Hi Drew
Every evaporator has an MSS (Minimum Stable Signal (Superheat control point)) this maybe 4k ,maybe 14K it all depends on the evaporator design, distributor pressure drop, air flow etc etc.
A Thermostatic valve can never work with a 1K superheat as the superheat spring would have little or no effect and the orifice would simply bounce open and close with no control at all.
Three forces make the TEV work , Bulb Pressure(Opening Force) Evaporator Pressure force and Spring force combined (Closing forces). The spring tension is the only thing on a TEV that you can adjust to change the S/heat control point. When you find the MSS point of the evaporator the TEV setting is at its Optimum setting. This setting will all depend on the plant set up and the evaporator design typically a good evaporator can work typically between 4 -7K.
Some years ago I introduced the Danfoss TQ electronic expansion valve to the UK , and York fitted them to their Gold Standard Chiller. York had several great tech guys and we decided to carry out deep R&D work on the chiller to see what Valve suited the chiller best. We ran tests with an ALCO tev , then a Sporlan hydraulic barrier TEV , and then the Danfoss TQ electronic valve.
The Chiller barrel had reinforced glass end cover plates plus 3inch thick Perspex covers to support the glass then a steel ring to hold the special end covers on to the barrel. This allowed us to watch the refrigerant being injected right the tubes through the 4 passes of the chiller.
The 2 Thermostatic valves worked well with the compressor on full load , but when the machine unloaded you could see the chiller start to dry out of liquid and the duty fall off was dramatic. With the TQ electronic valve the full load duty with the valve set at 5K superheat was slightly better than the TEV's but the part load difference was dramatic , even at part load the chiller was still fully flooded up with no liquid carry over at all.
We then started looking at the liquid distribution through the chiller, and you could see large swirling venturi like liquid hitting the end covers, but you also see large tube areas of pure gaseous vapour near the swirling liquid. To try to help mix the liquid and vapour back up, in each end cover in the 2nd ,3rd and 4th pass of the chiller a heavy metal gauze was fitted. When we ran the plant the gauze acted like an oil separator and you could see oil becoming entrained in the chiller, so the gauze was removed. So then the gas carrying tubes were blocked off, some 17% of the tubes, and the plant re run . The chiller still did its design duty but now we actually ran it at 1k superheat. It was run fully loaded then unloaded at 50%, 25% 75% up and down at 1K and there was never any signs of liquid or foam in the compressor oil . The control was spot on at all capacity levels with the TQ valve. The pressure drop through the chiller was slightly higher but with 17% of the tubes the chiller still performed perfectly.
Air coolers are notoriously of poor design ie a square coil block with round fans is a bad start, but only the small commercial coolers go through any degree of R&D , whereas with larger bespoke coolers the only R&D done is by guys like you & me when it doesn't work properly. Many of the largest installations for cooling have the totally wrong air flow design ductwork . These design guys think are turns right angles , well believe me it doesn't.
Electronic valves will always beat a TEV but as the old saying goes You cant make a silk purse from a Sows ear.
So I have proved Electronic valves can work at 1K superheat, but the chiller was slightly supped up from the normal design, but the TEVs failed badly at this point.
As I said in the previous mail you need to know how to best set up an electronic valve, otherwise you are wasting your time and your customers money,. So get your Carel or Danfoss rep to give you some in depth training and then you will see the benefits and so will your customers. I ran a training course on electronic controls just before I retired in Preston , 2 of the engineers were using the ETS version valves and when we started to go through the parameters , the only ones they changed were the refrigerant type and the valve size , everything else was left at factory settings . When we discussed the valves speed , its superheat settings and mode of control they didn't have a clue what changing the settings could do. So their company was using the latest technology but they might as well have stayed with a hand regulator.
This chiller is now in a university in Scotland, there were many videos of the chiller taken but they were never released , but they were a fantastic training aid , and helped to give me an in depth knowledge of evaporators and liquid injection its a fascinating subject .