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Thread: AKV versus TEV

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    AKV versus TEV



    This is the answer I will give to the engineering office tomorrow.
    My conclusion to them is that AKV's are not worth to install in this application (evaporators of 3 kW till 12 kW)
    Will you look for anomalies?
    Tried to explain it simple because they're som sort of all round building engineers and no Refr. engineers.

    Peter

    I think it’s wrong to premise a client energy savings which are made by the manufacturer himself – Danfoss in casu – without the fact that they have been tested completely and independent and/or are proven by thermodynamically calculations.
    In many cases where savings, Danfoss was sponsor of test or test were done under supervision of Danfoss. This is not correct.


    Important remark beforehand: every compressor manufacturer insist - for obvious reasons - a SH of at least 5K. Owing to this, the whole theory of an AKV with his low superheat becomes doubtful.

    The only energy savings that can be made are perfectly plotable in a log-P diagram.
    This savings can be accomplished by increasing the specific mass of the sucked gas and/or preferable both, decreasing the high pressure through which the pumped mass will increase.
    The more LP and HP comes together, the more we will achieve the ideal Carnot process.

    Danfoss states in his selection tables that a minimum DP of 2 is needed.
    This is not realistic. Why?
    Let’s take a room-temperature (RT) of 0°C and evaporating temperature (TE) of –8°C at evaporator or –9°C at compressor (friction losses), so…this setup should condensate with a DP of 2 bar at 2°C (R404a) This is unrealistic seen the average annual temperature of 11°C in Belgium.
    For a freezer it’s even impossible at all without a compound or a 2 stage system.

    If we take the mean Belgium outside temperature of 11°C, the we have 6,5 DP over the TEV when evaporating at – 9°C
    This is far more realistic and savings has to be seen with a DP of 6,5 bar.

    SH can be reduced to 4 K in an installation with long lines and/or suction accumulator.
    A TEV can function perfect with a DP of 6 bar.
    When DP increases to a DP of 12 bar (summer conditions when temperature outside reaches 30°C), TEV capacity increases with 20%. (TEX5-OR4)

    A DT of 2 K for an AKV is perhaps to rosy, (compressors manufacturers demand 5 K) , let’s assume that this is possible and we compare this wit a SH of 5 K for a TEV and a RT of 0°C.

    We wish a DT of 8K or evaporating at –8°C. The suction gasses of an AKV will be 3 K colder then those with a TEV, so denser.
    Both – AKV and TEV – evaporate at the same evaporating pressure, but compressor will suck denser gasses with an AKV, so a bigger specific mass.

    If we simulate this with the software of Bitzer with an input of 10 kW, condensing at 20°C;, 0K SC and suction temperature of –3°C. This gives a COP of 4,6 (2DC-2.2-40S)
    If we increase to 2°C (5K more or a TEV) then the COP doesn’t change.

    The only benefit an AKV gives, is the fact that a larger part of the coil will be used for evaporating and less for superheating.
    This larger coil area will result in a bigger capacity.
    This larger capacity will result in a shorter running time of the compressor (energy savings)

    Or explained in another way, we can evaporate a little bit higher for the same initial capacity. (increased COP)

    How much higher can we evaporate due to this smaller SH for an equal capacity as previous by which the compressor will run with a higher COP?

    This question can’t be solved easily. All the capacity tables and selection software uses a by the manufacturer predetermined SH which in most cases can’t be changed.

    Therefore, I asked this question to some evaporator manufacturers, all Eurovent certified: Kuba(Germany-GEA group), Goedhart(Netherlands largest and well know in the EU), Helpman and Heatcraft – Friga Bohn.

    Their answers:
    Kuba: when evaporating with a DT of 8, and a decrease of SH from 5 K to 2 K (in other words TEV to AKV), capacity will increase 5,3 %.
    An evaporator of 10 kW will give 10,5 kW.
    But, trough this bigger capacity, we can slightly evaporate a little bit higher for the same initial capacity (10 kW in our case)
    This will result in a – increase of only 0,5 K.
    So an increase in performance of some tents of degrees.

    Searle (UK) says: “from testing done some years ago, the increase in performance when movingfrom 6 K to 2 K superheat with 8 K TD is approximately 10%. In theory, this would mean that with 0.2 K superheat the 6K superheat capacity could be achieve at 7.3 K TD.

    “In practice, 2 K superheat is virtually impossible to achieve without flooding. This is because refrigerant distribution is never perfect and it not helped by a pulsing electronic expansion valve. The average superheat may be 2 K but the actual value will vary between circuits and over a pulse cycle of the valve.”
    “Further to this we would add from an applications point of view that if asked to offer a selection on this basis we would decline. As although it is possible in theory it would not be feasible rely on this effect in practice.”

    Helpman (Netherlands) gave almost exactly the same figures as Kuba: in increase of 7% in capacity and an increase of evaporating temperature of 0,4 K.

    So,… BIG peanuts.

    Same profit can also be achieved with a larger evaporator which will work with a smaller DT.

    Conclusion for this section: installing an AKV will not give the promised savings. The cost for the AKV will never payback.

    Compresor side.
    I made here a theory that was proved to be correct because they were crosschecked with evaporator manufacturers.
    These entire where efforts to optimise the filling of an evaporator.

    But then, all his is only valid and only then if the compressor or pack can also follow these very narrow benefits, if the evaporator temperature at the compressor can be adjusted to such a small variations in evaporating pressures.
    Otherwise, this theory will not hold water.

    Without a frequency inverter, it’s almost impossible to achieve a stable evaporating pressure.
    This stable pressure has to be measured at the service valve of the compressor, not on the evaporator. The differential pressures – measured over the compressor - mainly determine the COP (and not measured over the evaporator)
    Therefore, installing a CVP at the outlet of an evaporator doesn’t solve the problem at all, no even an electronically version of it. The COP will then even decrease when working with a single compressor.

    Most packs have a death band wit coupled with it a delay time. Otherwise, the compressors will switch to frequently.
    During en due to this delay time, pressure can and will vary around the ideal set point. On those moments, the pack will run with a worse COP.
    Technically, This can’t be prevented.

    This also means that the predicted benefits at evaporator side will be completely cancelled due to the inherent but necessary slowness of the pack.

    We can build the most beautiful theory around perfect injection in evaporators, and the possible benefits this will give, if the other components can’t adapt constantly and perfectly to these new situations, and then it will be of no avail.

    If a standard pack will be installed, AKV's are useless because the benfits of them are annuled by the necessary slowness of the pack.

    Lowering condensing pressure.

    In the whole argument, there never has been spoken about lowering the possible benefits of subcooling the liquid.
    Practical field and lab test (Miller 1981, Linton 1992) showed an increase in cooling capacity for each 1K additional subcooling.
    These savings can easily plotted or proved in a log-P diagram

    So, subcooling can give substantial savings.

    A condenser is prescribed working with a DT of 10 K. A standard condenser is seldom foreseen of a subcooling coil.
    We must be satisfactory with a SC of 3K in most cases.

    The liquid vessel can be installed on the roof whereby it can add to the subcooling. But in our case, it’s placed on the ground floor. Liquid lines leave then the vessel back to the ceilings.

    The savings for a lower HP are valid for all expansion valves. It’s a saving on the compressor side and has nothing to do with the sort of expansion device.

    The only thing that can be said when lowering the HP is that the chance becomes bigger that flashgas appears in the liquid lines which will affect negative every expansion device.

    The subcooling achieved in the condensor will be lost again.

    This subcooling kan only be prevented by subcooling the liquid (after the liquid receiver and not after the condenser) or by increasing the liquid pressure without increasing the HP (isenthalpic)


    Simulation
    of a real situation of 10 kW, condensing at 35°C, R404a log-P diagram and let’s compare the COP, the only parameter which tells how efficient our compressor is running.

    TV SH SC COP
    1 -7 5 10 3,563
    2 -7 2 10 3,555
    3 -10 5 10 3,242
    4 -10 2 10 3,233
    5 -7 5 2 3,231
    6 -7 2 2 3,216
    7 -10 5 2 2,936
    8 -10 2 2 2,921
    9 -10 2 0 2,841

    Table deformed after posting: TV-7 or -10°C, SH is 5 or 2, SC is 10 or 2 and onc 0, figure wuith the decimal is the COP

    In this table (1/ 2) can be found that a reducing of the SH from 5K to 2K (= difference verschil between a AKV and a TEV) there is almost now COP improvement.( 0,2%)
    For a TV of –7°C and a SC of 8K gives a saving of 10%(1/ 5 of 3/7)
    At an equal SC and SH, the efficiency increases with 9% (1/3) if TV 3 K can be increased.

    Subcooling can give thus serious profits.

    After this, I gave also the explanation of Marc's Hy-Save and the theory behind it (4 pages). The benefits of it can be plotted and proven in a log-P diagram.
    Curious what they will answer and also curious in your feedbacks.
    Last edited by Peter_1; 25-04-2004 at 01:35 PM.



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    Re: AKV versus TEV

    Haven't had a chance to read ALL the talk on AKV's lately as I just moved house and got my PC setup, but, as much as I do like them they are only as good as the algorithm that runs them, that is the controller and the evaporator that they feed. In my experience with danfoss, (have assisted them over here in an energy savings trial in a supermarket - bit different though not AKV vs TXV but Danfoss rack and case control vs Abbotly energy savings module) turned out AKV control with their own controllers was very poor , definitely not as good as CPC Einstein case controls.
    If money savings the big sell, LPA's gotta be the way to go doesn't it?

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    Re: AKV versus TEV

    Hi marc
    What about controlling the condensor superheat, I know that Danfoss rack controllers do have the facility, what effect does that have on savings and /or on the system?

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    Re: AKV versus TEV

    Condensor superheat?
    Should it not be condensor subcool or evaporator superheat?

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    Re: AKV versus TEV

    They apparenty control the superheat of the CONDENSOR this is a Danfoss product

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    Re: AKV versus TEV

    Presure and temp on the outlet of the condensor that is how they measure the superheat and that controls the fans in or out

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    Re: AKV versus TEV

    Always thought they once should re-invent the warm water again at Danfoss.
    Where have you read that Allan?

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    Re: AKV versus TEV

    Their Sales man gave it to me but i think it is on their website it it the adap kool comp rack controller

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    Re: AKV versus TEV

    The Danfoss rack controller has an option to control air cooled condensers on pressure or saturated condensing temp, the first uses only a transducer reading discharge pressure and the sat. cond. temp. uses discharge transducer (converts to temp in controller depending on refrigerant selected) and monitors liquid line temp out of condenser, you tell it the design TD of the condenser and put in a setpoint in degrees C or F for the condensing temp, I thing there is also setpoints for max subcooling allowed, will have to simulate the program on my laptop when I get a chance. Remember giving it a shot once and found it to be no better control than the standard setup. To answer it simply, its not superheat its subcooling, mind you they do make superheat controllers... Case controllers.

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    Re: AKV versus TEV

    I speak under correction but according to my knowledge subcooling is the difference between the liquid temperature reading at the outlet of the condensor and the temperature of the liquid prior to entering the expansion device. So my question is this the change of state of the gas in the condensor is called what? If the change of state after the expansion device is superheat. Another point is that they dont have a temperature reading at the liquid entering expansion device to measure the sub cooling so i dont know, may be someone can help me understand this.

    Secondly they (Danfoss) do make controllers that operate their pulsating expansion valves which control the superheat and trends set on history. Not only does the valve work on actual readings but operates the valve according to previous history of the cabinet or valve.

    So if subcooling is the reduction in temp of the liquid from the exit of the condensor to the entry point of the expansion valve and the change of state in the suction side is measure between the sst and the temperature exiting the evaporator is called superheat then what is the change of state in the condensor called?

    Please know this, i understand the principles but what is it called is what i'd like to know. If you can have exactly the right amount of THR then surely one will have a full head of liquid which one can possible subcool ( heat exchanger) and achieve much better results than just bringing in cond fans according to head pressure.
    Last edited by allanbaker; 18-05-2004 at 05:08 PM.

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    Re: AKV versus TEV

    Change of state in a condensor - gas to liquid - is called condensation.
    Your explanation of subcooling is correct.
    They measure outside temperature (suction at the condensor) and condensing pressure.
    I still haven't found something about superheat of a condensor, is in fact thermodynamically impossible in a normal setup.

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    Re: AKV versus TEV

    I speak under correction but according to my knowledge subcooling is the difference between the liquid temperature reading at the outlet of the condensor and the temperature of the liquid prior to entering the expansion device.
    You are nearly correct with the definition. Subcooling is the amount of cooling applied to the refrigerant after saturation point. It does not apply to whether it is as it leaves the condenser or not it is actually after the saturation point. This might happen half way down the condenser (dependant upon the ambient) or right at the exit from the condenser, etc.

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    Re: AKV versus TEV

    These particular controllers also have an option to monitor de-superheat temp, if your installation uses a de superheater - possibly a two stage setup. I think this is just a mix up in terminology, the controllers monitor desuperheat temp, they also control HP via saturated condensing temp.

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    Re: AKV versus TEV

    [QUOTE=Peter_1]Change of state in a condensor - gas to liquid - is called condensation.
    Your explanation of subcooling is correct.
    They measure outside temperature (suction at the condensor) and condensing pressure.
    QUOTE]i think that you misunderstood what i was trying to say
    I know it is called condensation but what is the equivelent name of superheat in an evap in the condensor (THR), for instant supercool (opposite translation) or de-superheat?
    Todays times look for innovative devices and i believe that it makes sense to control the condensor correctly to achieve the best liquid temp without any flash gas and the product that Danfoss have come out with seems to do just that. Most controllers control on pressure and not on " de superheat" and creates a full head of liquid going to your TXV or AKV. In conjunction with the AKV one can really save large amounts of cash on energy bills.

    just another thing that may help is, here in South Africa electricity is mesured in maximum demand and units used, where the maximum power consumption during any 30 minute period during the entire month is charged at the highest peak for the whole month. if the maximum demand can be controlled then one can set the energy bill. So what we have done is taken a store and measured the maximum demand and coupled it with the refrigeration and airconditioning system and when peak times arise a digital signal is sent to activate that the suction presure is raised cutting out one or two compressors, just by doing this we were able to save the supermarket 32% of his electricity costs. The feasiblity study showed a payback period of 9 Months, there after savings big time. Here is another option and quite easy to do

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    Re: AKV versus TEV

    I know it is called condensation but what is the equivelent name of superheat in an evap in the condensor (THR), for instant supercool (opposite translation) or de-superheat?
    Todays times look for innovative devices and i believe that it makes sense to control the condensor correctly to achieve the best liquid temp without any flash gas and the product that Danfoss have come out with seems to do just that. Most controllers control on pressure and not on " de superheat" and creates a full head of liquid going to your TXV or AKV. In conjunction with the AKV one can really save large amounts of cash on energy bills.
    All the points right of the saturated vapor curve are superheated. It becomes only more superheated then it was due to the compression.
    You can de-superheat it again with for example a heat exchanger to make up warm water.

    The same system for peak control exists also in Belgium but it's measured in the coldest 4 months from November till February and from 11:00 till 13:00 and 17:00 till 19:00.

    The best liquid temperature for a TEV and AKV, or better the whole system is the lowest liquid temperature possible and as long as you stay left of the saturated liquid curve, there will be no problems with flashgas.

    As far as I know, you walways need a full head of liquid going to your TXV or AKV. If not, your valve will function not properly.
    In a 6 sec cycle, the moment the AKV opens and no liquid is on the inlet, what then?

    Thermodynamically seen, the lower the condensing pressure, the better.
    And the further you go with your condensation line to the left, the better (larger enthalpy)
    So no fancy electronics for this. Just condensers big enough or other means to do this.

    A Danfoss controller can't prevent flashgas in the lines. No way. You correctly stated " it seems...".
    This can only be done mechanically: adding somewhere a subcooler, preferable after the liquid receiver.
    How it's subcooled is another story.

    We service a big installation (slaughtery) with flash prevention (subcooling) from 1960, long before the electronic age. And it still works.
    Without any transistor or resistor in the whole installation.

    I'm a real electronic enthusiastic (did some microprocessor programming with PIC's and Stamp) but they're sometimes used - or better pushed - in some refrigeration applications where they better hadn't done it.

    Explain me and prove why an AKV can save on energy bills?
    Nobody so far could convince me AND prove it thermodynamically. We can't change the nature laws.

    Let's take a real 6 sec cycle of an AKV: the moment it injects some seconds, then I see no problems. But after the opening, the valve closes again. So on that moment there is at the intrance of the coil no liquid entering, so no heat exchange, at least in some rows.

    At a speed of some m/sec, some rows of the coil will starve out of *****. Perhaps the injected amount of ***** goes that fast in the tubes that it meet somwehere further in the coil the previous injection and bumps onto it. (?)
    Or are they passing through the tubes at the same speed and will they never meet each other?

    But on the low pressure, you will see a +/- equal LP pressure due to the slowness of the system.

    Perhaps a good idea for a school: mount an AKV on see-trough high pressure tubes so that you can see what happens inside.
    Last edited by Peter_1; 26-05-2004 at 04:15 PM.

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    Re: AKV versus TEV

    hi guys,

    I 've been folowing this thread from the begining and it looks endless.

    I am not trying to judge anyone but I've been using both systems and all I can add the discussion is that from first investment AKV is much more expensive and not as relyable as TEV.

    the problem of full head of liquid can be solved easy by design the right size condenser and liquid subcooling system and adding speed controled fans.

    I dont belive for a second that a unit operated with AKV will cool a given subtance, faster then the same unit doing it with TEV!!

    one the big differences is that you do have to know is how to adjust a TEV and to use the right orifice.

    I belive that in that point lays the difference. please all those who can do it perfectly, raise one hand.

    AKV takes a few seconds to do it on a computer or the hand held monitor.

    the last thing about saving energy puzzles me. I have learned and worked all the years that insulation is the biggest energy factor.

    so two units with the same capacity, operating with two refrigerant control methods will not produce any differences in power drawing.

    I will stick to peters idea and no, I didnt get a free coffe at the tearoom.

    chemi

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