mp 39 system with past leaks

[kim]

Had a walk-in this summer with real high pressures. All I could do was tell the owner about the situation and recommend evacuating and recharging. They did not know how long they would be keeping that store.

I just went out there when it was 20F. (that is cold here) Walk-in was running warm. thermostat and soleniod seemed to be working so I go outside. Cut-in was set at 50psi. I dropped the cut-in to 25 and it started and ran. Looked like good R12 pressures for cold outside and warm box.

Here is my question: If this system is charged with 409 & 1/2, could it have normal evap pressure when it is cold and high evap pressures when it is hot?

[Marc O'Brien]

Here is my question: If this system is charged with 409 & 1/2, could it have normal evap pressure when it is cold and high evap pressures when it is hot?


Indeed it would,

The condenser coil capacity is proportional to TD or temperature difference. If it is designed for 30°C ambients and to condense at 40°C then that is a 10K temperature difference.

If the ambient drops to 10°C then the TD increases to 30K giving the condenser coil a rating 3 times its design. The result is a greater heat loss and lower saturated condensing temperatures.

Also the liquid is cooler giving the refrigerant a higher net refrigeration effect meaning the valve has to close to to maintain superheat. The problem is though, this doesn't get to happen, in fact the reduced pressure loss across the valve overrides the net refrigeration effect gain resulting in an underfed evaporator. The compressors volumetric efficiency increases with lower condensing temperature on account of the lower pressures there so it can also pull vapour faster from the evap.

Head pressure control is needed and if not used then the LP switch needs to be set for the lowest imagined operating conditions. Or a time delay LP override can be used at start-up if you do not wish to set the LP too low in case you lose leak protection.

[kim]

I did not measure the SH or SC, because they just wanted it to run and not pay much.

Suction line was cold though. I thought it was too cold for a 55F box. Sight glass was full. I did not look at it as the system started, so I could not say anything about start-up flash.

Basically I thinking the high condensing pressure was driving the high evap pressure this summer. I do not recall the evap pressure this summer, but it was condensing at 255 PSI. I assume the high cut-in was to keep the discharge low during pump-down. (Low being a relative term) Of course, at that head pressure volumetric efficiency could not be good.

The compressor was relatively new the evap looked like it was about 25 years old in an old Norlake box. The evap fan motors were on the outside of the fan guards. A real POS system.

[Marc O'Brien]

Looking R409A (FX56) (MP39) temperature/pressures...

For a box 55°F (12.7°C) which might be veg or beers etc.

Perhaps a suction pressure around 29psi would be normal. That's a mid point of about 26.6°C (-3°C)

For a 104°F condensing temperature, mid point, the pressure should be about 142psi.

Why might eh pressure be higher in summer?

It might be that the system has an accumulator and that the TEV somehow floods the accumulator in summer which causes internal fractionation where only the higher pressure refrigerants are left in circuit outside of the accumulator. Especially though if the LP switch was set so high. The unit would not pump the evap down properly which can result in start-up flooding to the accumulator causing the lower pressure component of the gas to stay their being suppressed by the now resulting higher average suction, and total, system pressures.

[kim]

When I have had a blended refrigerant in the past with strange pressures, I would generally evacuate and recharge. I assumed it was charge with the can up fractionualizing the gas. That would generally look like a restriction that somebody overcharged to try to fix. Or it had a leak and fractionualized. Either way, fresh gas charged correctly fixed the problem.

This is the second time I have seen this in a large system requiring lots of gas (hence money) to repair. Both times the owner refused the repair. This is the first time I have seen the system in the heat of summer as well as the cold of winter.

There is no accumulator on this system, but I see your point about the folly of incomplete pump-down.

[Marc O'Brien]

Here is my question: If this system is charged with 409 & 1/2, could it have normal evap pressure when it is cold and high evap pressures when it is hot?

Kim, what does "409 & 1/2" mean? And if 409 means MP39 what then does "& 1/2" mean?

[kim]

It means I suspect it was charged with 409, but it has been fractionualized by some means or another. The coil TDs were way off with decent SH and SC this summer.

[Marc O'Brien]

It means I suspect it was charged with 409, but it has been fractionualized by some means or another. The coil TDs were way off with decent SH and SC this summer.

Okay, yeah, internal fractionation is very common with high glide blends that have very low superheats or control problems that cause the occasional low suction line superheats. But an accumulator is necessary too.

The accumulator collects the last refrigerant to boil off which is a also the first to condense. Perhaps during a long off cycle some collected in the oil separator long enough for you to see odd pressures. But then you should have noticed a little too much foaming and cooling of the sump oil. Otherwise a leak late in the evaporator or early in the condenser will cause a loss of predominantly the lower pressure component.

You say all you could do was recommend a recovery, vac and recharge - did you do that?

Oops, have just gone down to reread your second last post and I see it has changed. Sorry, last night when I replied to it, it was an identical repeat of your first post. Okay, I see you did put fresh refrigerant and you do suspect fractionation which you originally stated anyway by saying 409 & 1/2.