AKV versus TEV

[Peter_1]

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.