Don't think "choking the expansion valve" will raise subcooling.... Unless some one removed the receiver.Maybe someone taught you that Subcooling is what it is? Did you know Subcooling plays a huge roll in capacity. Ontop of that did you know you cannot achieve a solid column of liquid to the metering device without Subcooling? You should change your line to "Why aren't you using Subcooling in refrigeration?". Subcooling = Capacity / Superheat = Efficiency
You sound like a supermarket installer. You've taken 4° SH and slapped it on every freezer application. Do you know where the job is located? If the walkin is inside or outside of the facility? What the humidity is? If the door has a kill switch. Tuning lower SH = more refrigerant in the Evap = lowering subcooling/capacity = faster frosting = less surface area to transfer heat = impeeding air flow = iced coil = liquid refrigerant reaching the compressor.
He said he cleared the coil and couldn't go below 22F because of frosting. His suction saturation averages at -6/-5F. Superheat reading averages 9°-10°. It's obviouse that since he's trending at 22F, SH was the right thing to check. Subcooling let's him know if he has refrigerant/capacity. Choking the expansion valve will lower refrigerant metered into the Evap = raising Subcooling/capacity = less frosting at the coil = more surface air to transfer heat = unrestricted air flow through the coil. At 22F he shouldn't be reading a 10° SH. Should be more like 16°-26° SH. Once he reaches set point (which I'm guesstimating is 0F) his SH should be closer to 8°-12°. From there he should tune SH to Evap spec if available. After which he should then charge the system and bring his Subcooling up to 8°-12° if needed.
If the thought of just lowering SH to 4° is going to do the trick then why not just go all the way with 0°SH? Maybe you can explain how lowering SH and metering in more refrigerant on a coil that ices up at 22F with 10° SH will help?
Regardless of what he does
-Lowering SH risks compressor damage.
-Raising SH risks a higher temperature trend.
If it was my 4 point of contact on the line, you best believe I'd skip the risk of compressor damage first.