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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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