This a long one - AKV

I think I found the article the engineering office was referring to when we were discussing he benefits of an AKV.
I read it critical and disagree with some points. I need some advice from the pro’s among us.
I ‘italiced’ the text from the article to make it more readable.
http://refrignet.asean.danfoss.com/SW/SGRA_ApplicationExamples/En/assessment_of_danfoss_adap-kool_System.htm?newguid={DF7E01DE-0365-4965-B643-DE548A87F1F6
Introduction

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Due to the number of components involved and the number of variables needed to be controlled, manual optimization is almost impossible for a varying demand situation.

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Such a big variations don’t occur with a simple cold room or with cabinets and bins in supermarkets. The loads there remains fairly a constant. Goods are delivered under storage conditions.
3. Regulation of Refrigerant Flow with Conventional Thermostatic Expansion Valve

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….Dry evaporator circuit must be designed so that no liquid refrigerant can get into the compressor suction. To ensure this, the TXV is throttled to maintain a constant superheat of around 5oC (BS 3122 Part 2 specifies 10oC for standard compressor test) at compressor inlet. When the load is reduced due to products removed from the cold storage cabin, there is not enough heat to boil the refrigerant and superheat reduces.

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When products are removed out of a coldstore, load is only reduced if goods have latent heat an/or where at a temperature higher then room temperature. In most cases, goods are on the same temperature and removing them doesn’t do anything to the load at all. Superheat will reduce a little bit but bulb will sense this and close the valve to maintain the same superheat. 10K (as stated in BS3122) then also valid for an AKV I suppose.

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The opening of TXV must be reduced to emit less refrigerant and the system establishes itself to a new equilibrium of lower evaporating pressure.

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If pumping capacity of compressor stays the same. With the proper regulation on the compressor, pressure will remain the same.
In the process of establish this new equilibrium, superheat can be over-corrected to a much higher value than the preset 5oC level.
Why should this occur? TEV will stabilise again to the pre-set superheat and it will take some seconds to do this . But also for an AKV: sensors must first measure the difference .

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From the energy efficiency points of view, the conventional TXV has the following shortcomings:
(1) The need to maintain a superheat, i.e., a higher inlet gas temperature to the compressor and therefore requires more compression power.

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The need to maintain a constant superheat is a need for the compressor and is not related to the expansion device you are using. And this is only valid for a single unit. With a pack, pumping volume will be decreased and superheat shall remain the same. Both a TEV as well an AKV maintain a constant preset superheat and the same (higher inlet gas temperatures) will happen for both. So, I don’t see the shortcoming for the TEV.

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(2) The need to operate at lower evaporating (suction) pressure and therefore higher pressure ratio with higher compressor power at reduced load.

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If LP lowers, then needed compressor power will also lower (less pumped volume) But also, less pumped volume is less condenser-load, so lower HP. Perhaps they will say that you can increase suction pressure a little bit higher due to the lower superheat. That’ is more correct.

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(3) At lower evaporating pressure, temperature in the cold storage cabin is also lower and that will promote heat transfer from the surroundings to the cooled storage space with increase in unnecessary cooling load.

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This is a stupid statement: when evaporating pressure lowers, room temperature will not lower. The air out temperature will lower (with a single unit) and room will be cooled faster. Thermostat will cut off the SV or compressor as soon the preset room temperature is reached. Suppose it should be through, the load through the walls is margin. As long as room temperature is maintained, whatever the evaporating pressure may be, heat losses remain the same.

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(4) The need to maintain a relatively constant pressure drop across the TXV such that the condenser pressure cannot be ‘floated’ down to save compressor power during low ambient conditions.

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The fluctuations in condensing pressures are not that sudden. They change with changing outdoor temperatures which goes slow in relation to the time needed for a TEV to adapt itself to the new lower liquid pressure condition. And even then, when pressure drops, capacity of a TEV will drop but never to such a level that in fact a bigger one should be installed. Let’s take a TEX2, orifice 04 for a needed capacity of 10 kW: normal DP is +/- 12 bar or 10.5 kW.
If DP drops to 6 (is condensing at 10°C or 0°C outside temperature), then capacity is 8.7. But installing a orifice 5 with this condition will give 10.5 kW at a DP of 5. The only thing that will happen is perhaps some hunting.

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As the components in a refrigeration plant are inter-related, it is extremely difficult to optimize the system with the existing TXV and simple pressure and temperature setting methods. This explains why most refrigeration plants with conventional control are operating at poor efficiency at part load.

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Why is it extremely difficult to optimise a system with simple pressure and temperature methods? And what do they mean with inter-related? A difficult expression to explain that all the components must fit together? This explains nothing for me. The reason why they run with a poor efficiency is the fact that when the load is lowered (is the load lowered that much in a standard application), the compressor load is not lowered for this new situation. This in fact many times not possible when running with a single unit. And as said in another thread: load in a common application remains constant for years (perhaps only after a defrost)

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An ideal control system for refrigeration plant must be able to:
(1) maintain a negligible or minimum superheat at all loads;

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Correct

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(2) maintain a fixed evaporating temperature (or pressure) at all load;

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This is something that not can be done with a TEV nor a AKV. Only wit a reducing compressor capacity or an evaporating pressure regulator.

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(3) float the condensing temperature (or pressure) downward during night time or raining days.

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Correct but why not run it also low during the day.

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Reduction in Superheat
As electronic expansion valves are known to give bare minimum superheat[1,2], its use can reduce the degree of superheat and therefore reducing compressor power requirement. The saving on compressor power will depend on (i) type of refrigerant and its flow rate, (ii) compressor operating pressures and (iii) degree of superheat with original TXV.

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For a given refrigerant, difference between TEV and AKV, needed compressor capacity is only related to superheat (flow is related to the load) There was a time ago a thread around low superheat and the problem that then arise with oil return.

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For refrigerant R22 operating at an evaporating temperature of –10oC, compressor power reduction due to decreasing superheat is estimated to be as follows:
Reduction in Superheat (oC) Reduction in Compressor Power (%)
5 1.8
10 3.7
15 5.4
20 7.1

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Normal superheat is +/- 6K for a proper adjusted TEV. So you can lower maximum 3K with an AKV.

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4.2 Control of Evaporating Temperature
With the use of electronic expansion valve and evaporator controller, it is also possible to maintain the evaporating temperature and the compressor suction pressure without unnecessary overcool at low load period. For refrigerant R22 with a design evaporating temperature of –10oC, lowering of evaporating temperature to –15oC will cause the compressor power to increase by about 10.6%. For every 1oC drop in evaporating temperature, compressor power can be expected to increase by 2%.

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Perhaps I understand this wrong but it’s jus the opposite way: when load reduces, evaporating drops. This can be prevented – at evaporator side – with an evaporator controller which will indeed stabilise evaporating pressure. But suction pressure will drop then dramatically. So the final result for the needed power will be the same. But they start to mix 2 different regulating devices, not only the AKV.

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4.3 Floating of Condensing Temperature
The combined applications of electronic expansion and the compressor controller enable the floating of condensing temperature according to outdoor ambient condition. When the outdoor temperature is dropping at night or during raining season, the condenser can operate at a lower condensing temperature.

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The combination TEV/Suction pressure regulator will give +/- same result.

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For air-cooled condenser used in most cold storage plants, typical condensing temperature is chosen at 15oC higher than air inlet temperature to the condenser. If the design air inlet temperature is taken as 32oC, the design condensing temperature would be 47oC. Assuming an evaporating temperature of –10oC, the compressor will be pumping from 0.353 MPa to 1.805MPa or a compression ratio of 5.11. By floating the condensing temperature (and pressure) downward, power consumption by the compressor could be reduced.

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We chose them at 10K. Those who select 15K asks for high power bills. Same should happen with a TEV, this statement is not only valid for an AKV.

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It can be concluded that with R22 designed to run between –10oC and 47oC, every 1oC drop in condensing temperature will save 1.5% of compressor power. In the Singapore context, maximum fluctuation in ambient temperature can be taken as 8oC. Energy saving in floating the condensing temperature can be substantial.

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This is through, but condensing pressures were taken high for a Tev and low for an AKV to enlighten the benefits (?) of an AKV.

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4.4 Defrosting Cycle Control
The evaporator controller can also be programmed to activate the defrost circuit on demand only rather than following a fixed schedule. Although the direct energy saving on this improvement may be small, it can help to reduce the extra cooling load needed to cool down the evaporating tubes after the unnecessary defrosting.

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Can also be done with other controllers.

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5.1 Saving Assessment
From our observation during the visit to the site, the installations of both control systems were properly done and both systems functioned normally and satisfactorily. The savings obtained during the different periods are relatively consistence (see table below). Savings of +/- 22%The percentage savings of energy for the different periods vary from 20.8% to 23.6% which are considered to be very consistent for this kind of energy conservation projects.

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Those figures don’t say anything if they don’t state at what conditions the compressors were operating, especially the HP side. I don’t say they’re manipulated but I don’t believe the savings. They’re too big. The savings don’t come from the AKV alone.

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5.2 Other Observations and Comments
(3) System effectiveness is dependent on the proper installation and control parameter settings.

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The same for a TEV

peter,

cant you see, they are making more money from selling AKV's!!

chemi

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Originally Posted by chemi-cool

peter,

cant you see, they are making more money from selling AKV's!!

chemi

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This is really correct!

Just noticed now the pro's never gave their opinion

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Originally Posted by Peter_1

Just noticed now the pro's never gave their opinion

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Maybe those days there is not much pro's as today! We will see in few days.;)

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Originally Posted by Peter_1

Just noticed now the pro's never gave their opinion

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Hello Peter:)

my opinion is that there is applications for AKV and TEV.
I would tend to use AKV for applications where suction pressure optomisation is possible, where multiple evaporators are used and where loads drop off at night or during low ambients. But only on the controller that uses a pressure transducer.
TEV I would use on large fixed loads, where the load is constant and where the ambients don't vary as much.


The advantage of AKV over TEV is it sets it's self up, the engineer does not need to optomise the superheat. If suction pressure optomisation is not required and the TEV's are properly set up I personally don't think there is any advantage using AKV's.

Just to note it is usually more cost effective to purchase AKV's and their controller when larger 30kw + per valve loads are to be considered.

One application where AKV's should not be used or should be not set up to modulate is where off coil humidity is important (fruit stsorage, serve over counters ect).


Kind Regards Andy:)