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.
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
….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.
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.
The opening of TXV must be reduced to emit less refrigerant and the system establishes itself to a new equilibrium of lower evaporating pressure.
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 .
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.
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.
(2) The need to operate at lower evaporating (suction) pressure and therefore higher pressure ratio with higher compressor power at reduced load.
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.
(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.
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.
(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.
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.
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.
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)
An ideal control system for refrigeration plant must be able to:
(1) maintain a negligible or minimum superheat at all loads;
Correct
(2) maintain a fixed evaporating temperature (or pressure) at all load;