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hookster
25-07-2009, 12:25 PM
Hi All
Could someone assist in shedding some light.
Cooling in a vacuum is achieved by boiling water, mostly off the surface of the product to be cooled.
At 0C, the saturation pressure is around 0.6 kPa.

Water vaporizes in the chamber under the low pressure. Until saturation is achieved. To increase saturation levels a condensing coil is fitted in the chamber.

What is the temperature required at 0.6 kPa for condensing to take place?
Why does water not flash off again as run off from the condensing coil occurs?

Any insight much appreciated.

Contactor
25-07-2009, 01:07 PM
The chamber is being maintained at saturation point and will be vapourising and condensing on all surfaces. There is no flashing off as such. Assuming your figures are correct, then the temperature required is therefore 0 c.

hookster
25-07-2009, 02:00 PM
Thanks but I would assume that there is some subcooling required on the condenser below saturation temperature.If water is vaporising at 0.

hookster
25-07-2009, 07:44 PM
At 6.5 mBar (5 Torr) water is boiling at 0 c.
Does anyone have a chart or nomograph for water at low pressures ie 10 - 0 Torr.

Gary
25-07-2009, 08:06 PM
When a liquid/vapor is at saturation, adding heat will cause the liquid to change to a vapor or removing heat will cause the vapor to change to a liquid. During this change of state, in either direction, it will remain at saturation temperature.

hookster
25-07-2009, 09:14 PM
Yes but to achieve condensation you have to change the state of equilibrium. With a condensing coil in the vacuum chamber heat given up is the latent heat of sublimation. Sublimation occurs when air saturated with water vapor is suddenly cooled below the freezing point of water.
My question is if water is vaporizing at 0 C what would be the required condensing temperature?

Gary
25-07-2009, 10:03 PM
Yes but to achieve condensation you have to change the state of equilibrium. With a condensing coil in the vacuum chamber heat given up is the latent heat of sublimation. Sublimation occurs when air saturated with water vapor is suddenly cooled below the freezing point of water.
My question is if water is vaporizing at 0 C what would be the required condensing temperature?

The required condensing temperature would be 0 C.

Gary
26-07-2009, 12:35 AM
Okay, to elaborate on this:

The source of heating must necessarily be more than 0 C, and the source of cooling must be less than 0 C, but the change of state is at saturation temperature (0 C).

Contactor
28-07-2009, 10:52 PM
It is vapourising and condensing (BOILING) at the same time at constant pressure and temperature. There is no sub cooling and no superheat. Sub cooling can only be achieved when 100% of the refrigerant is below saturation point (latent heat of condensation). The refrigerant can only be superheated when 100% of the refrigerant is above saturation point (latent heat of evaporation).

The conditions you describe are no different to a pot of boiling water at 100 c in your kitchen. Only the pressure has changed.