View Full Version : cold room temp & the product temp

18-09-2007, 12:19 PM
It is a well known fact that the cold rom temp is generaly the return air temp on the evap coil. many a times the client gets worried when the door is opened & the temp almost imediately shoots up. Though the product temp is unchanged, he is always worried.
Is there any technical literature explaining this phenomenon?

18-09-2007, 12:34 PM
The air temperature will always rise when a door is opened and external air is allowed inside.

The product temperature will remain more constant because of its mass.

Try changing the temperature probe from an 'air' sensor to a 'product' sensor or improve the the door seal arrangements to reduce the effect of door openings.

The Viking
18-09-2007, 01:03 PM
Another option is to put the sensor for an elextronic thermometer in a 500ml bottle filled with glycol mixture (screwed to the wall).

When the customer looks at the thermometer, he will see the "mean" room temperature wich will be close to the temperature of the products in the room.

18-09-2007, 04:14 PM
Dear Viking,

Seems to be the practical suggestion. We can surely use that in our new projects.

18-09-2007, 05:42 PM

If its for future projects then look at your choice of controller.You could opt for a controller that controls the temp on return air but displays a % off Air on /Air off giving a more stable display temp.Or leave the the digital display inside the the controll panel and fit a box guage.

However if the customer is leaving the door open to long then your just masking the problem,it might be better to fit an annoying door alarm that activates after 60 secs.They,ll soon close the door.:)

Regards Bernard

20-09-2007, 03:22 PM
thats a good idea,

old gas bottle
20-09-2007, 05:52 PM
or put a micro switch on the door so it stops the fans and pumps the unit down,with the probes being on or arround the blower they should stop stable untill the door is closed,but then they should expect it to go up a bit. this helps with icing up problems on freezers but if its a chiller i would go with the slow acting probe or the vessel with fluid in.;)

John Hunter
21-09-2007, 05:50 AM
Hi smpsmp45.
If you can check out ASHRAE Refrigeration Hand Book Chapter 12 " Infiltration Air Loads" , gives a good insight in to the effect of Warm Air flowing in to Freezer rooms. Frost accumulation and the formation of ice crystals can be a more adverse effect that product temperature rise.

21-09-2007, 07:59 AM
Dear John,

Many thanks

21-09-2007, 11:46 PM
Hi smpsmp45

The room tempertaure is formed for a 3D set of isotermal curves . The lowest temperature is leaving the evaporator and the highest is in the return (inlet of the evaporator). When you open a door this pattern changes and the highest temperature could be close to the door. A new isotermal pattern is formed. Normally the product has an inertial temperature constant that depend of many factor incluiding the enclousure. So the poduct do not react so fast to the temperature changes.It is a logarism aproch
If you instal a electronic temperatur sensor far from the door and in the return side of the farer evaporator It may help to have a better idea of the temperature.
You also could install hawain plastic courtain or even fast acting door together with horizontal openning door see www.ritehite.com (http://www.ritehite.com)