New!!! 3 April 2017

ANTI CONDENSATION GLAZINGS (1)

ANTI CONDENSATION GLAZING (1)

SIMPLIFIED   THEORY   OF LOW E  ICE FREE GLAZING. ( THEORIE GEL)
PATENT DE 28 33 234  /FR 2  399  331/GB 1 598   924 (  g.LAFFAY AND AL. 1978).

In conclusion, that type of low E windscreen is very efficient on winter on evening to return from an outdoor parking of the job or kino and in autumn/spring, on morning to go to  the job office from an outdoor parking.
In summer, it lowers the greenhouse effect (10°C see  DE  28 33 234)  inside the car.
LAFFAY GUY on 28 january 2012. Condensation in Astronomia. (Albireo78 Abstracts). 

Ce type de vitrage est basé sur le  ralentissement du refroidissement de sa face extérieure grâce au dépôt d’une couche à faible émissivité (Low E).

IL est utilisé en face n°1 dans le bâtiment sur les double/ triple vitrage super isolants qui condensent beaucoup.
Dans les portes  des armoires frigorifiques. La face n°1 Low E est celle tournée vers le froid.
Dans les pare brise des véhicules, en face avant n°1 et les vitres latérales.

Application sur les pare-brise Windscreen-Ice-free

1)BALANCE OF TEMPERATURES  DURING THE COOLING

Powers concerned.

A motionless windscreen on winter,  during a clear night lose  many calories by radiating effect (Infra-red 5->15 microns) so that it can be cooler than the ambient  air and therefore get calories by conduction/convection and condensation of humidity into water or ice

Loss of calories by emitting  effect of an ordinary windscreen

(noted s after strahlung).
The  temperature of the windscreen   is    ~0° or 273°K (maximum of radiating ~ 11 microns and emissivity ~ 0,9)  so STEFAN  formula  give us  the loss energy     0.9*5.7*10^-8*273^4=284w/m2
The sky is  ~minus 20°C=253°K (maximum of radiating ~12 microns and emissivity  ~0,9) ,  the windscreen get  only  0,9 of incoming downward sky radiation.
0,9*0,9*5,7*10^-8*253^4=190 w/m2
The formula   Swinbank/Goforth  which don’t use sky temperature , generally badly  defined, but temperature of windscreen and humidity  percentage , give us   for a clear night.
On winter  8,78*10^-13*(273^5,852)*(95^0,07195)=220 w/m2
On  autumn/Spring    8,78*10^-13*(273^5,852)*(85^0,07195)=218 w/m2
The windscreen  get  0,9*219=197 w/m2 (mean).
So the radiating loss  is 284-197=87 w/m2 or 87/20=4,3 w/m2/°C     

           
Loss  of calories by emitting effect of a low E  windscreen.

Emissivity=0,2
The loss   is   0,2*5,7*10^-8*273^4=63w/m2
Emission of sky is the same  219 w/m2. Windscreen   gets   0,2*219=44 w/ m2
The emitting   loss is 63-44=19w/m2  ie 1w/m2/°C
Emissivity=0,1 loss by  emission  is 32 w/m2.  Windscreen   gets  219*0,1=22 w/m2
The final loss 32-22=10 w/m2 ie~ 0,5 w/m2/°C

Gain of calories by konduktion  /konvektion noted  k after

These gains are computed   by clear   and calm weather  ie generating ice.
They are estimated  11w/m2/°C (v<1,5 m/s). If  there a lot of wind v, the gains are much greater
But  the windscreen  don’t ice. The formula from   Granqvist (1987) and  al  is  5,7+3,8*v.

Gain of calories by  kondensation  of humidity

The part of condensation  warming  the windscreen  is  estimated   537 calories/g of  water
Ie  2244 J/g     if  a  condensation  of  thickness    (ep)  in     (  1/10 mm)   is deposed on the windscreen  (surface 1 m2)    in 1,5  hour,   It means  that a volume    ep/10 liter or 100*ep in g is deposed  ie 41,4*ep  w/m2.
That quantity  of    warmth  is a major  contribution.

Temperature balance  during the  cooling of an ordinary windscreen on a clear calm sky

During the cooling, power P concerned is P=konvektion*delta(tk)-strahlung*delta(ts)
Delta(ts)=tg-ts   tg  temperature of the windscreen  (g for glass) and ts=  -20°C emitting
Temperature of sky (s  for strahlung)
Delta(tk)=ta-tg   ta=0°C ambient temperature
When  reaching  the balance state P=0,      0=11*delta(tk)-4,3*delta(ts)
Or  delta(tk) + delta(ts)=20
11*delta(tk)=4,3* (20-delta(tk))
Delta(tk)*(11+4,3)=4,3*20
Delta(tk)=86/15,3=5,6°C   delta(ts)=14,4°C
The temperature of windscreen   is decreasing  to -5,6°C   but is icing before if   humidity HR>65%
If HR, by example, is 85% icing is taken place at -2°C,  then cooling is according to ,if ep=1  ( thickness
Of water condensation)          0=11*delta(tk)+41,4-4,3*delta(ts)
11*delta(tk)=4, 3*(20-delta(tk))-41,4
Delta(tk)=44,6/15,3=2,9°C
The windscreen with ice  stop cooling at -2,9°C

Temperature balance during the cooling of a windscreen LowE=0,2

P=0=11*delta(tk)-1*delta(ts)
Delta(tk)+delta(ts)=20°C
11*delta(tk)=1*(20-delta(tk))
Delta(tk)*(11+1)=20    therefore  delta(tk)=1,6°C
Temperature of windscreen It is   -1,6°C.    It  is icing if HR>90%
If  Emissivity=0,1  delta(tk)=0,87°C  temperature of windscreen is -0,87°C. It is icing if HR>95%.
So in our example no icing.
The differences of surface temperature  2,9-1,6=1 ,3°C   and 2,9-0,87=  2°C are those mentioned
In   patent DE 28 33 234

2)KINETIC   OF THE COOLING

Very approximately, we have  ( Hour TU+1, end of twilight, the first night of a period of nights of clear sky , frequent humidity) latitude 48,7.

January  18,2 H   90->99% the windscreen  has to cool 1 degree to attain the  dew point
February   19H   90->99% 1 degree.
Mars  20H   90->99% 1 degree
April  20,5 H    80->89%  2 degrees
September  20 H   80-> 89% 2 degrees
October  19 H   80->89% 2 degrees
November  18 H   90->99%  1 degree
December  18 H   90->99%   1  degree

End of twilight is one hour after   sunset
We saw from  C. Granqvist (1987)  that conduction/convection coefficient  k=5,7+3 ,8*v
V  wind  velocity in m/s.   If v= 0   k=5,7 and if v=2 m/s  k is 13,3 w/m2/°C
During the cooling in winter, on a clear night (90% HR), difference of temperature (windscreen Low E and air) is ~ 1 degree.
So if the wind is 0 m/s, the windscreen get from the air 5,7 w/m2
If the wind is 2 m/s , the windscreen  get  13,3 w/m2
If we consider  a windscreen of  surface  1 m2 thickness=5 mm, its weight is  12,5 kg.
The massic heat of glass is 720 j/kg/°C.  So 720*12,5=9000 joule are necessary to cool that windscreen 1 degree.

Emissivity   0,1.   We saw that the radiating loss is about 10 w/m2 so with a wind  1, 33 m/s
The windscreen conduction/convection and radiating loss are identical.
the time to cool 1 degree   is  then 9000/0=infinite. The windscreen  don’t ice.
If wind velocity is 1,2m/s, the gain  is 10,26 w/m2, the radiating loss is  10,6 w/m2
Ie global loss   is 0,34 w/m2  or  0,34 J/s/m2
The  time to cool 1 degree will be 9000/0,34=26470s or 7,3 hours.
If the velocity of the wind is 1,12m/s the time to cool is 4 hours.
If wind  is 0,m/s the gain is 5,7 w/m2, the radiating loss is  10,6 w/m2, the global loss
4,9 w /m2 or 4,9 J/s/m2.

The time to cool 1 degree  will  be 9000/4,9=1838s ie 0,5 hours.
A  minor variation of wind  velocity between  0 and 1,33 m/s induce ice with a delay of 0,5 hour to infinite. On a period  of several  hours,  we have to choose mean temporal  delay. It has been estimated  very approximately   from   experiments  with mean velocity  of wind  1m/s,  that, on a clear night (period from end of twilight to dawn) of nov, dec,  jan fev ,mar, a windscreen  of a car outdoor without coating is cooling  1°C/hour and a windscreen coated with 0,2 emissivity (epsi)   0,2°/hour.

0n april,sep, oct a car without low E is cooling 0,9°C /h and with low E   0,18°C/H.
With these values, we can estimate and report the time when the windscreen  are icing.

Estimation DE 28 33 234

Epsi=0,9                                           Epsi=0,2

Jan 18,2+1=19,2h                            18,2+5=23,2h
Feb 19+1=20h                                  19+5=24h
Mar 20+1=21h                                 20+5=25h
Apr 20,5+2/0,9=22,7h                      20,5+2/0,18=7,6h Am**Sunrise(6h)
Sep 20+2/0,9=22,2h                         20+2/0,18=7,1h Am**Sunrise(6h)
Oct 19+2/0,9=21,2   h                       19+2/0,18=6,1h Am**Sunrise(7h)
Nov 18+1=19 h                                  18+1/0,2=23 pm
Dec 18+1=19  h                                 18+1/0,2=23 pm

Globally the delay of icing with low  E , compared to without Low E is 4 hours in winter and 8/9 HOURS in autumn/spring.

On Apr,Sep,Oct, there  are minor icings on Sunrise with LowE  0,2. With  low E 0,1 there  are no icing.
 « Applied Optics » vol 26 n°11 from Hamberg/ Claes Granqvist  tackled the subject in 1987.
Granqvist and al  don’t point out the delay of icing with low E till midnight. All the meteo data are registered at 7 am.That delay is appreciable.

The efficiency according to Granqvist is 1-6/29=0,8 (calculated with a number of favorable icing days), the same given in DE 28 33 234 (0,8/0,9 at 7 am with true measures). We have to add that a low E windscreen is easier to de-ice than an Ordinary one because the ice is always thinner.

In conclusion, that type of low E windscreen is very efficient on winter on evening to return from an outdoor parking of the job or kino and in autumn/spring, on morning to go to  the job office from an outdoor parking.
In summer, it lowers the greenhouse effect (10°C see  DE  28 33 234)  inside the car.
LAFFAY GUY on 28 january 2012 .Condensation in Astronomia. (Albireo78 Abstracts).

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New!!! 18 April 2017

ANTI CONDENSATION GLAZINGS (2)

(Windscreens, windows glazings, cold chambers) ** guy Laffay patent 1977 DE 28 33 234_théorie G.E.L
See schemes in annexe 1 and 2

Les pare brise givrent couramment l’hiver sur la face avant dite n°1 mais aussi les façades verrières des bâtiments dans le nord de l’Europe du fait que l’isolation actuelle, maintenant excellente empêche la moindre perte de chaleur pour réchauffer la face N°1. Le verre anti condensation permet de garder par tous les temps la transparence du verre qui est sa qualité principale.

Windscreens on outside parking and multi glazings windows, on cloudy nights don’t ice because the temperature of their exterior surface named N°1 and the exterior air temperature are the same.
It is not the case of a simple glazing which gets an important lot of heat from the interior ~+20°C and when adding sheets of glass or plastic to obtain a multi glazings one, the N°1 surface is getting now the behaviour of a windscreen’s n°1.
if suddenly, the clouds disappear, the radiating loss
toward the sky (here -20°C) of an ordinary windscreen is 90 watts/m2 and the gain conduction/convection
induced is 5,7+V*3,8 w/m2/°C ie 11 watts /m2 if the
face n°1 of the windshield and exterior air have a difference of 1 degree and 0 w/m2 at the beginning. (V is by example 1 m/s.).
The heat transfer of n°1 of a triple glazing when k’=0,7
Is ~ 0,7*18=12 w/m2 if exterior temperature is 0°C and interior 18°C.
1/k’=Sum(1/k)

So we can dispose now of the first point on the cooling diagram.
Axis Oy describe radiated power lost and conduction/convection power gained in w/m2.
Axis Ox being difference of temperature of surface N°1 relevant to temperature of exterior air.
The crossing of Oy,Ox is the zero point of the cooling of the windshield when clouds disappeared ;
Concerning a triple glazing, the point 12w/m2 is the first point on Oy if clouds disappeared.
The evolution continue then with the gain of conduction/convection according to 5,7+V*3,8 when the sky is always clearing.
The evolution stop when that gain reach the loss by the radiating surface n°1, function of its emissivity.
( 0,9/0,2/0,1 etc).

IF condensation appears, specific heat (40 w/m2 )is added according to a parallel line to Oy like heat coming from the interior in the case of multi glazing windows .Emissivity is getting now the value 0,9 (emissivity of water) and evolution stop on line 0,9.
Parallels to Oy concerning dx equal -1/-2/-3°C define relative humidity ie the dew points.
Concerning cold chambers multiglazing windows if k’=1 by example, the interior being minus 20°C ie similar to the sky (but always clear) and if the exterior temperature is +18°C, the heat entering the cold chamber through the surface n°1 is 38w/m2. We see that an emissivity of 0,45 is enough to avoid condensation and see through the window all the time.
LAFFAY guy on 28 january 2012 ; Condensation in astronomia. (Albireo78_Abstracts).