Cooling my A/C condenser - Page 1
Forum section: electronics / projects

author: DanaK
author_rank: Newbie
timestamp: June 24, 2012, 05:04:33 pm
content: We're well in to the HOT period of the year here in West Texas and I'm in the process of putting together a project that I've thought about for a couple of years now. I was tired of listening to our fairly new house A/C unit run and run and run at night and during the day even though we keep the inside thermostat set in the high 70's. The new unit is about a half ton more than the old unit was, not to mention more efficient, but that didn't achieve as much of a cooling increase as I believed it would. The installers assured me that it was well matched to our house's square footage. So, besides putting in more insulation in our attic, I thought putting a water mist assist in proximity to the outside condenser unit couldn't do anything but help get even more heat out of the house.I've put together a collection of parts in a weatherproof PVC electronics enclosure that includes a sprinkler system controller by Orbit, a wall box with a single gang wall socket for power and a relay who's 120v coil runs off of one leg of the 220vac power that runs the condenser unit and its N.O. contacts pass 48vac to the sprinkler solenoid when they close. The way the whole thing is supposed to work is the sprinkler controller has one section setting to run for the max amount of time which is 240 minutes. When that section times out another picks up until you get out of the heat of the day and things cool off to less than 95 or so. There's six sections available so this ought to allow enough time to run from about 11am to 8pm or so. The sprinkler controller puts out 48vac for the solenoid valve all the time its set to be on so the single leg of the condenser unit that is energized when the condenser is running closes the relay during the run and opens it when the condenser shuts off thus causing an interrupt in the power to the solenoid and shutting the water flow to the misters off when the condenser shuts down.So here's my couple of questions for anyone out there:1. A more elegant solution to this would be to replace the sprinkler controller with a thermostat but all the ones I know about work off of 24vac, not 48. Are there any out there that are affordable which use 48vac?2. I wonder about 48vac feedback to the sprinkler controller on the legs to the other sections not running at the time for the max amount of time. Would this cause a burn out?3. I was wondering about using a relay that senses 120vac rather than actually tapping a 120vac source for isolation but those relays that I've found are pretty expensive. Does anyone know of some out there in the $20 to $30 range? I'm on a budget here...

author: SeanB
author_rank: Super Contributor
timestamp: June 24, 2012, 05:47:50 pm
content: Spraying water onto the coils will cool them, but will corrode them to nothing in under a year. The coils are copper, with the fins being aluminium, and this makes the perfect electrochemical cell when wet to corrode the aluminium to powder in short order.Better is to make sure the outdoor unit is raised at least a foot off the ground on a firm stand, and is a minimum of 3 feet from any wall. Place a roof over it, with at least 2 feet of space between, and big enough that there is a overhang of at least 3 feet as well on all sides. If it touches the house wall then leave a foot of space there as well. Keep all vegetation at least 5 feet away, and make sure that it is all well shaded.Keeping the outdoor unit cool is good, but you must keep it dry as well. Adding a water mist will be very bad for it, in many ways.

author: DanaK
author_rank: Newbie
timestamp: June 24, 2012, 06:05:17 pm
content: Hmm. That sounds pretty logical. We have very hard water here so I was also wondering about calcium deposits forming and blocking the fins. The calcium has really done a number on the fixtures in the house. Just had to replace the float valve in the master bath due to this.I was wondering, though, if I kept the mist far enough away would it be effectively fully evaporated before it gets to the coils and the additional cooling achieved? The condenser unit is right under the eve of the house and has had water falling on it through all of the rains we've had for the last 5 years. There's also a euonimus (I know I spelled that incorrectly) hedge within 3 feet of it at a 90 degree angle but I thought water evaporating off of it would feed the cooler air into the unit more effectively.The exhaust from the unit blows up against the eve of the house and recirculates through the condenser so I was also going to put a flat panel on the top of the unit to redirect the exhaust away from the eve. Just haven't got the material for that yet.The unit has not been placed very well but the installers were working with the location of the old one and didn't have much to work with.

author: SeanB
author_rank: Super Contributor
timestamp: June 24, 2012, 06:09:49 pm
content: Better is to sort out the location of the unit before anything else. You need airflow through the unit, and this has to be clear on input and output sides. What unit is it anyway?

author: DanaK
author_rank: Newbie
timestamp: June 24, 2012, 06:13:43 pm
content: The condenser unit is a Ruud Achiever unit built back in 2004. I'm pretty sure it's a 5 ton unit.

author: SeanB
author_rank: Super Contributor
timestamp: June 24, 2012, 06:25:41 pm
content: Big unit, at least for a single split. Get the airflow right as a first thing, and place some shade on the outdoor unit. This will help a lot to keep it cool. If you drop the air coming in by 10F and drop the case temp by the same it will be a much better unit cooling wise. The thing with rain dripping on it is thast it stops, and then the hot coils then dry out. misting them keeps the moisture in intimate contact deep in the coil.It will help too to rinse out the coil while the unit is off, using a gentle spray from a hose ( not a high pressure cleaner, as this will buckle the fins, just a fine spray that you can bear easily on your skin) and rinse the accumulated dirt from the fins. You should be able to see light through the fins somewhat, if not then they are blocked. If they are the type that looks like thick tinsel ropes then you are SOL if they are blocked, you cannot clean these easily.

author: DanaK
author_rank: Newbie
timestamp: June 24, 2012, 06:35:47 pm
content: I have the fin type cooling coil thank goodness! Can't imagine the tinsel type coils lasting too long in production as they'd be proven too much of a design "fail" right off the bat. It's been a long while since I've seen one of those. Think it was a window unit but it's been a while.I do clean my coil every spring with a garden hose spray head and the unit usually stays idle throughout the winter months. When it gets dry and dusty around here the coils certainly do plug up. We also have a lot of live oak leaves that accumulate in it over the winter so that needs cleaning out as well. It's a couple of shovel fulls!I'll work on that air flow problem for sure. It may also be a good time to have the service people out to check the coolant level and make sure it isn't leaking.Thanks.

author: SeanB
author_rank: Super Contributor
timestamp: June 24, 2012, 06:52:16 pm
content: I live near the sea, and certain coils only last 2 years before they turn into bare pipe and a mass of small aluminium pieces. Replacement is then cheaper than a new unit, so I do it, even though the coil is nearly 1/4 of the price of the complete unit at trade price. I normally take 3 days to do them, as i paint the base while it is apart, and do a lot of rust prevention on the rest of the unit.

author: DanaK
author_rank: Newbie
timestamp: June 24, 2012, 07:14:16 pm
content: Yes, I finally saw your South African flag there. You DO live in a very different environment than I do, for sure! I guess the salt in the air there messes things up like it does cars in the North Eastern U.S. states with their anti-icing treatments they put on the roads up there. You never see a vehicle there over ten years old that isn't a pile of rust! Out here in West Texas I have old pipe and drill stems from the oil field lying half buried in dirt that's accumulated around them over the years at the ranch. We use it for building frames and fences but it gets its coating of rust and then just sits with no further accumulation for decades. There's an old metal wagon frame in one pasture that I know is at least a hundred years old I want to pull up and preserve some way but it's still 90+% there.The entire southern point of Africa seems to be fairly well forested. I've always wondered at what point inland it became dry there like it is here.... Heck, what TIME is it there? Are you a night owl?

author: Simon
author_rank: Unknown
timestamp: June 24, 2012, 07:46:45 pm
content: Are we all certain this is the copper coil type with aluminium fines ? there are also condensers that are made in a similar fashion to automotive radiators out of aluminium tubes and fins (and "tanks"). It might be worth putting in two condensers ? but then I know you can't just change thing in AC systems without knowing what your doing as it's a funny old world with the same substance in a system being gas and liquid.

author: G7PSK
author_rank: Super Contributor
timestamp: June 24, 2012, 08:48:44 pm
content: I have often wondered why plate condensers are not used in air conditioning systems. A few years ago I made some essential oil distillation equipment and a large plate condenser was used. it was fed with water at ambient temperature from a large tank (45,000 Gallons) through a 4 inch pipe the steam was fed into the other side via a 4 inch pipe at atmospheric pressure. When the system was purging the air coming out of the steam exit was so cold that if you held your hand in the blast it would get frozen.

author: DanaK
author_rank: Newbie
timestamp: June 24, 2012, 09:54:20 pm
content: Hi Simon and G7PSK!Simon - that is one thing I do not know for sure about the make-up of the condenser coil. The fins are painted black and I don't remember what the tubing looked like the last time I was into the unit.As far as adding another condenser on it or extending the condensing tubing I'm pretty sure that extra add-on isn't offered over here. At least I haven't heard of it.I'll be honest about something here and say that this project is something I haven't discussed with the A/C installer, even though we haven't talked since it was installed. Talking about something like this is like talking to your doc about herbal remedies. It's pretty much of a slam-dunk that you're going to get shut down hard with threats of warranty violation and such. The unit is out of warranty so I feel pretty safe on that count, though.G7PSK - I'd like to know more about the plate condenser. What was the steam source you were working with? I was a machinist mate in the Navy so I understand the steam cycle as well as the refrigeration cycle. Is this basically just a flat metal plate dividing a cooling chamber with water on one side and steam condensing on the other? Was there a choke point on the steam side that may have acted like an expansion valve in a refrigeration expansion coil?

author: IanB
author_rank: Super Contributor
timestamp: June 25, 2012, 12:40:58 am
content: I'm not sure whether the "plate condenser" is going to be applicable here. This kind of heat exchanger is a totally different design from the one used in domestic A/C units and you can be sure the designers didn't ignore an obvious improvement in performance just for the heck of it. Remember there are all sorts of rules and regulations encouraging A/C units to be as efficient as possible these days.As a chemical engineer I can tell you it is easy to get all sorts of wrong intuition about the performance of heat exchangers. There are many "traps for young players" in this arena.Fundamentally your A/C condenser unit is going to be limited by the mass of air the fan can move through it and the temperature of that incoming air. The earlier advice is sound. The unit needs to be positioned so that the supply of air is as unconstrained as possible (the unit should be free standing off the ground, far from walls and other obstructions). The unit should be shaded so that the heat of the sun does not undo the cooling effect of the air. The air close to the ground is hot, so elevating the unit will find cooler air. The unit should be kept clean and free from blockages so that the air can move as freely as possible through the fins.Once that is done you need to look elsewhere for improvement. Try to make sure the house is as insulated as possible (not only attic, but also walls and windows). Double glazed windows will keep heat out in summer and heat in in winter. Put awnings over the windows to prevent sunlight getting in.Here in California I have noticed that houses are built with all the substance of a tent. The walls are paper thin and drafts are everywhere. Keeping such a dwelling warm in winter and cool in summer is near enough impossible.

author: Unknown
author_rank: Unknown
timestamp: June 25, 2012, 12:59:16 am
content: Commercial wet pack assistance devices have been on the market for a long time they do and don't work depending on situation. They are never a replacement for a properly designed air cooled condenser. Evaporitive cooling for your condenser.A simple sprinkler may get you through a few over design days, but the water consumed to achieve useful results will be substantial. Even recirc system willuse a considerable amount of water.Recirc cooling pads will always reduce airflow, most condensers fan systems are incapable of fulling or pussing through much more static than the coil itself.Corrosion is a problem but generally not a huge one for well designed outdoor condensers.End of the day you issue is you need a large enough mass of whatever to transfer energy into. Plate heat exchangers are common solution, they efficiently transfer heat away but you still require somewhere to dissipate it. In miserable climate like the UK co-gen systems can quite effectively reuse condenser waste heat for water and space heating needs in other parts of a complex. Not much help in TX or AU or SA though. Same applies to heating swimming pools unless they are substantial in size.One project I work on quite effectively uses the (substantial) decorative fountains and water features to dissipate A/C heat, it works really well, but is not something that is replicable in a domestic situation.Like most thinks in life and engineering there is no free lunch, the only real solution is correctly sized condensers and adequate air and water flows both of which come at a price. There is lots can be done but none of it will be cheap to achieve.

author: Unknown
author_rank: Unknown
timestamp: June 25, 2012, 01:27:29 am
content: Quote from: IanB on June 25, 2012, 12:40:58 amThere are exchangers commercially available, but they wont be cheap. neither will running additional water pumps etc. They are common here for pool heating and hot water. Efficiency will not be the issue, price will.QuoteToo true! Equally there are many design considerations needed in the design of efficient air conditioning. Despite the plethora of appalling garbage split and package systems being sold.QuoteThere you go, you're doing it again! Comfounding the polishing of turds by applying science to the problem.QuoteAll true! Air cooled condensers can be designed for all types of environments too. 50°C air on etc. The keyword being designed, it is not a matter of fitting the biggest condenser you can find. Over condensing creates just as many problems for a system.QuoteThere you go with all that common sense again. And when passive measure can do no more and mechanical means of cooling is required design considerations apply. No POS split console is going to provide sufficient air changes to effectively cool anything but a tiny space, regardless of what the Coolermaster2000 salesman or some retired cricketer told you. Undersized inverter systems run from dawn to dusk aren't going to save a lot of electricity either.QuoteThat is because of two factors, number one CA like East Coast Australia has a warm but relatively mild climate. Sure it gets hot and cold, but the reality is humans can an do survive in outdoors in cardboard boxes. The over ever present factor is home design is price. These places are mostly flogged of by developers who don't care about the efficiency of heating or cooling the thing. Price driven as always.

author: Simon
author_rank: Unknown
timestamp: June 25, 2012, 06:06:23 am
content: I'm guessing plate condensers which I assume is what we call plate and bar at work are not used as they are horribly heavy compared to other types particularly if bought from some Chinese suppliers.You can see if you condenser is aluminium or copper tube and aluminium. Have a look at it, An all alumunium unit will be flat about 20mm deep or maybe more than one 20mm unit stacked. It will also be large like 2 foot square and is best suited to being blasted by a large fan.The copper tube and aly fin type tends to be deeper and often of a smaller area, it will also have capillary tubes coming out of each circuit and joining externally into a "distributor", the tubes will be going through the fins rather than the fins being in rows between the tubesI can out photos us later if you want

author: IanB
author_rank: Super Contributor
timestamp: June 25, 2012, 06:32:39 am
content: By plate heat exchanger I understand something like this:http://www.process-cooling.com/Articles/Feature_Article/4f4dbdedf75b7010VgnVCM100000f932a8c0____However, these have many tiny channels and are not the easiest thing to squeeze a large volume of air through.Consider that our A/C condenser is air cooled and air has a very large volume for a given mass. Since the ability to remove heat depends on the mass of the fluid, the air cooled condenser must shift very large volumes of air through the system to carry the heat away. This is done by having big fans and large open areas for the air to flow through. You can see this in practice, for example in this document:http://www.mcquay.com/mcquaybiz/literature/lit_ch_ac/IMOM/Iomacaq.pdfAlthough plate heat exchangers can be very efficient, they do have limitations on exactly where they can be used.

author: Simon
author_rank: Unknown
timestamp: June 25, 2012, 07:01:39 am
content: ah I think what you refer to as plate cooler is a dual circuit job which we at work tend to call heat exchangers as they use two seal circuits and a cooling medium although technically any radiator is a heat exchanger. Yes they are much ,more compact and space efficient.

author: IanB
author_rank: Super Contributor
timestamp: June 25, 2012, 07:13:11 am
content: The typical A/C condenser is very similar in concept to a car radiator. Indeed they are both in fact air cooled heat exchangers.

author: T4P
author_rank: Super Contributor
timestamp: June 25, 2012, 07:54:40 am
content: Quote from: IanB on June 25, 2012, 07:13:11 amEspecially turbocharger intercoolers

author: Simon
author_rank: Unknown
timestamp: June 25, 2012, 08:10:10 am
content: Quote from: DaveXRQ on June 25, 2012, 07:54:40 amno actually the manufacture and requirements are different. An intercooler is typically made with plate and bar methods and the tubes are wider than in a plate and bar oil cooler as high flow rates are essential to not limit the air flow of air you just just spent energy compressing.On the other hand a condenser needs a specific flow rate that must not be too high or low, if too high the gas will get through before condensing

author: G7PSK
author_rank: Super Contributor
timestamp: June 25, 2012, 10:15:50 am
content: This is the type of plate condenser I am referring to.http://www.alfalaval.com/solution-finder/products/alfa-cond-plate-condenser/Documents/alfacond.pdfhttp://www.canaley.com/canaleypages/alfaphe.html

author: Simon
author_rank: Unknown
timestamp: June 25, 2012, 11:42:31 am
content: they are water cooled and not cheap, I doubt they are used in home instalations

author: G7PSK
author_rank: Super Contributor
timestamp: June 25, 2012, 09:25:27 pm
content: Quote from: Simon on June 25, 2012, 11:42:31 amIn my original post I wondered why something along these lines was not used. they would not cost as much once they were mass produced and air ti air works very well to. I once used some heat exchangers that were all plastic and and produced remarkably cold air with just two one and a half kilo watt fans.

author: Unknown
author_rank: Unknown
timestamp: June 25, 2012, 10:02:06 pm
content: Quote from: G7PSK on June 25, 2012, 09:25:27 pmWhat the OP is referring to as a condenser is actually acondensing unit. A little more than a coil and a few fans. There is a compressor and some other bits in those common dustbin boxes. The coils in them are relatively efficient as is, albeit generally undersized during selection.

author: NiHaoMike
author_rank: Super Contributor
timestamp: June 26, 2012, 03:24:47 am
content: Try painting the roof white (or at least a light color) if you can. Also shade windows letting in direct sunlight.The idea of a "hybrid" A/C is a great one but converting a regular one isn't going to work well since it's not designed for such low condensing temperatures. Done properly, however, it would work very well indeed. My friend Brittany Benzaia developed a hybrid A/C that uses a switched reluctance centrifugal compressor so it can use water as a refrigerant. (There's a sealed loop as well as an open loop.) The fact that the compressor is free flowing when off means that "bypass mode" (evaporative cooler only) is done by switching off the compressor, and there's almost no transition between bypass and normal mode. She claims that in Arizona (very hot and dry for those who don't know), it gets the equivalent of 40 SEER.As an aside, A/Cs originally used toxic chemicals like methylene chloride and ammonia. Because of the obvious safety issues, they found that CFCs were safer alternatives. Then came the ozone depletion problem and they went to HFCs. Then they claim that HFCs cause global warming, so now they're trying to find further alternatives. (The issue with HFCs in air conditioning is moot when aerosol products put orders of magnitude more HFCs into the atmosphere than leaky A/C equipment.) Water (R718) makes a great refrigerant for air conditioning, except its low vapor pressure calls for a high volume flow rate at a given capacity. Direct off line induction motors are limited to 3600 or 3000 RPM, meaning the compressor would need to be impractically large to get a useful capacity. Inverter driven switched reluctance motors (or any inverter driven motor for that matter) are not subject to that limitation and can operate at very high speeds. The compressor in the hybrid A/C can rev as high as 100kRPMs or more if it needs to.