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Siamesed HVAC

Like multi-speed compressors, but cruder
 
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I like the multi-speed compressor and multi- and variable-speed blowers for the level of efficiency they can wring out of an HVAC system. But if that single compressor goes kerflooey, where are you? You have a no-speed system.

As a handyman maintaining dozens of airconditioners, some of which serve gigantic mult-occupant spaces with no backups, I'm a big proponent of redundancy. If one goes, you can limp along with what remains. In our facility, we have a couple of truly enormous spaces that are served by air conditioner units that have within them more than one compressor, but share the refrigerant lines. I call this a multiplexed system; it can run at sharply reduced capacity on just one compressor. That is a very slow ramp up to uncomfortable temperatures. When all four fire up, everything cools right down in a hurry.

But this sort of thing isn't represented on the residential scale, and for the sake of uninterruptible service, maybe it shouldn't, at least not in every detail. I originally envisioned this as using multiple compressors but one refrigerant line, and heat pump reversing valves adapted to switch compressors in and out of the refrigerant loop as necessary, but that would be bad: a burned out compressor would poison and kill the other healthy compressors by contaminating the shared refrigerant; retaining some cooling capacity in spite of equipment failure is the whole point here.

My idea uses several small AC/R compressors, comparable to what is found in a window air conditioner or maybe even a refrigerator. Each separate compressor has its own refrigerant circuit, and each compressor and circuit is set up to operate at a slightly different temperature, so that as the conditioned space slowly overpowers a single cooling system, another joins up, and another until finally all the conditioning systems are working in concert. When the conditioned space is idle, a single unit would probably suffice to maintain the setback temperature. Delay timing circuits would prevent all the compressors and fans from firing up at once to prevent unpleasant demand spikes on the power system.

The concept lends itself nicely to a PTAC installation, using already-existing window HVAC compressors and their associated coil packs. Using longer refrigerant lines, it would just as easily translate to a split system with the air handler within the building envelope, and the noisy bits removed closer to the neighbors.

The concept would require a somewhat larger outdoor unit than what is typical, even for a PTAC system. It wouldn't be unnecessarily unattractive, being a long, low monolithic shape with cooling grids on the front and fans behind. It would have to be larger to provide space for the multiple condenser coils and fans, but would in general be quieter in operation, with the possible exception being when all units are running simultaneously.

One potential problem I have thought about: all the compressors and fans running simultaneously may create an unpleasant periodic harmonization.

elhigh, Aug 16 2007

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       For those that don't know - HVAC = Heating/ Ventillation/ Air-Conditioning.   

       My question is - why not go to a chilled water system? Each compressor has it's own refrigerant circuit, and is totally independant of all other compressors. Temperature controll in the live space is performed by regulating coolant (chilled water) flow, and by fan speed at the heat exchangers. Control of the compressors can be as simple as a thermostat. These systems are superior in almost every way, and are also more robust because of the closed-loop nature of the parallel compressors.   

       Oh, and with almost any mechanical system - larger units means higher efficiency, if only as a result of adiabatic heat transfer. Generally more efficiency can be dragged out of a large compressor as opposed to several small ones. For instance, larger HVAC compressors are appearing with electromagnetic bearings - a significant efficiency increase.
Custardguts, Aug 16 2007
  

       //all the compressors and fans running simultaneously may create an unpleasant periodic harmonization'.....miaowing?// Jericho.
Zimmy, Aug 17 2007
  

       I agree with cus in that the compressors need to be separate. One of the things I've had go wrong is freezing of the coils from running too long. thermostat broke or something.
dentworth, Aug 17 2007
  

       The point is supplying HVAC service to an area where constancy of service is of greater importance than ultimate efficiency, e.g. a server farm cool room or a large dorm room. The very obvious and very basic solution would be to have two or more complete systems serving the same space; my idea asks the air handling equipment to do for all, while multiple independent refrigerant circuits do their thing within the air handling circuit.   

       Ultimate efficiency of the complete system will never equal that of the well-designed single large compressor, I agree. However, the individual efficiency of each compressor unit would be quite high, and the ability to bring cooling service up in stages as demand increases would help prevent temperature swings.   

       I don't see how the chilled water system is much different from the existing large system.   

       I've always perceived the harmonization of multiple motors as more of a "wowing."
elhigh, Aug 17 2007
  

       Sorry, I was trying to address your issues with the existing system, which I percieved as being a) temperature control with differing load, b) reliability and (as we should all be constantly aware of) c) efficiency.   

       Chilled water systems excell at a, b and c. Oh well, this is the 1/2 b, so I'll sign off now, what with my logic and reason and such tomfoolery.
Custardguts, Aug 19 2007
  

       For all of the issues except reliablity, a variable speed drive is your answer. They have become quite cost effective and highly controlable and have longer service lives, taking ECM motors in fan powered terminal units as an example. Having multiple compressors certainly adds reliability, but you're not just adding another compressor - you need to add condenser coils, evaporator coils, likely another fan (or you'll be wasting energy by blowing air over unused coils), and likely another filter (as one fan will be used more than the other, only one piece of your filter will get dirty). Why build one strange custom unit with effectively two units inside? It would be strongly more cost effective to buy two standard units, and set two different setpoints. This, of course, is widely baked.
Worldgineer, Aug 20 2007
  
      
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