Hey Gary,
I overdid on a volunteer project and have had to lay low. My health is pretty shaky.
While waiting for parts and supplies, I've been trying to get up to speed on the relationship between pressures, temperatures, and refrigerant flow rate, and efficiency. I've come a long way, though my memory problems makes it a challenge. COP or efficiency is a matter of keeping superheat close to the safe minimums. I must achieve a lower superheat and lower suction side temperature (60-65F max) in order to have the COP I need (to get 10K BTU's out of the water with 4.61A current) plus keep the compressor cool.
It's still unclear if just increasing refrigerant flow alone will result in low enough suction vapor temperature for compressor cooling, and for a low enough superheat to get a decent COP.
Swep is a company that makes brazed plate heat exchangers. They have some very helpful design articles.
This is a a good summary that relates well to my situation of too restrictive (stock) capillary tube, poor performance at rated power (my added italics):
"When a SWEP BPHE is used as an evaporator, a secondary gas or liquid is cooled as it loses heat to the refrigerant. The refrigerant boils and is converted into a gas, absorbing more energy. A SWEP evaporator provides a good, stable boiling process with a small temperature difference between the refrigerant and the secondary fluid. A low temperature difference means that a higher evaporation temperature is possible, which corresponds to a higher pressure. Reducing the pressure difference between the low-pressure side (evaporator) and the high-pressure side (condenser) will reduce the energy use in the compressor. The higher evaporation pressure will also increase the density of the refrigerant gas. For each stroke, the compressor will therefore transport more refrigerant through the system. Lower electricity consumption and higher refrigeration capacity will increase the total system efficiency (COP). "
They also have a good article on capillary and TXV valves. Alas, they don't specifically address my water temperature being so high. Another problem with high water temperatures is that the suction side vapor temperature is above the normal 60-65F maximum temperature for cooling the compressor. The return vapor is run inside the housing for pump cooling, and is the primary means of pump cooling.
I did think of a plumbing cheat that might be useful; by adding a bypass valve on the water to and from the BPHE/evaporator, this would allow the BPHE water (and suction side) temperature to be lower, while keeping the temperature going to house above 58F (to avoid manifold condensation and mold). This might allow the use of a standard TXV and operate at lower (high side) pressure. The normal TXV line pressure and temperature balance will be outside the normal design range otherwise. This also gets suction temperature below the 60-65F that is specified to provide compressor cooling. Right now, if water temp is 78F, so is the suction side, and that's BAD news for the pump temperature and life. This could also be helpful for a capillary tube expansion valve, in that it allows for the return vapor temperature to be within specs for compressor cooling. One issue with this water bypass method is that water flow may have to be changed to the same direction of flow as the refrigerant to lower suction vapor temperature. Right now, suction temp tracks inlet water temp...too high!
I found that there are no manual/hand expansion valves rated for R410a pressures. They were used and are still being made for R22 level pressures.
I did find a teflon packing, high pressure needle valve that could be used in addition to an intentionally short cap tube to make up an R410a rated manual adjustable expansion valve. Most high pressure needle valves use viton o-rings/seals and refrigerants eat Viton. EPDM is fine, but they don't seem to use that.
Today's project is to build my reclamation pump with the used R410a compressor and filter/dryer. I'll still use ice water in a bucket around the reclamation tank.