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    Re: AWHP superheat & sub-cooling



    Quote Originally Posted by Gary View Post
    This is not an even trade. Lower flowrate will give you higher temp water, but less heat transfer. It is a mistake.
    This is true, for the condenser heat-balance, as we saw in Drew's swimming pool heat-pump.

    The best way to actually test the real effect of these changes is to measure the heat-up time for water cycling around a heat-pump loop. In practice, I've actually found little noticeable difference in tank heat-up time using slower, or faster flows.

    What happens is that the lower temperature of the entering water can be set to maintain the same log-mean-temp-difference across the condenser, so that the condensing temp is not actually affected.

    In Drew's case, my test was set a Tc,sat=50'C, then the water flow was closed off - the Tc,sat rose to compensate. This is not a realistic test for circulating water, though. What typically happens is that at a similar Tc,sat~70'C the whole heating cycle finishes - with less passes through the heat-pump circuit, but more dTw in each pass.


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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by desA View Post
    This is true, for the condenser heat-balance, as we saw in Drew's swimming pool heat-pump.

    The best way to actually test the real effect of these changes is to measure the heat-up time for water cycling around a heat-pump loop. In practice, I've actually found little noticeable difference in tank heat-up time using slower, or faster flows.

    What happens is that the lower temperature of the entering water can be set to maintain the same log-mean-temp-difference across the condenser, so that the condensing temp is not actually affected.

    In Drew's case, my test was set a Tc,sat=50'C, then the water flow was closed off - the Tc,sat rose to compensate. This is not a realistic test for circulating water, though. What typically happens is that at a similar Tc,sat~70'C the whole heating cycle finishes - with less passes through the heat-pump circuit, but more dTw in each pass.
    Edit: Oops... I need to re-think my statement... I'll just remove it and maybe nobody will notice... lol
    Last edited by Gary; 15-07-2009 at 04:14 PM.

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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by desA View Post
    For most of these AWHP's, the condensers are either tube-in-tube coils, or plate heat-exchangers. Both can be treated in essence as counterflow units.

    For a condensing phase unit, the cross-flow correction factor is taken as unity, in the phase-change region. It is only when you get to the sensible heat-transfer regions (de-superheating & sub-cooling), that the effect of cross-flow, counter-flow, or parallel flow, is noticed.

    In general, the HX's are taken as counterflow.
    Counterflow and crossflow can give us equal heat transfer, but counterflow gives us a water temperature gradient, which is far better for our purposes.

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    Re: AWHP superheat & sub-cooling

    I am trying to envision the water loop. The water leaves the condenser, travels around the point of use areas, then to a storage tank, then back to the condenser. Is this correct?

    Where in this loop is the pump? Where is the makeup water? Where is the expansion tank?

    I'm thinking a piping diagram would be useful here.

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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by desA View Post
    This is true, for the condenser heat-balance, as we saw in Drew's swimming pool heat-pump.

    The best way to actually test the real effect of these changes is to measure the heat-up time for water cycling around a heat-pump loop. In practice, I've actually found little noticeable difference in tank heat-up time using slower, or faster flows.

    What happens is that the lower temperature of the entering water can be set to maintain the same log-mean-temp-difference across the condenser, so that the condensing temp is not actually affected.

    In Drew's case, my test was set a Tc,sat=50'C, then the water flow was closed off - the Tc,sat rose to compensate. This is not a realistic test for circulating water, though. What typically happens is that at a similar Tc,sat~70'C the whole heating cycle finishes - with less passes through the heat-pump circuit, but more dTw in each pass.
    Okay... let's try this again:

    As water flow is increased there should be more heat transfer (decrease in SCT), however... every condenser has a point where a further increase in flow will result in equal or less heat transfer rather than more. If you can reach this point, your pump is oversized for the condenser.

    To put it more simply, if lowering the water flow results in the same heat transfer, then you can save energy and initial cost by lowering it permanently (smaller pump). And if lowering the water flow beyond this point lowers the heat transfer, then why do it? Just enough and not too much.
    Last edited by Gary; 15-07-2009 at 06:56 PM.

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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by desA View Post
    Basically an inter-cooler strapped across suction & liquid lines, plus pressure control pre-TXV.
    With the HX, I could then reduce the evap footprint = good!
    I would suggest that once the capacity of the coil is maximized, the coil could be sized such that ambient temp of 25C would result in Te,sat of 15C thus riding the upper limits of the compressor.

    Of course, as the ambient rises the Te,sat will try to rise. So... with a variable speed fan, the airflow can be slowed to bring the Te,sat back down, holding it at a steady 15C, riding the compressor limits throughout the entire ambient range (25C-35C) and beyond.

    As a back-up precaution to safeguard the compressor, I would suggest a crankcase pressure regulating valve (CPR).
    Last edited by Gary; 17-07-2009 at 07:08 AM.

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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by Gary View Post
    I am trying to envision the water loop. The water leaves the condenser, travels around the point of use areas, then to a storage tank, then back to the condenser. Is this correct?

    Where in this loop is the pump? Where is the makeup water? Where is the expansion tank?

    I'm thinking a piping diagram would be useful here.
    There are a number of variations on this water loop concept. Each system designer has their own ideas for what they prefer.

    From a heat-pump perspective, the simplest to work with, conceptually, is as follows:
    Tank->pump->condenser->tank. (Pump-around loop)

    The water make-up feeds into the tank on level, as separate water pump can be used to pull out of the tank, as hot water is needed. This is a mixed tank concept & is inexpensive.

    Some folks prefer different systems of water storage - some pressurised, others not. The least expensive route used over here is a simple pump-around loop - much like the petrochemical industry.
    Last edited by desA; 17-07-2009 at 12:45 PM.
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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by Gary View Post
    Okay... let's try this again:

    As water flow is increased there should be more heat transfer (decrease in SCT), however... every condenser has a point where a further increase in flow will result in equal or less heat transfer rather than more. If you can reach this point, your pump is oversized for the condenser.

    To put it more simply, if lowering the water flow results in the same heat transfer, then you can save energy and initial cost by lowering it permanently (smaller pump). And if lowering the water flow beyond this point lowers the heat transfer, then why do it? Just enough and not too much.
    I agree with going for as small a pump-around pump as is necessary to just do the job - with some safety margin for wear & tear. It makes no sense to waste pumping power.

    There seem to be two very different schools of though when it comes to heat-pumps:
    1. Set dT,water across condenser in range of 3-5K & adjust water flow-rate accordingly (pump-around);
    2. Pass water through heat-pump only 1 time - raise from say 20-65'C in one pass. Here the water flowrate is incredibly small.

    With (1), the heat-pump cycle is changing continually throughout the heat-up cycle, whereas with (2), the heat-pump run is a steady-state operation.

    If anyone can clearly state which option is the best from overall energy conservation & operability points of view, the frosties will be on their way.
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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by Gary View Post
    I would suggest that once the capacity of the coil is maximized, the coil could be sized such that ambient temp of 25C would result in Te,sat of 15C thus riding the upper limits of the compressor.

    Of course, as the ambient rises the Te,sat will try to rise. So... with a variable speed fan, the airflow can be slowed to bring the Te,sat back down, holding it at a steady 15C, riding the compressor limits throughout the entire ambient range (25C-35C) and beyond.
    Touche'... You tell me you're not a designer...

    As a back-up precaution to safeguard the compressor, I would suggest a crankcase pressure regulating valve (CPR).
    What types of compressors would this work on?
    How does this work? I've never worked with these yet.
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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by desA View Post
    There are a number of variations on this water loop concept. Each system designer has their own ideas for what they prefer.

    From a heat-pump perspective, the simplest to work with, conceptually, is as follows:
    Tank->pump->condenser->tank. (Pump-around loop)

    The water make-up feeds into the tank on level, as separate water pump can be used to pull out of the tank, as hot water is needed. This is a mixed tank concept & is inexpensive.
    It seems to me that you would want to feed the make-up water directly to the heat pump to avoid cooling the tank water.

    tank > check valve > feed water > pump > condenser > tank

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    Re: AWHP superheat & sub-cooling

    ^ That's what can be done, in practice. Would need a little more instrumentation - although not really a problem. It will only differ by (T,in+dT,w,cond) though.

    The other problem with storage tank mixing, is that all the water ends up at a single mixed temp.

    Some folks look at layered storage tanks which feed top-to-bottom in series. The colder water goes to the heat-pump & the warm water stays in the hot part - ready for use. There is a temp gradient across the tank. Problem is though, that the slightest inlet disturbance & the tank partially mixes. Not everyone likes this method.

    Horses for courses.
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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by desA View Post
    Touche'... You tell me you're not a designer...
    I have zero experience in system design, but several decades of making problem systems work reasonably well despite the design, turning lemons into lemonade.

    I find this particular project fascinating in that the temperature/humidity of the evaporator leaving air is not the end product... and that changes everything.

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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by desA View Post
    What types of compressors would this work on?
    How does this work? I've never worked with these yet.
    A CPR valve places an upper limit on its outlet pressure, which is the inlet pressure for the compressor. It can be adjusted such that the maximum load can be placed on the compressor without overloading it.

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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by Gary View Post
    A CPR valve places an upper limit on its outlet pressure, which is the inlet pressure for the compressor. It can be adjusted such that the maximum load can be placed on the compressor without overloading it.
    Can this be applied to a scroll compressor?

    Is this a valve on the suction line, then? If so, how is it set, or where does it take it's control signal from?
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    Re: AWHP superheat & sub-cooling

    I find this particular project fascinating in that the temperature/humidity of the evaporator leaving air is not the end product... and that changes everything.
    AWHP's are fascinating beasts & that is what got me into the technology in the first place. They are a little counter-intuitive at first, but, once you get going, they're a fascinating design challenge.

    To make the machines in ultra-compact format is a real challenge - especially given the size of the current evaporator technology (archaic technology).
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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by desA View Post
    Can this be applied to a scroll compressor?

    Is this a valve on the suction line, then? If so, how is it set, or where does it take it's control signal from?
    Yes... it is on the suction line near the compressor.

    As far as I know, the CPR is applicable for any type of compressor.

    There is an adjustment stem on the CPR. Its control/sensing is mechanical and internal.

    The fan speed strategy and CPR valve are redundant. Either could be used to optimize the compressor load.

    The fan speed strategy has the added benefit of reduced energy for the fan motor. However, if the fan stuck in the full speed position in high ambients this could be disastrous for the compressor. Its a belt and suspenders kinda thing.

    As to type of compressor, the only type I would tend to eliminate out of hand would be the rotary, because the compressor shell is part of the high side and runs very hot. We don't want to lose that heat to the air surrounding the compressor. We want that heat to go to the condenser.

    What type of compressor are you currently using?
    Last edited by Gary; 17-07-2009 at 05:10 PM.

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    Re: AWHP superheat & sub-cooling

    What type of compressor are you currently using?
    Scroll compressor - reputable brand.
    Last edited by desA; 18-07-2009 at 08:57 AM.
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    Re: AWHP superheat & sub-cooling

    Another load limiting strategy would be a TXV w/MOP (maximum operating pressure) charge in the power element.

    Given the desire to ride the upper limits of the compressor, I'm surprised that some form of load limiting is not commonly used in these systems.

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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by desA View Post
    There seem to be two very different schools of though when it comes to heat-pumps:
    1. Set dT,water across condenser in range of 3-5K & adjust water flow-rate accordingly (pump-around);
    2. Pass water through heat-pump only 1 time - raise from say 20-65'C in one pass. Here the water flowrate is incredibly small.

    With (1), the heat-pump cycle is changing continually throughout the heat-up cycle, whereas with (2), the heat-pump run is a steady-state operation.
    It seems to me that filling/heating the water in the storage tank would be a one time thing.

    From that point on we are heating the feed water and/or maintaining the storage tank temperature.

    This being the case, I vote for strategy #2, with the system automatically switching between the former and latter duties in accordance with the water level in the tank.

    feed water > check valve > condenser > flow regulator > tank

    -OR-

    tank > pump > check valve > condenser > flow regulator > tank

    The flow regulator would automatically restrict the water flow to maintain a constant 65C leaving water temp.
    Last edited by Gary; 18-07-2009 at 06:05 PM.

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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by desA View Post
    There seem to be two very different schools of though when it comes to heat-pumps:
    1. Set dT,water across condenser in range of 3-5K & adjust water flow-rate accordingly (pump-around);
    2. Pass water through heat-pump only 1 time - raise from say 20-65'C in one pass. Here the water flowrate is incredibly small.

    With (1), the heat-pump cycle is changing continually throughout the heat-up cycle, whereas with (2), the heat-pump run is a steady-state operation.
    I would contend that the energy consumed is pretty much the same either way, with one exception: The energy consumed in pumping the water.

    If the feed water is fully heated before it reaches the storage tank, then the local water supplier has paid to pump it, therefore not running the tank water pump to heat the feed water is a savings to the end user.
    Last edited by Gary; 20-07-2009 at 08:26 PM.

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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by Gary View Post
    Of course, as the ambient rises the Te,sat will try to rise. So... with a variable speed fan, the airflow can be slowed to bring the Te,sat back down, holding it at a steady 15C, riding the compressor limits throughout the entire ambient range (25C-35C) and beyond.
    Thus far, assuming everything works as envisioned, we have a system which absorbs a very stable amount of heat in the low side.

    In the high side, a high percentage of that heat is transferred to the water in the condenser, while a small percentage is transferred back to the waste air stream via the pressure reduction coil.

    Perhaps we can control the variable speed fan to sense/minimize this waste heat, dropping the SST to pump only the heat that can be currently utilized by the condenser.

    Then look for ways to improve the condenser heat transfer.

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    Re: AWHP superheat & sub-cooling

    It occurs to me that the pressures and loads being stable, the system is now ideal for a cap tube. A cap tube is not only less expensive, but it uses less refrigerant.

    And the fan can be controlled off discharge line temp using a thermistor, rather than using a more expensive transducer to control off low side pressure.

    Somehow, this beast keeps evolving... lol
    Last edited by Gary; 25-07-2009 at 04:36 AM.

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    Re: AWHP superheat & sub-cooling

    Apologies for not replying earlier - I've been away for a few days.

    Quote Originally Posted by Gary View Post
    Another load limiting strategy would be a TXV w/MOP (maximum operating pressure) charge in the power element.
    Can you explain more on how the MOP option works? I've seen the option on offer - although not common in my present location.

    Given the desire to ride the upper limits of the compressor, I'm surprised that some form of load limiting is not commonly used in these systems.
    I agree here. If you look at some of the products coming out of Asia, I'll be very surprised if their compressors last any decent time at all.

    Conservative design is absolutely essential, if the compressors are to have a decent lifetime.
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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by desA View Post
    Can you explain more on how the MOP option works? I've seen the option on offer - although not common in my present location.
    Where there is both liquid and vapor in an enclosed container an increase in temperature will cause an increase in pressure.

    This relationship continues until all of the liquid becomes vapor, at which point the pressure becomes fixed regardless of any further increase in temperature.

    By precisely manipulating the amount of refrigerant in the TXV bulb, a fixed bulb pressure limit can be set.

    The TXV judges superheat by comparing the pressure in the coil to the pressure in the bulb.

    An increase in coil pressure, when compared to a fixed bulb pressure is interpreted as a decrease in superheat, which tends to reduce refrigerant flow, which in turn reduces the coil pressure.

    Thus equilibrium is reached at a predetermined coil pressure. The coil is at its maximum operating pressure.

    When the evaporator load decreases, the bulb temperature decreases, liquid droplets form in the bulb and everything goes back to normal.
    Last edited by Gary; 26-07-2009 at 04:30 PM.

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    Re: AWHP superheat & sub-cooling

    Hi desA
    The primary consideration is compressor operational conditions, for longevity of system integraty.
    Did the TEV superheat check I suggested stabalize performance, it has worked for me for decades.

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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by Magoo View Post
    Hi desA
    The primary consideration is compressor operational conditions, for longevity of system integraty.
    Did the TEV superheat check I suggested stabalize performance, it has worked for me for decades.
    Hi desA
    Interesting post/topic, with evap superheat testing, setting the TEV is critical for system performance. Start by reading the air on temp., and the actual evap pressure converted to temp., this is system TD. Then read suction temp at TEV bulb versus the evap pressure/temp. The superheat of TEV should be 60 > 70 % of system TD. Can be set up during pull down or at design, any adjustments to TEV wait 15 minute for TEV to stabilize. Doing this method of checking gets rid of all the "rule of thumb " ideas
    magoo
    Hi Magoo,

    Thanks so much for your follow-up. I'll be running up my lab machine tomorrow, with a pre-calculated mass charge. I'll apply your TEV rules in setting the superheat & report back on the performance.
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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by Gary View Post
    By precisely manipulating the amount of refrigerant in the TXV bulb, a fixed bulb pressure limit can be set.
    I should add that for your purposes, given the non-conventional pressures you need in order to maximize coil heat absorption, you would probably need to special order your MOP charge.

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    Re: AWHP superheat & sub-cooling

    You may have noticed that refrigeration coils are sized for TD's of 10-15F/5.5-8.5K, while A/C coils are sized for 35-40F/20-22K TD's. There is a very good reason for this: If an A/C coil were sized for 10-15F/5.5-8.5K TD it would be incapable of achieving acceptable humidity levels. Your home would be a cold swamp.

    You have no such dehumidification needs in this system, therefore you can achieve much higher COP by sizing your coil in accordance with refrigeration practices as opposed to A/C practices.
    Last edited by Gary; 27-07-2009 at 05:59 PM.

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    Re: AWHP superheat & sub-cooling

    On the other hand, conventional sizing calculations assume a portion of the coil is used for flashing off the liquid. Since we are taking steps to eliminate flashing this changes everything.

    I'm thinking you are going to have to size and adjust every component through a step-by-step trial and error process.

    On the bright side, at the end of this process you may be in a position to devise your own set of unique formulas for this particular industry niche.
    Last edited by Gary; 27-07-2009 at 06:50 PM.

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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by Gary View Post
    I would contend that the energy consumed is pretty much the same either way, with one exception: The energy consumed in pumping the water.

    If the feed water is fully heated before it reaches the storage tank, then the local water supplier has paid to pump it, therefore not running the tank water pump to heat the feed water is a savings to the end user.
    A very useful thought...

    I wonder how many users will still prefer to pump around on a mixed tank, rather than plan the inventory properly & heat incoming.
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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by Gary View Post
    Thus far, assuming everything works as envisioned, we have a system which absorbs a very stable amount of heat in the low side.

    In the high side, a high percentage of that heat is transferred to the water in the condenser, while a small percentage is transferred back to the waste air stream via the pressure reduction coil.
    Perhaps we can control the variable speed fan to sense/minimize this waste heat, dropping the SST to pump only the heat that can be currently utilized by the condenser.
    This is very cunning. I've sourced a range of fan speed controllers that can allow fan speed optimisation. I'll work further on this - very interesting.

    Then look for ways to improve the condenser heat transfer.
    The current condensers are already pretty good in terms of their heat-transfer capacity. Very, very compact. This, plus a few other tricks, has allowed the current prototype model to reduce down to 54% of the previous box volume. Practically, this can go down a fair bit further still, judging from the piping layout.

    I'm absolutely loving the challenge so far. It always gets interesting when you're spending your own money on such development projects.
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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by Gary View Post
    It occurs to me that the pressures and loads being stable, the system is now ideal for a cap tube. A cap tube is not only less expensive, but it uses less refrigerant.
    This is a very useful observation, indeed. A change of heating philosophy leads to a more compact, simpler system. Thanks for that.

    And the fan can be controlled off discharge line temp using a thermistor, rather than using a more expensive transducer to control off low side pressure.
    Agreed. I do prefer this method. My current prototype has allowance for two active thermistors. The second can be put to good use here.

    This is fun...
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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by Gary View Post
    Where there is both liquid and vapor in an enclosed container an increase in temperature will cause an increase in pressure.

    This relationship continues until all of the liquid becomes vapor, at which point the pressure becomes fixed regardless of any further increase in temperature.

    By precisely manipulating the amount of refrigerant in the TXV bulb, a fixed bulb pressure limit can be set.

    The TXV judges superheat by comparing the pressure in the coil to the pressure in the bulb.

    An increase in coil pressure, when compared to a fixed bulb pressure is interpreted as a decrease in superheat, which tends to reduce refrigerant flow, which in turn reduces the coil pressure.

    Thus equilibrium is reached at a predetermined coil pressure. The coil is at its maximum operating pressure.

    When the evaporator load decreases, the bulb temperature decreases, liquid droplets form in the bulb and everything goes back to normal.
    Thanks so much for the detailed explanation. That would be quite some juggling act the bulb-manufacturer has to take care of. I'd imagine that the bulb location would also be critical here.
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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by Gary View Post
    I should add that for your purposes, given the non-conventional pressures you need in order to maximize coil heat absorption, you would probably need to special order your MOP charge.
    Ok, that will probably make for an interesting supply discussion.

    Which manufacturers of TXV's would be open to supply optimised MOP options? In my view, this would actually be a useful service component in terms of service support. Perhaps also give a bit of competitive edge as well.

    Some very interesting thoughts coming out of this discussion.
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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by Gary View Post
    You may have noticed that refrigeration coils are sized for TD's of 10-15F/5.5-8.5K, while A/C coils are sized for 35-40F/20-22K TD's. There is a very good reason for this: If an A/C coil were sized for 10-15F/5.5-8.5K TD it would be incapable of achieving acceptable humidity levels. Your home would be a cold swamp.

    You have no such dehumidification needs in this system, therefore you can achieve much higher COP by sizing your coil in accordance with refrigeration practices as opposed to A/C practices.
    Can you walk me through this, step-by-step? I'm assuming by refrigeration coils & A/C coils, you're referring to the evaporator coils?

    If we are to size for a smaller TD (e.g. 10-15F/5.5-8.5K TD), would it not end up being a lot larger - volume, or area, for the same heat-transfer rate?

    This is no problem, in practice, as the fin heat-transfer coefficient can be improved dramatically with a new design concept currently on the drawing board.
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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by Gary View Post
    On the other hand, conventional sizing calculations assume a portion of the coil is used for flashing off the liquid. Since we are taking steps to eliminate flashing this changes everything.

    I'm thinking you are going to have to size and adjust every component through a step-by-step trial and error process.

    On the bright side, at the end of this process you may be in a position to devise your own set of unique formulas for this particular industry niche.
    Gary, thanks so much for the incredible input & insights you have provided on this thread. Between you & Magoo you have given me a huge amount to think about & experiment with. I really do see that innovations need to be developed in this market niche. The end result will hopefully be passed on in terms of smaller size & lower cost to the end user.
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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by desA View Post
    Ok, that will probably make for an interesting supply discussion.

    Which manufacturers of TXV's would be open to supply optimised MOP options? In my view, this would actually be a useful service component in terms of service support. Perhaps also give a bit of competitive edge as well.
    This would be a moot point if a cap tube is used. And a cap tube will be much better for this application. The cap tube can be used for both pressure reduction and HX and this will reduce weight, expense and footprint. It is ideal.

    The key to using a cap tube for this application is the evap heat load stabilization provided by the fan control.
    Last edited by Gary; 28-07-2009 at 04:30 PM.

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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by desA View Post
    Can you walk me through this, step-by-step? I'm assuming by refrigeration coils & A/C coils, you're referring to the evaporator coils?

    If we are to size for a smaller TD (e.g. 10-15F/5.5-8.5K TD), would it not end up being a lot larger - volume, or area, for the same heat-transfer rate?
    We already have a target TD. At 25C incoming air temp, we want the SST to be 15C, therefore 25-15=10K.

    Our ideal TD, assuming a minimum incoming air temp of 25C, is 10K.

    If we were designing for cooler ambients, say 20C incoming air, we would want to size our coil for 20-15=5K TD. In trying to get our TD lower than 5K we would hit a point of diminishing returns.

    The machine would work well in cooler ambients than we are designing for, but the SST would drop and we would no longer be riding the upper limits of the compressor, thus the COP would be reduced as the ambient temp reduces.

    In other words, if we were designing for all ambients, we would want the evap TD to be 5K.
    Last edited by Gary; 28-07-2009 at 04:25 PM.

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    Re: AWHP superheat & sub-cooling

    ^ What happens when incoming air temp rises to 35'C, as is common in Asia & Africa?
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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by desA View Post
    ^ What happens when incoming air temp rises to 35'C, as is common in Asia & Africa?
    The fan slows down to compensate for the rise in heat load. Less evap airflow = less evap heat load.
    Last edited by Gary; 28-07-2009 at 04:46 PM.

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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by Gary View Post
    The fan slows down to compensate for the rise in heat load. Less evap airflow = less evap heat load.
    Agreed.

    This fan control concept is also applicable to the unsteady heat-pump cycle situation.
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    Re: AWHP superheat & sub-cooling

    What are your thoughts on COP,hp optimisation?

    How to force the heat-pump system to operate on maximum COP,hp throughout the heating cycle?
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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by Magoo View Post
    Hi desA
    The primary consideration is compressor operational conditions, for longevity of system integraty.
    Did the TEV superheat check I suggested stabalize performance, it has worked for me for decades.
    I did the following today, on the lab machine:
    1. Weighed in the calculated refrigerant charge sufficient for all internal components at selected operating conditions;
    2. Let the system settle;
    3. Tuned the TXV using the 0.6-0.7 times TD rule;
    4. Let system settle between TXV adjustments;
    5. Ramped system up from ambient water temp to 60'C.

    The system ran as sweet as a bird. Smooth. The TXV tuning rule seems to be bang on, Magoo. I'm a very happy camper.

    Tomorrow, I plan to run the machine up to my standard test point, hold it & then further fine-tune the TXV a 1/4 turn at most from its current position.

    Thanks Magoo - I owe you a few cold ones.
    Last edited by desA; 29-07-2009 at 02:28 PM.
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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by desA View Post
    What are your thoughts on COP,hp optimisation?

    How to force the heat-pump system to operate on maximum COP,hp throughout the heating cycle?

    Now that we are maximizing the heat absorption on the low side, what remains is to transfer that heat to the water flowing through the condenser.

    As heat is transferred from the refrigerant to the water, the refrigerant temp drops, having lost heat, and the leaving water temp rises, having gained heat.

    The two temperatures "approach" each other. Thus the difference between the SCT temp and the leaving water temp is called the approach temp.

    By installing a water regulating valve at the condenser outlet (which senses and controls SCT) we can regulate the flow to give us a steady SCT of 75C, which is the upper limit of the compressor.

    Since the SCT is thereby fixed at 75C, the variable in our approach becomes the water leaving temp. The better the heat transfer, the higher the leaving water temp.

    We need to have a condenser large enough to give us 65C leaving water temp, giving us an approach temp of 75-65=10K approach.

    By increasing the condenser size beyond this minimum we can increase water temp and reduce the approach, but this again involves a point of diminishing returns.

    I'm thinking a reasonable approach target would be about 5K (75-70=5K).
    Last edited by Gary; 29-07-2009 at 02:57 PM.

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    Re: AWHP superheat & sub-cooling

    By install a water regulating valve at the condenser outlet (which senses and controls SCT) we can regulate the flow to give us a steady SCT of 75C, which is the upper limit of the compressor.

    Since the SCT is thereby fixed at 75C, the variable in our approach becomes the water leaving temp. The better the heat transfer, the higher the leaving water temp.
    This is excellent system design logic. Thanks for this.

    I'd envisage a simple water flow control valve, taking its signal off the condenser. For a concentric-tube type, this should be no contest as long as the temp probe is well-secured.

    For a plate-type condenser, I wonder whether there would be a decent place to pick up the condensing temp SCT @ 75'C on the body? May have to infer the SCT from another suitable temperature, or perhaps convert the SCP pressure, in the controller.

    My current design approach is typically around 10K, for an economical design. The cost effect can be easily developed versus approach - I'll look into that.
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    Re: AWHP superheat & sub-cooling

    The only problem I can foresee at this point is in maintaining the water temp in the storage vessel.

    Since the condenser incoming water temp is high, we may not be able to get enough flow through our condenser to prevent the SCT from exceeding the 75C limit.

    If this is the case we may need to override the fan control, shutting down the fan to decrease the heat transfer... but let's cross that bridge if/when we come to it.

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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by desA View Post
    I'd envisage a simple water flow control valve, taking its signal off the condenser. For a concentric-tube type, this should be no contest as long as the temp probe is well-secured.
    I would control the flow off high side pressure, rather than temperature. This is a common strategy for water cooled condensers and the valve is readily available. Controlling off pressure rather than temp assures that we are riding the upper limit of the compressor but not exceeding it.

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    Re: AWHP superheat & sub-cooling

    The storage vessels are usually fairly large, for the larger heat-pumps. Some could be in the region of 4000L or so. There would be a fair amount of thermal lag there.

    A sensor signal from the tank can be input into the heat-pump controller & action taken to shut back the pump, fan etc. Typically the water temp is used to shut the heat-pump off, when the pre-set temp has been reached.

    Hi/low pressure trips are also installed. If the water flow drops too low & SCT rises above the critical value, the hi/low should catch it, if correctly set - to protect the compresor.

    Basically, since the hot water is generally blended with cold water by the end user, the terminal tank temperature can undershoot, or even overshoot a little, without much problem - in general.
    Last edited by desA; 29-07-2009 at 03:48 PM.
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    Re: AWHP superheat & sub-cooling

    Quote Originally Posted by Gary View Post
    I would control the flow off high side pressure, rather than temperature. This is a common strategy for water cooled condensers and the valve is readily available. Controlling off pressure rather than temp assures that we are riding the upper limit of the compressor but not exceeding it.
    Ok, good. That makes good sense.

    Do you have any manufacturers, or model numbers that may be of use in this application? I'll research it.
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    Re: AWHP superheat & sub-cooling


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