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AC condenser question

  1. Aug 20, 2010 #1

    Ivan Seeking

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    In a typical full-sized home AC system [central air] is there a problem with oversizing the condenser? In particular, the idea is to add a pre-cooler just before [above] the condenser. This would provide additional cooling as well as increasing the volume of the system.

    I tend to assume that the additional cooling capacity wouldn't be a problem as the expansion valve would still continue to regulate the temp of the expansion coil. Also, presumably the condenser fan would only operate as/if needed, so the precooler could go on or offline without causing any problems. Yes? No?

    Not sure about volumetric concerns. My inclination is to assume that there is a new proper charge level [ozs of freon], and everything would otherwise operate nominally.

    I need to steal heat from the condenser. Unfortunately, I can't say any more about the specific goal at this time. But I can describe what happens functionally.
     
    Last edited: Aug 20, 2010
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  3. Aug 20, 2010 #2

    russ_watters

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    Correct on basically everything. The condenser fan runs on head pressure, so if you precool the refrigerant (basically add another condenser section), you drop the head pressure and the fan never turns on. You do want to make sure your precooler isn't too oversized though becuase it is possible to drop the head pressure so low that your evaporator is no longer fully evaporating the refrigerant - then you get liquid going back to the compressor.

    Remember, the compressor is (typicall) inside the condenser, so there isn't a real easy way to try to pre-cool the refrigerant before it hits the refrigerant coils. I've kicked-around a few schemes for precooling (such as using the heat to pre-heat my hot water), but the fact that I'd need to rip apart my condenser and re-pipe the refrigerant snaps me back to earh.
     
  4. Aug 20, 2010 #3

    Ivan Seeking

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    Thanks, Russ. Clearly this is right up your alley.

    So I guess the trick is to estimate the difference in the amount of heat removed with the condenser fan running, as opposed to not running, and then size the heat exchanger to that BTU load, as a max?

    Wow, really? I've never seen such as setup. How bad is this? I was allowing for the copper work, but how bad is it to get to the compressor discharge? At this point it becomes a cost issue. I have about a $2000 budget for the install.

    Btw, this is not for a hot water preheater.
     
    Last edited: Aug 20, 2010
  5. Aug 20, 2010 #4

    Ivan Seeking

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    The geothermal systems come to mind - the ones that use the ground temp for condenser cooling. What do they do in this case? Are these a/c and heat pump systems unique in that they use an exposed compressor? Or are they modified at the factory to allow for easy connections?
     
    Last edited: Aug 20, 2010
  6. Aug 20, 2010 #5

    Ivan Seeking

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    Also, is there an easy way to determine the minimum acceptable refrigerant temp going to the expansion valve. Wouldn't the expansion valve simply close if the temp got too low? Or would this have to be calculated according to some minimum refrigerant flow value, and the min heat load on the expansion coil?

    Do these systems typically use hot-gas bypass, or variable speed compressors? I see many listed as variable speed, but I don't know if that relates to the duct air flow velocity, or the compressor speed. I assume this refers to air flow?

    PS: This is a worthy project and not a matter of fast cash for me.
     
    Last edited: Aug 20, 2010
  7. Aug 20, 2010 #6
    I'm an ex Trane employee and can give you a little of what I learned concerning this prob.

    The predominate capacity of your system is determined by the size of the compressor. It has a volumetric flow rate that determines how much gas is pumped.

    The gas just coming off the compressor is superheated. If you brush up against the outlet line, you'll get a very nasty surprise. So, the initial job of cooling the superheated gas is easy - it's hot and even with a poor heat exchanger, it will easily dissipate into the air.

    A combination of gas and liquid is present in the condenser's heat exchanger, so foam forms and creates loss due to the pressure drop. AC manufacturers deal with this by placing several condensate lines in parallel. Typically 4-5. They then squeeze a tiny bit more efficiency by running the cooled liquid through a final circuit to cool the liquid refrigerant slightly.
    Heat exchangers for geothermal apps typically have a cylinder with several coiled pipes within. The refrigerant is in the cylinder and the water flows within the pipes.

    The expansion valve on a good machine reacts to the outlet pressure as well as the temperature on the return gas from the evaporator coil. Ideally, the gas leaving the evaporator will be a few degrees warmer than its boiling point.

    Some energy is thrown away at the expansion valve. As the gas goes through, there is an increase in entropy, but no change in enthalpy. A little turbine there could likely pull out 40-100 watts of electricity (I forgot the value...). At the same time, the gas would be cooled proportionately.

    An old technician's trick is to purchase an over sized condenser from salvage and swap the compressor with a smaller one. This decreases the head pressure, decreasing the load on the compressor / electrical power.

    It's well known in the trade that builders do a horrible job with the duct work. Any given home is likely losing between 20% to 40% of their AC capacity due to poor design / workmanship. Crawling through the attic, looking for leaks, kinked ducts, poorly insulated ducts, and even ducts that were just left open, will help.

    Oversize AC leads to waste. Humidity is much better regulated with an increased duty factor. Also, there is a loss associated with each on/off cycle.

    Finally, one of the most efficient, most comfortable systems you will find with be a variable speed, multi evaporator, mini-split system. Think Japanese. Sanyo, Mitsubishi, or my fav, Diakin.

    Best luck,

    Mike
     
  8. Aug 21, 2010 #7

    Ivan Seeking

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    Thanks, Mike
    I would like to know more about the physical configuration wrt the compressor and condenser. Were your compressors typically accessible, or were they integral to the condenser unit as Russ described. Keep in mind that I want to insert a bypass at or near the compressor discharge port. How hard would this be in the systems you're familiar with; say in the 3 ton range?

    How do they tie in with the geothermal systems? They have precisely the same requirement that I do. Do they still use an air-cooled condenser in addition to the ground system heat exchanger, and provide a bypass mode, or do they only use the liquid heat exchanger. Also, at what point are the geothermal systems unique? Do they simply modify existing systems, or do they build systems specifically for this application?
     
    Last edited: Aug 21, 2010
  9. Aug 21, 2010 #8
    Hello again, Ivan,

    We owned a company that marketed under a different name, and kept one of the test units in the building (actually, our building had a LOT of test units cooling it)
    I suspect the thing was originally boxed, but being a herd of engineers and techs, the box was long gone by the time I saw it.

    It was a fairly compact affair. Roughly the size of what we used to call 'hat box' air conditioners, which were compact, but grossly inefficient condensers. The sort of stuff builders would stuff in 70's apartment buildings.

    Anyway, it contained the compressor, heat exchanger, and the sundry plumbing and electronics. I don't recall seeing a water pump - I think that was supposed to be in another part of the system.

    Anyway, ours used the chilled water system from the building as the source and dump for heating and cooling.

    I imagine someone could build one, and I did have an employee who tried. The thing is, you have to know a bit about the secret sauce. And, before you're done, you may spend more than you intend.

    My tech made a gross inefficient heat exchange, because he thought a simple coaxial counter flow exchanger would do the trick. He put one copper pipe inside another and ran the refrigerant through the center one. The coupling was poor, and I'm sure he had a terrible problem with foam in the inner pipe.

    The one we had used about an 8" X 24" cylinder with water carrying tubes all wound about inside it. The affair was tilted slightly, and the liquid refrigerant would simply be collected at the lowest point.

    Of course if you do this, you need an amazing amount of heat exchange area in the yard, or you need a body of water.
     
  10. Aug 22, 2010 #9

    Ivan Seeking

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    Thanks,
    This is not for a geothermal system but it has the same basic requirements. No secret sauce. No special plastics. I guess I need to take a run over to the geotherm supplier and see what they do.

    I am trying to establish a baseline cost for the bypass system. I know how to design a heat exchanger, but, as always, I will buy something off the shelf if possible. What is news to me is this business of an integrated compressor-condenser unit. I need to determine if these can be modified or not, and how common they may be.

    Incidently, I do plan to put in geothermal system for us. I am going to be reworking some retaining walls [very long runs] and can easily put the pipes in at the same time. But this project has nothing to do with that. This is pure R&D for work.
     
    Last edited: Aug 22, 2010
  11. Aug 23, 2010 #10

    russ_watters

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    Yes. You might be able to figure it out with some measurements or observations - or check the manufacturer specs to see if they list the airflow. I suspect that when it is warm outside, the fan accounts for a very high fraction of the heat dissipation - 75% or more.
    It depends on the condenser, but most have at least some way to access the compressor for servicing/replacement.
    Virtually water source heat pumps are single package: the compressor is in the same case as the air handling unit. Things like walk in refrigerators sometimes use water cooled condensers, but it is rare in air conditioners.
    It isn't the temperature at the expansion valve that's the issue, it's the temperature/pressure at the compressor. And while you can read it off a table or the service manual for the air conditioner, you'll probably need to measure the temp and pressure.

    Heck, with that budget, it may be worthwhile to pick up an HVAC testing/monitoring device: https://www.amazon.com/Mastercool-99772-Digital-Manifold-Gauge/dp/B0018RVX7S

    Besides just making sure you don't kill your compressor, it'll measure/monitor performance for you.
    No, most are just a fixed orifice plate.
    What systems - residential? I've never heard of a residential condenser with hot gas bypass or variable speed compressor. A good fraction of commercial compressors do, but unlikely in a residential size. Residentially, some have two-stage compressors though:http://www.residential.carrier.com/products/acheatpumps/ac/infinity.shtml
    Residential, it is almost certainly evaporator (duct) airflow.
     
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  12. Aug 23, 2010 #11

    russ_watters

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  13. Aug 23, 2010 #12
    The now defunct Tran XV1500 was a series of variable rate compressors. Now, I don't think any Americans build true variable speed residential condensers, but there are a number of two speed compressors and at least one dual compressor system, again a Trane.
    If you want a true variable speed, you can get one from a number of Japanese manufacturer's Toshiba, Sanyo, and Diakin come to mind.
    I've seen condenser units with the fan off, and it's purely a short term transient condition as the compressor is quickly turned off by the high side pressure trip. Without the fan, the condenser unit has next no heat dissipation.
    At least most good condensers have low and high side trip switches to reduce the chance of you destroying them :)
     
  14. Aug 24, 2010 #13

    Ivan Seeking

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    Sure, but I was thinking of the min liquid temp/press going to the expansion valve as a function of overcooling at the condenser - with too little heat load on the evaporator - ultimately to avoid getting liquid back at the suction port of the compressor. I guess your point is that the compressor suction and discharge pressures and temps tell me that?

    It sounds like this is a significant concern as there is no bypass mode [as I understand it]. As Mike mentioned, there are low/high pressure cutoffs, but I assume these are designed to prevent damage in a failure mode, and not standard control devices?

    It sounds like I really can't avoid monitoring the head pressure and regulating the heat load on the exchanger accordingly -mimic the condenser fan control. In principle, I could tie directly into that system for the control, but I'm sure the practical side of this will vary from system to system [P.S. No system has been specified as yet].

    I would tend towards an analog head pressure sensor, and PID control on the exchanger heat load; with head pressure as the process value, and the midrange [Min pressure + (max-min)/2] of the existing fan control, as the setpoint. Beyond cost, is there any reason not to do this? I can't think of any advantages in using pressure switches, beyond the low cost.

    Does this all make sense?

    Thanks, yes, time for some new toys.

    Years ago I had the top Snap-On brand gauge set. But when I returned to school, they went into a storage locker that was later robbed. :mad: They got my OMG, standard and metric, coarse and fine, tap and die set, as well. IIRC, the pair were worth almost $2K [well, I think that's what I paid Snap-On :biggrin:].
     
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