Temp inside evaporator tube and outside

[Johan Malan]

How big can the temperature difference be between the evaporating refrigerant inside the pipe and the actual temperature meassured on the outside of the pipe? Some people say up to 6K....? Can someone please help.

[Andy]

Hi:)
No 6td would be the temperature difference between say air and the refrigerant not the inside to the outside of the tube. The differance in this case will be that small as to be nearly un-measurable.

Kind Regards. Andy:)

[US Iceman]

As Andy suggested the temperature difference across a tube wall can be very small. It does vary depending on the tube wall thickness , metal conductivity, and the Reynolds number of the fluid flow on each side of the tube.

Thinner tubes would have a lower temperature difference due to a lower wall thickness. The same applies if copper is used instead of stainless steel.

The faster the fluid flow is on either side also helps to break down the boundary layer which inhibits heat transfer. If the fluid velocity is too slow, this film resistance increases, therefore you need more temperature difference across the tube to drive the heat transfer.

PS. Welcome to the RE forum. That was good question to start with. ;)

[Dan]

This question begs the reasons for mounting TEV bulbs at 4 or 7 o'clock. At 6 o'clock, the oil may insulate the bulb. At 12 o'clock, there may be a layer of vapor. This implies that heat transfer does differ around the circumference of the pipe at least to the point manufacturers think it could affect TEV valve operation.

[US Iceman]

Right you are Dan.

This is what makes the RE forum so interesting...

One person reads the question and approaches it from one area while another comes in from a completely different angle.

The same question can generate multiple answers that can make a person think.

[Lc_shi]

This question begs the reasons for mounting TEV bulbs at 4 or 7 o'clock. At 6 o'clock, the oil may insulate the bulb. At 12 o'clock, there may be a layer of vapor.
A visual description is easier to grasp,ASHRAE book say it's at 3 or 9 o'clock. I think it's same :)

regards
LC

[Johan Malan]

Thanks for the answers guys.

[US Iceman]

Glad to help Johan. Did you get the information you were looking for?