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Thread: This a long one - AKV
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16-04-2004, 08:05 PM #1
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
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.
3. Regulation of Refrigerant Flow with Conventional Thermostatic Expansion Valve
….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.
The opening of TXV must be reduced to emit less refrigerant and the system establishes itself to a new equilibrium of lower evaporating pressure.
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 .
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.
(2) The need to operate at lower evaporating (suction) pressure and therefore higher pressure ratio with higher compressor power at reduced load.
(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.
(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.
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.
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.
An ideal control system for refrigeration plant must be able to:
(1) maintain a negligible or minimum superheat at all loads;
(2) maintain a fixed evaporating temperature (or pressure) at all load;
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16-04-2004, 08:06 PM #2
Re: This a long one - AKV
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.
(3) float the condensing temperature (or pressure) downward during night time or raining days.
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.
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
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%.
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.
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.
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.
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.
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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5.2 Other Observations and Comments
(3) System effectiveness is dependent on the proper installation and control parameter settings.
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16-04-2004, 08:44 PM #3
Re: This a long one - AKV
peter,
cant you see, they are making more money from selling AKV's!!
chemi
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26-12-2008, 01:39 AM #4
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26-12-2008, 07:39 AM #5
Re: This a long one - AKV
Just noticed now the pro's never gave their opinion
It's better to keep your mouth shut and give the impression that you're stupid than to open it and remove all doubt.
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26-12-2008, 08:50 AM #6
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26-12-2008, 11:47 AM #7
Re: This a long one - AKV
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 AndyIf you can't fix it leave it that no one else will:rolleyes:
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