Super what???

Im sick and tired of hearing people spouting off about superheat settings and the correct way to commission a valve. Every explanation or method ive read in books, magazines,web sites etc seems to differ. The more i read the more confounded i become.
My question is this.

Where is the best position to take our td readings? Outlet of the tev and oulet of the evaporator coil.......or outlet of the evaporator coil and saturation temperature/pressure at compressor suction valve? Surely they are different and which one is correct???? Also why is it that on modern Japanese splits which are pre charged the valves are preset no matter what the length of pipe run? It can be anything between 10-30m but the valve setting remains the same as there is no adjustment necessary according to the pioneers.

Whose first to put this matter to sleep??

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Where is the best position to take our td readings?

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When you are measuring evaporator superheat via temperature difference (td), you are attempting to measure evaporator temperature by placing a temperature probe somewhere on the evaporator surface where one would expect two-phase (boiling) refrigerant. Placing this probe somewhere half-way thru the evaporator will normally be ok. If the evaporator is single circuit, one can place the inlet of the coil, but you'll typically get a more accurate reading more towards the middle of the coil. If the evaporator employs a distributor, you cannot measure temperature upstream of it and expect it to be anywhere near evaporator temperature. This is due to the pressure drop across the distributor.

Measuring superheat in this manner is not as accurate as using the pressure-temperature method. With this meathod, one evaporator pressure (with an accurate gauge, hopefully), and convert this pressure to temperature with a P-T card. If evaporator pressure is measured at the compressor suction service valve, the expected pressure drop across the suction line (1 or 2 psi is typical) should be added to your pressure reading before converting to temperature.

Once you've figured out evaporator temperature, it is a simple matter to subtract this temperature from the vapor temperature you read at the outlet of the evaporator.

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Also why is it that on modern Japanese splits which are pre charged the valves are preset no matter what the length of pipe run?

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This is a good example of KISS methodology... :)

On the modern japanese kit the metering device is always (with the exception of vrv) on the outdoor unit so the "liquid line" is part of the evaporator.
This is why the liquid line is insulated.
On the small splits the indoor/outdoor length is determined to suit the metering device. Small splits nornally have a cap restricter and this is reflected in the small indoor/outdoor lengths. The larger splits have EEV's which are driven by the electronics to suit the superheat etc.

You cannot compare a japanese split a/c system to a standard refrigeration system as the controls are miles apart.

There is NO adjustment necessary or available on split type conditioners as they do not use TEV's.

Thanks for the info prof. So basically we are sayig that the most accurate measurement of the temperature at the tev outlet is defined by reading our suction gauge pressure/temp at the compressor suction valve. Measure the temperature of the evap outlet with touch probe and subtract the gauge temperatre?

What about if we have a colossal pipe run? Does 2psi still carry the rule of thumb?

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Measure the temperature of the evap outlet with touch probe and subtract the gauge temperatre?

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You got it! :)

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What about if we have a colossal pipe run? Does 2psi still carry the rule of thumb?

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With colossal suction line runs, it is a good idea to have a pressure tap near the evap outlet. A pressure tap in the TEV equalizer line works well for this purpose.

If your colossal suction line run is properly sized, 2 psi should cover the line loss for most medium temp applications. A/C applications could have a tad higher pressure drop. But the Prof is from the "Show Me State", and would prefer having a tap near the evaporator outlet in the event the line is marginally sized.

If you used the 1 or 2 psi rule with regard to evap pressure by taking the pressure at the compressor.
Why can't you do the same and take a temp reading at a point close to the compressor inlet and make an allowance.At least on short to med piperuns that are properly insulated.

You could check at the beginning and at the end.
The reason I ask this as usually the evaps(AC & Refrig) are not always in the most accessible locations.

If you wish to know superheat at the evaporator, then measure it there. If you wish to know superheat at the compressor then measure it there.

There is no good advice assuming what happens between the two. Field piping is a variable within which there are no rules of thumb.

hmmmm. Not exactly. For example, if you have no superheat at the compressor, you know you have no superheat at the evaporator.

Compressor manufacturers recommend a minimum superheat of 25 deg F (I made that up, but I think it is close). What that says to me, is that they worry anything closer to saturation pressure could be misread in the field.... like out of caliber guages, thermometers.....and other variables such as evaporators icing up, and such.

High superheats also worry compressor manufacturers. Geez....

I go back to the beginning. If you want to know... you have to measure it where you want to know about it.

Accesibility, or lack thereof is no excuse.... it is a pain in the ass, however.

Just to give you actual numbers, Dan. Tecumseh recommends 15F minimum and copeland recommends 20F minimum at the compressor inlet. The idea is to cool the compressor (extending it's life) without flooding it (shortening it's life).

Dan, thanks for comments.