starting of defrost time

[pirestani]

Dear experts,
Is there any new applicable technic or somehow usual device to recognize the defrost starting time of a subzero evaporator in cold room or in blast freezer?
Although I have heard a bit little somting about :
*photo-electric eyes switch
*ultrasoanic device
*barometric pressure-state&with delta(T) controller in PLC
I would be thankful if I can get advises?

[NoNickName]

Photo-coupler is the most recent application: when the photo-coupler recognises a light reduction of approx 40% to the nominal, defrost is started.

Otherwise, a more common combined defrost can be used. Time, pressure and temperature can be combined to get the most appropriate defrosting start.

[Peter_1]

I doubt there's a reasonable pay-back time for any system.
Where do you need to measure the frost build-up?
What about electronics ion a blast freezer?
What if the the system let the coil build up the frost every time a little bit more. All the savings this system probably gives is then lost.

Go for hotgas defrost, proper terminated.

[US Iceman]

It sounds like you are asking if there are any good "demand" defrost systems. In other words, the evaporator is defrosted only when it needs to be.

Hot gas defrost is by far the best as Peter said. You can record the time the liquid solenoid valve is ON and adjust the seasonal requirements for the defrost time. This works pretty well.

A big concern with hot gas defrost systems is how the defrost liquid is controlled. ***** systems try to do something different than ammonia systems (but I can't figure out why, unless it is due to costs).

[pirestani]

thanks for attentions .
at first let me say that I can not use hot gas and this isnt my problem.
indeed if I exactly catch when the frost layer perform
bitween the fins or coils of blast freezer evaporator, I would have much shorter time to freezing.
by the way as Mr. NONICK mention ,could u please introduce some photo couple brands.
thanks again.

[NoNickName]

More info on photocouplers can be found on Volume 29, number 2, 2006 of International Journal of Refrigeration.
The photocouplers used in the article were Infrared type and can be found in any electronic components store. It's a photodiode and its cost is just a fraction of a euro. But then the signal from the diode has to be conditioned and interpreted via a dedicated electronic board, and the analogue output interfaced to the heat pump microprocessor.

[A_Ahed]

thanks Mr NONICK ,
I try to follow ur advise.