Temperature of water inside Olympia Cremina boiler? - Page 3

#21: Post by **rpavlis** » November 16th, 2015, 6:53 pm

I just wrote this little C program. This takes atmospheric pressure and initial and final temperatures of a boiler and gives final pressure. It assumes that the gas volume in the boiler is constant and that there are no leaks! The constants involved require T in Kelvin and Pressures in kPa.

#include <stdio.h>
#include <math.h>
#define Tc 647.096
#define Pc 22064
#define A1 -7.85951783
#define A2 1.84408259
#define A3 -11.7866497
#define A4 22.6807411
#define A5 -15.9618719
#define A6 1.80122502

double getpressure(double);

/*This is a program to take a temperature and compute the total pressure
inside a constant volume with liquid water in equilibrium when the container
initially contains air. Tinit is the starting temperature, Tfin is the
final temperature.

ln(p/pc) = (a1τ + a2τ1.5 + a3τ3 + a4τ3.5 + a5τ4 + a6τ7.5)Tc/T



τ = 1 − T/Tc  Output T in Kelvins, input in KPa */

int main()
{
  double T,Tinit, Tfin,Tatm, Patm, Pair,Pairf, Pwf, P0, Ptot, Ptot_fin;
  int n;
  printf("input starting Temperature:\n");
  scanf("%lf", &Tinit);
  P0=getpressure(Tinit); /*initial vapour pressure of water*/
  printf("input final Temperature:\n");
  scanf("%lf", &Tfin);
  printf("input atmospheric pressure:\n");
  scanf("%lf", &Patm);
  Pair=Patm-P0;   /*initial partial pressure of air in container*/
  Pairf=Pair*Tfin/Tinit; /*final partial pressure of air in container*/
  Pwf=getpressure(Tfin); /*final partial pressure of water*/
  Ptot_fin=Pwf+Pairf;
  printf("final total pressure is:%lf\n", Ptot_fin);
}


double getpressure(double T)
   {
     double term, tau,p;
     tau=1.0-T/Tc;
     term=A1*tau;
     term+=A2*(pow(tau,1.5));
     term+=A3*(pow(tau,3.0));
     term+=A4*(pow(tau,3.5));
     term+=A5*(pow(tau,4.0));
     term+=A6*(pow(tau,7.5));
     p=Pc*exp(term*Tc/T);
    return(p);
    }

Executing this program with 1.0 bar atmospheric pressure and 20C initial and 86.5C final temperature predicts 78 kPa or 0.78 bar.

#22: Post by **CoffeeBar** » November 17th, 2015, 5:45 am

Wow, Just Overwhelming for me. Anyway I set my pressure stat at 0.6bar and immediately after finished my brewing( Lever down ) The coffee puck measured at 92C ( Group Temp at 165F )

#23: Post by **rpavlis** » November 17th, 2015, 7:32 am

I hate to make HB into a physical chemistry site, but another thing that goes into the actual temperature before bleeding is the fact that molecules in the gas phase only travel a short distance before colliding with other molecules. This makes gases thermal insulators, that is why styrofoam insulates because it is mostly air and the air cannot circulate. In a boiler when there is air present, the movement of water molecules is thus restricted, so the water vapour near the walls condenses unless there is no heat loss from the boiler. That means the space near the walls has a lower concentration of water vapour, so the pressure inside is less than it would otherwise be, so the pressurestat does not turn off, and the actual temperature is thus higher than equations based on pV=NRT and vapour pressure equations would predict. The real temperature is thus an extraordinarily difficult thing to predict.

Once all air is gone, however, water vapour has unrestricted contact with the walls, and water vapour condensation keeps the wall temperature right at boiler water temperature. If one measure the temperature of the top of a boiler and the bottom part that is filled with water the temperatures will not be the same when the boiler is not bled. After bleeding the entire system under pressure is essentially at the same temperature.

It really is impossible to maintain a stable temperature with air in the system.

The equations in the computer program assume that the vapour area is homogeneous, which is not the case under normal espresso machine boiler conditions. The groups of second generation La Pavonis with their steam heat system are cool until the system is bled!

#24: Post by **vit** » November 17th, 2015, 9:40 am

Process within the boiler is surely quite complex; formulas show only theoretical situation that doesn't exist, so every part of the boiler has own temperature, which is obviously highest around the heater (when on) and lowest on the walls

Other important thing is that heater, when switched off by pressure-stat or thermostat, is still very hot and is continuing to heat water for a while, which is quite important in entry level espresso machines with small boiler like mine (didn't find it mentioned anywhere where temp. surfing was discussed)

#25: Post by **OldNuc** » November 17th, 2015, 9:50 am

The reason all of what you have mentioned is not discussed is that it flies so far over the heads of many and is actually of minimal significance with a manual lever, small boiler machine. The actual initial water level in the boiler will have a larger impact on the rate of change in group average temperature during heat up and the concurrent noncondensible venting process than the rest of these unconsidered variables.

#26: Post by **vit** » November 17th, 2015, 10:30 am

Sure - I forgot to mention that my machine is actually entry level vibe pump machine

- where it is important

#27: Post by **OldNuc** » November 17th, 2015, 11:58 am

You need to determine how exactly obtain a reasonably accurate boiler outlet to group temperature. You will get a slug of cold/cool water on any initial water flow from sitting so a flush to drain is critical as it has to be long enough to purge cold water and heat the pump etc to boiler outlet temperature. What all is involved in doing this with some repeatability depends on the physical layout of the hardware.

#28: Post by **vit** » November 17th, 2015, 12:06 pm

Yeah, it took considerable time to find the best way (of course I did several flushes before) and various ideas I read about temp. surfing were misleading

#29: Post by **rpavlis** » November 18th, 2015, 8:23 am

This morning I did an interesting and revealing experiment with my pressurestat equipped 1999 (generation II) Europiccola. The room temperature was 19 C. I removed the normal boiler cap and replaced it with my gauge equipped boiler cap. I had the boiler filled to about 1 cm below the top of the site gauge. I turned on the machine and let it warm up. The lamp in the switch indicated that the pressurestat turned off the current at 0.75 bar gauge. (1.75 bar true pressure.) I removed the drip tray and unplugged the machine. I tipped the machine on its side and shook it. The pressure shot up to 1.25 bar gauge, (2.25 bar true pressure!)

I turned the machine right side up and reconnected the power and bled it thoroughly using the olive jar test to ensure complete bleeding. The pressurestat turned off the current, once again, at 0.75 bar gauge, 1.75 bar true pressure. I had not replaced the drip tray, so I simply unplugged the machine and tipped the machine over and shook it again. The pressure remained unchanged, right at 0.75 bar gauge! It did not deviate at all!

This little test shows just how important thorough bleeding is to prevent erratic temperatures!!! As explained above, the air trapped in the boiler does not condense when it hits the walls of the boiler. (b.p. nitrogen is -196, argon -187, and oxygen -182!) These air molecules get in the way of the water vapour molecules and prevent their contact with the walls of the container, so the region right against the walls is much cooler than else where in the boiler. They cannot diffuse in rapidly enough to keep equilibrium. When I shook the container, the vapour contacted the splashing liquid very thoroughly, and brought the system to equilibrium. The full pressure from Dalton's law developed in the boiler. (Pressure is the sum of vapour pressure of water and air pressure.)

Now after I bled the system, there was no air inside to form a gaseous "film" near the walls, the water vapour had unrestricted contact with the walls. I measured the wall temperatures after bleeding, they were right at 115-116 degrees, which indeed corresponds to the vapour pressure of water at 1.75 bar or so.

The important lesson of this experiment: Be sure that bleeding is complete to avoid erratic temperatures. The problem is worse when ambient temperatures are low, because then more water vapour needs to be condensed to keep walls hot. Once the machine is properly bled water vapour will condense on all internal surfaces practically instantly. Everything touching vapour then has the same temperature.

#30: Post by **OldNuc** » November 18th, 2015, 10:12 am

Proper big boiler startup from cold is to continuously vent as the boiler comes to full operating pressure. Failure to do this produces results similar to your observations other than the difficulty of tipping the boiler over. Monitoring external shell temperature with contact RTDs shows this stratification and excessively cold vapor space clearly. The LaPavoni can be continuously vented during heat up if the steam wand is not full open effectively to insure a complete purge.