main problem with water-cooled residential systems. The main problem is capital cost and maintenance. If you really want to save energy in places where water-cooled systems give the most advantage (like Las Vegas) then you should just use an evaporative cooler or use an evaporative pre-cooled air conditioner. They work well, in fact my house has both an evaporative cooler and an air conditioner.If you want to actually calculate how much water (evaporative) cooled air conditioners will save you compared to air-cooled systems you can download the Carlyle (a division of Carrier) compressor rating software at http://www.carlylecompressor.com/corp/details/0,,CLI1_DIV24_ETI,00.html In this software there is a BIN analyzer which allows you to specify the degree of approach of the condensing temperature to the heat sink temperature. The heat sink is selectable as either dry-bulb (air) temperature or the wet-bulb temperature. You also put in the minimum condensing temperature (to take care of the problem of the TEV minimum condensing temperature). Select the location (or the location with a similar climate) and the system will go through the weather data hour by hour and calculate the energy used. The compressors that they have are more small commercial compressors but the COP's or EER's are basically the same as residential. After you do this then you need to add the additional water pumping energy, water treatment energy, and water usage energy to the energy used by a water-cooled air conditioner.Don't get me wrong, I am a big believer in evaporative condensers (for industrial systems) and cooling towers (for large building systems) but there actually are some reasons why air-cooled systems are used. It is hard to get small and medium commercial air conditioning users to go to water-cooled systems let alone a residential user. Remember that whenever I have mentioned "water-cooled" in the above I actually mean "recycled evaporative cooled water for the condenser".
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(Mechanical)
(OP)
22 Oct 07 11:12Thanks to both imok2 and gepman. I was curious about a couple of points brought up. Imok2: Is there more info that can be provided on "the trend in Las Vegas." I would like to find out more about this. Also, it would be interesting to get more detail on the experiemntal results you claim as gepman pointed out.
gepman: you also seem to know a great deal about this subject. What is your background if I might ask.
(Electrical)
22 Oct 07 13:16congo40
I am a registered mechanical and electrical engineer (California requires an engineer to take the test in each discipline in which they claim to be proficient), I have spent a longer time than I wish to say working for large food processing companies designing, installing, and commisioning large food processing plants in the electrical power, mechanical equipment (including ammonia refrigeration system design), and process controls disciplines. The last five years I worked for a medium size (200 engineers) consulting firm in the food processing, general industrial (hydropower generation, mining, manufacturing), and oil production fields. However the last six months I have been analyzing refrigeration systems (industrial and commercial) for California electric utilities. I probably visit and analyze two to three refrigeration systems a week (for free, paid by the electric utility) from little .5 TR hermetic condensing units to processing plants with TR using 750hp screw compressors.
(Mechanical)
22 Oct 07 15:23gepman,congo40, I refer you to a study performed in Hong Kong by a Professor W.L.Lee and Research Associate Hua Chen at the Hong Kong Polytechnic UniversityThe web site is the 4th article down http://www.google.com/search?q=water+cooled+resenditial+air+conditioning+&hl=en&ie=UTF-8&start=10&sa=N The4th article down
(Aerospace)
22 Oct 07 15:53A correct spelling of "residential" seems to pull up some more relevant links:
TTFN
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(Mechanical)
22 Oct 07 16:36Irstuff, Even with the wrong spelling we get the same links,
could it be because phonetically they sound alike? (smile)
(Electrical)
22 Oct 07 17:49I read the study. I don't disagree with the primary conclusion that lowering the condensing temperature will increase the COP. The study did not state but I hope that it included the power consumption for the water pump and the fan in the COP calculations. I have concerns with some of the statements and methodology in the study:
1) That the efficiencies of the units could not be predicted mathematically. I have most of the major compressor manufacturer's sizing software (Bitzer, Copeland, Danfoss (Maneurop) and it is relatively easy to determine the energy consumption given the condensing temperature.
2) It didn't describe the COP of the particular base air-cooled unit.
3) I am actually more interested in the degree of approach to wet-bulb. With this information one can determine if an air cooled unit could hope to match the performance of the water cooled unit.
4) The decrease in energy consumption was about 19.5% at best efficiency point. One would have to determine if the additional water usage and expense is worth the savings.
5) The unit was a through the wall 1 ton unit which is not much like a typical house unit in the US.
6) I think that they went to a lot of trouble to prove the obvious, that lowering the condensing temperature increases COP.
7) They did not address if they had a water bleed on the condensing water. Cooling towers require a water bleed which if they did not have would at least double the water use rate. Cooling towers also require water treatment.
8) They did not address maintenance issues.
(Mechanical)
22 Oct 07 22:12I shudder to think of the real-world fouling factor in a condenser served by a homeowner-maintained evaporative cooling system, used for maybe 3 - 4 months of the year and idle the rest of the year, not likely any chemical treatment, and god knows what kind of water in the first place. Anyone who has maintained a swamp cooler would not suggest something like this! I don't see any way the maintenance required would pay for the energy savings unless power was well over 15 cents/kWh.
(Mechanical)
22 Oct 07 22:29I believe you have some valid points and as I'm not involved in research I can only comment on what I read. As far as the unit is concerned it's not supposed to be like your common air cooled system, but be that as it may, you,me ,every one has some diferent opinion and depending on many factors in setting up the research one can come up with different conclusions.That said, I still believe that a water cooled system even with a small cooling tower is the best alternative. I appreciate you critical analysis and if you would like to persue your analysis further may I suggest you your concerns to the Professor he may be able to answer your questions.
(Mechanical)
22 Oct 07 22:39RossABQ The concept of water cooled system is not so far feched as fa as water treatment is concerned many homeowners take care of the chemestry of their swimming pools were not talking about a 100+ ton cooling tower here
I have been taking care of my pool for 20 years and it takes me about 5 minutes to check the chemicals. Now my grandsons take care of the pool...(for a fee of course)
(Mechanical)
23 Oct 07 17:35There are numerous manufacturers that manufacture small water cooled AC units, but these are typically used in high rise residential buildings with large cooling towers located on top of the building. Water cooled systems generally are not economically viable for system much less than 500 to kWr.
Water cooled systems imply cooling towers and cooling towers require water treatment to prevent corrosion, scaling, and biological control (including legionella). Most cases of legionalla originate in small poorly maintained cooling towers, spas, etc. unfortunately the size of the cooling tower is not proportional to how deadly the legionella can be is to die a 'little' from it they would still be totally dead!
Evaporative cooling of the air is very much climate dependant and only wok in dry climates. in hot humid climates you would get minimal evaporative cooling and the last thing you want to do is add more moisture to humid air inside a building!
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