TX Troubles

[440hardtop]

Hi all, I have a qestion about when is best to take your superheat readings to set your TX valve. I have a freezer that is set at -25c, Lets say the ambient temperature +30c when the freezer is turned on, as it starts to cool down when should i be taking my superheat readings?

[Brian_UK]

When the freezer is at -25°C.

Do NOT adjust the TXV unless absolutely necessary.

A search on this forum will find a great range of discussions on superheat and not adjusting the TXV.

http://www.refrigeration-engineer.com/forums/showthread.php?t=5135&highlight=superheat

http://www.refrigeration-engineer.com/forums/showthread.php?t=11903

[wambat]

That also applies to fixed metering devices always at the control set point.

[440hardtop]

If it isn't to much hassle could somenoe explain to me why this is the best time to check your superheat?

[Gary]

The whole point in using a TXV is to have high refrigerant flow when the refrigerated space temp is high and low refrigerant flow when the space temp is low.

The refrigerant flow can only be high when the superheat is high and can only be low when the superheat is low.

High space temp = high flow = high superheat.

Low space temp = low flow = low superheat.

Normal space temp = normal flow = normal superheat.

You should expect to have design superheat only when you have design space temperature.

[440hardtop]

Cheers Gary, ive had it explained to me before but never had it put that simply, it's all making sense now. Thanks again.

[cretan]

From what I have read,around 10f evaporator superheat is good.Some say to shoot for 20f sh at compressor.Others say that 20f at compressor is not important as long as you have some sh at the compressor.

[Gary]

From what I have read,around 10f evaporator superheat is good.Some say to shoot for 20f sh at compressor.Others say that 20f at compressor is not important as long as you have some sh at the compressor.

TXV superheat should always be adjusted according to evaporator outlet superheat.

Compressor manufacturers want at least 15-20F superheat at the compressor inlet. Proper superheat at the evap outlet will generally result in sufficient superheat at the compressor inlet.

But that isn't the whole story. Many compressors are refrigerant cooled, so high superheat at the compressor inlet can also be a problem.

If the TXV is adjusted for proper evap outlet superheat and the compressor inlet superheat is high, then better suction line insulation and/or mechanical de-superheating may be needed.

The TXV should never be adjusted to achieve compressor inlet superheat, but rather to achieve evap outlet superheat.

[oupak]

Hie,first time post. Iwould like to ask
1.what is definition of sensible heat of an air handling unit (chilled water) and how do you determine it?

[nike123]

Hie,first time post. Iwould like to ask
1.what is definition of sensible heat of an air handling unit (chilled water) and how do you determine it?
http://www.daikin.co.uk/faq/items/sensible-latent-heat.jsp