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Old 09-16-13, 06:38 PM   #41
jeff5may
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I believe the near-future improvements in residential HVAC will come in the form of variable-capacity inverter compressors (like the mini-split units use) as well as the split-evaporator designs (like the ejector-split design in the prius). The stacked-compressor idea was pretty much put to death by Hallowell and the Acadia units. Although not a bad idea on paper, very few refrigeration engineers will want to go out on a limb after the miserable failure of the Acadia units. Like the hyper-heat units by Mitsubishi, they favored raw BTU output at low temps over high COP.

For us hacking types, the stacked compressor design remains a good source for experimentation, especially with air-source units. With water-source units, inverter technology and digital controls have much room for improvement to fully realize the energy that can be saved.

In college courses, especially at the university level, speculation does not always balance correctly with reality. The professors intentionally say things in class to lead you down a path of failure, just to see how sharp you may be. In some courses, the ability to sniff out the BS early on is the only thing that allows some to pass while others fail. In their minds, this prepares the students for the chaos they will regularly have to sort out as engineers or scientists or lawyers or executives or whatever. Some of these ideas are ancient in nature and have already been disproved or revised to match reality. The lesson learned is not to believe everything you hear, to think critically before wasting time needlessly chasing your tail. Many brilliant people drop straight out of school for this very reason. In their minds, they would rather just get on with their own progress than sit through years of endless lectures that may or may not be useful to them in the future.


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Old 09-16-13, 08:39 PM   #42
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Quote:
Originally Posted by jeff5may View Post
I believe the near-future improvements in residential HVAC will come in the form of variable-capacity inverter compressors...
As long as we're dwelling on future HVAC improvements, another issue to consider is that there seems to be a move toward considering the merits of organic refrigerants.

I have seen quite a few papers written fairly recently on the environmental advantages of organic refrigerants, and in some cases, the efficiency advantages. I have seen these papers from Europe, China, Turkey and Australia.

I think that the real reason that synthetic refrigerants are predominate is that organic refrigerants are not patentable, so less money is to be made from their manufacture.

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Old 12-27-13, 11:09 AM   #43
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I am reviving this thread in hope of a rethink on the subject. Parallel compressors have been used in commercial refrigeration for many years. Kysor Warren builds most of Walmart's refrigeration units and as far as I know they use this method exclusively. I spent a few months at their Columbus assembly plant where I saw several models in various phases of construction.

Multiple compressors require suction accumulators and liquid receivers to handle varying load requirements. Residential HVAC systems rely on the can around a hermetic compressor to store idle refrigerant. I don't think this is practical with multiple compressors. FWIW, commercial compressors are not hermetic units.
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Old 12-27-13, 02:45 PM   #44
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With residential AC units and most heat pumps, the indoor and outdoor coils ARE the liquid receivers. If the unit is a smaller, cap-tube metered setup, it will probably have a "bullet" filter/dryer on it right before the cap tube, which kind of serves the same purpose. Medium-sized portable/window units have a section of each coil where the split flow merges, generally at the bottom of the coil. The section is situated a couple feet worth of piping from the compressor side fitting. Also, there is space at the bottom of the compressor that acts as a catch-can for oil or liquid slugs.

As the units become larger and split, around 2-1/2 to 3 tons of capacity, expansion valves and suction accumulators begin to enter the picture. The manufacturers have deemed units smaller than this size not worthy of these luxuries. On some heat pump units, manufacturers have plumbed in a "charge compensator" (aka "quiggle-ator") to try to minimize liquid refrigerant from overly backing up the condenser coil during heating season. Compared to commercial rigs, none of these components are very large or high-capacity.

Very few residential units have employed multiple compressors. Rather, the mfr's have come up with a few good designs and fed the masses with them. Air-source units in general have evolved into largely maintenance-free devices, so they would rather not mess a good thing up by changing time-tested designs. The public will not stand for a unit that requires any real preventive maintenance.

One such design is the twin-single compressor. It basically has two rotary/scroll assemblies in the pot, like a clothes washer. On low speed, one of the assemblies rotates while the other freewheels. On high speed, the motor reverses direction and both assemblies rotate. Works like a champ.

Another predominant setup is the variable-speed, or inverter drive compressor. After a slow start, this design is beginning to show up in lots of new units. Apparently, it took awhile to convince the industry that they actually work well.

The auto industry in general went with the variable displacement compressor first. The residential mfr's have been experimenting with the idea, but AFAIK, nothing has made it to mass production.

The multiple-compressor idea has pretty much been abandoned by the residential HVAC manufacturers. Oil and refrigerant flow controls are needed, due to the multiple refrigerant paths. Additional electronic controls are needed to switch and start multiple compressors. More copper plumbing is needed for the refrigerant circuit. All these extras incur costs that the mfr's must consider, both for building and providing service after the sale.

In the commercial sector, customers care about serviceability, redundancy, and ROI. Businesses have no problems paying extra for a system that will run at reduced capacity while they swap a burnt compressor, or that will save at least X amount of dollars a month running on low. Not so much in the residential market. Joe Snuffy wants a cheap, bionic unit that will live at least a week longer than he will, that's all. This formula adds up to the most basic, foolproof design possible at a certain level of performance. And no more.

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Old 12-27-13, 05:07 PM   #45
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Jeff;

Agreed. For the residential market an inverter makes much more sense than multiple compressors. Electronics are cheap at that level. The hacker is faced with a different problem. Inverter HVAC technology is going to be proprietary, and proprietary electronics are the bane of the hacker.

Even something as simple a three phase compressor and VFD are not practical at the DIY level. The wiring is straightforward and the drive is cheap. Three phase compressors are horribly expensive due to the smaller market. The targeted market makes it unlikely we will find them at garage sales or even scrap yards. The few that do come up are 480V from what I've seen. I replaced one last year. The only reason we used a 480 three phase pump was to avoid going through the already loaded 208 transformer.

Then there is the question whether a three phase hermetic will run well at reduced speed.

Thus my renewed interest in parallel compressors. It seems like a hacker friendly approach,
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Old 12-27-13, 07:20 PM   #46
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It's time to develop an inverter that can drive a standard split phase compressor as the asymmetrical 2 phase induction motor it is. There is an inverter out there (Shannon Liu Quadrature Drive) that does just that, but only for reciprocating compressors as it relies on torque pulsation to detect rotor speed, which will not work on a scroll. A good solution is to glue a piezo to the side of the compressor to pick up the vibrations.
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Old 12-27-13, 08:34 PM   #47
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I don't know if it's been covered before, but why can't we just use an automotive compressor coupled to a three phase motor? Hermetics may more efficient but inverter hermetics are not within our reach. Perhaps the ability to vary speed is more important than better efficiency at one speed.
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Old 12-27-13, 08:51 PM   #48
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Believe me, with the onslaught of technology that has been thrown into this sector during the last decade, there is no shortage of inverter-driven designs heading to the scrap yards every day. With the continued penetration of inexpensive, proprietary mini-split units into the market, the number of techs blaming "that SAMSONIC JUNK unit" they have no idea how to fix is increasing the number of purely hackable units being condemned. They are even showing up on craigslist and freecycle!

From a hacking or experimental standpoint, it is not a major undertaking to rig up multiple compressors in parallel. Like you said, a suction accumulator is needed to prevent slugging the compressor/compressors, especially during defrost. Some attention must be taken to plumb the compressors for proper oil flow. Extra control circuitry must be devised to avoid overcurrent or overpressure events from happening. But it has all been done before, you just need to know where to look for information.
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Old 01-02-14, 08:49 AM   #49
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Quote:
Originally Posted by jeff5may View Post

... to rig up multiple compressors in parallel. Like you said, a suction accumulator is needed to prevent slugging the compressor/compressors, especially during defrost. Some attention must be taken to plumb the compressors for proper oil flow. Extra control circuitry must be devised to avoid overcurrent or overpressure events from happening. But it has all been done before, you just need to know where to look for information.
Sorry,I have to confess this chat upon multiple compressors goes beyond my comprehension when/if residential heating concerned.Is there sound need
for extra compressors,I might ask ?

The point of "defrost" is essential ... and oil return during low ambient temperatures.

How this "nuisance" is recently handled then ?

I take here a case upon latest features on "new" thinking how "defrost"
and the related "off-cycle" is treated by a xxx-company in one of their split EEV-inverter models.They call it G10-system.The idea is that the compressor will not stop att all during defost cycle .At least 1Hz is prevailing (see attach.). Of course,the evaporator/accumulator is somewhat huge and EEV is a must! Permamagnet twin BLCD-compressor as a standard!

This system has been field-tested in southern-Finland with exellent perfomance behaviour .The Defrost cycle times abt. 3 hours at around minus 25degC /RH >70% acc. to logging data shown below.The defost is not triggered here egg-clockwise but ... on demand (oil return ?).

Innova H121 mittauksia

My point is that we will see soonest major invasion of sound competition based on EEV-widgets focussed in heating problematics instead of cooling , where in fact the main market (>90%) could be found.

R410a will be turned down and HC/or R32 -related gadgets will occupy the market within the foreseen future.





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Old 01-02-14, 04:27 PM   #50
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dc€x,

The Gree unit you referenced is a perfect example of the type of units the OEM's are producing now. Obviously, the manufacturers have done the research and know how to control these units. Electronic valves, variable frequency or DC brushless motors, and digital controls with temperature sensors everywhere inside the unit.

EDIT: I was not aware of the massive progress in this realm.

On the industrial side, the big mfr's have "universal superheat" controllers that can be rigged to basically any vapor compression system. Emerson, Sporlan, and Danfoss all have kits you can buy to convert whatever you have to electronic, seat-of-your-pants-efficiency. Basically, the older and more complicated the system is, the better the new control scheme and valve will improve it. The controls go together like erector sets, and are self-learning.

On the consumer/residential side, the OEM's use programmable controller chips much like found in TV sets and microwaves to make the units behave however they want them to. From one basic chassis, many unique models are produced. The OEM simply mixes and matches from a menu of approved components, then flash the controller with firmware to enable whatever unique features that unit possesses. The unit is loaded with sensors, which feed back to the self-learning controller. Once the unit runs for a few minutes, it optimizes its own efficiency.


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