| Sažetak | Poznavanje procesa prijenosa topline neophodno je za pravilno projektiranje toplinskih
aparata koji se koriste u naftnoj, kemijskoj, prehrambenoj i farmaceutskoj industriji te
energetici. Kondenzacija vodene pare uz prisutnost nekondenzirajućih plinova predstavlja
proces prijenosa topline koji je često zastupljen u navedenim industrijskim granama. U ovom
radu se provodi eksperimentalno istraživanje i analiza procesa kondenzacije vodene pare u
vertikalnim spiralnim cijevima izmjenjivača topline rekuperativnog tipa. U svrhu provođenja
istraživanja razvijena je i izrađena mjerna linija koja omogućuje ispitivanja toplinskih
karakteristika izmjenjivača topline u kondenzacijskom i toplovodnom režimu rada, uz i bez
prisutnosti nekondenzirajućih plinova. Opisani su postupci i metodologija mjerenja u oba
režima rada, definiran je parametar negativnog utjecaja prisutnosti nekondenzirajućih plinova,
te je detaljno prikazana metodologija usporedbe negativnih utjecaja prisutnosti
nekondenzirajućih plinova na kondenzaciju vodene pare u spiralnim cijevima s drugim
konvencionalnim izmjenjivačkim površinama. Dodatno je prikazana metodologija razvoja
proračunskog modela prisilne konvekcije oko spiralnih cijevi koji je potreban za provođenje
prethodno navedene usporedbe.
Mjerni rezultati prilikom kondenzacije vodene pare unutar spiralnih cijevi uz prisutnost zraka
kao nekondenzirajućeg plina prikazani su i interpretirani u odgovarajućim tablicama i
dijagramima. Nadalje, prikazani su i interpretirani rezultati mjerenja u toplovodnom režimu
rada, uz prikaz razvijenog proračunskog modela prisilne konvekcije vode oko snopa spiralnih
cijevi. Rezultati provedenog eksperimentalnog istraživanja kondenzacije vodene pare uz
prisutnost zraka u spiralnim cijevima su zatim uspoređeni s rezultatima izračuna kondenzacije
vodene pare uz prisutnost zraka kao nekondenzirajućeg plina prema proračunskim modelima
kondenzacije u ravnim horizontalnim i vertikalnim cijevima koji predstavljaju druge
konvencionalne izmjenjivačke površine, a sve uz pridruživanje izračunatih sastavljenih
standardnih mjernih nesigurnosti. |
| Sažetak (engleski) | A water vapor condensation in the presence of non-condensable gases (NCG) is a very
common phenomenon in process plants of various industries. The presence of even a small
amount of NCG during water vapor condensation can significantly reduce the heat transfer
coefficients and the efficiency of the heat exchanger. Heat exchangers with spiral tubes are
widely used as condensers in process plants. Knowing the above mentioned heat transfer
mechanism is necessary for optimal design of thermal appliances (apparatus) and process plants
in general. A review of the open literature revealed that there are a number of works dealing
with the water vapor condensation with and without the presence of NCG in straight tubes of
all orientations. There are number of works dealing with condensation of refrigerants in spiral
tubes used in refrigeration technology. However, when we talk about water vapor condensation
in spiral tubes, it is evident from the review of the open literature that this area is still not
sufficiently investigated, especially if NCG are present in mixture with water vapor. For this
reason, in the present study an experimental analysis of water vapor condensation in vertical
spiral tubes in the presence of NCG was conducted.
Based on theoretical knowledge about the positive effects of the appearance of secondary flows
in spiral tubes, and the negative effects of NCG presence on the condensation, the hypothesis
of this work is that the negative effects of NCG presence during the water vapor condensation
in spiral tubes are smaller comparing with other conventional geometries of heat transfer
surfaces.
The experimental research and analysis was performed with the following objectives:
- Develop an experimental line for enabling an examination of water vapor condensation in
heat exchangers with spiral tubes, with and without the presence of NCG.
- Based on the experimental analysis, quantify the negative effects of NCG presence on the
water vapor condensation in heat exchangers with spiral tubes.
- Analyze and compare the negative effects of NCG presence on the water vapor
condensation in spiral tubes and other conventional geometries of heat transfer surfaces.
The experimental investigation and analysis resulted in the following:
1. Multi-purpose experimental line for enabling an investigation of thermal behavior in heat
exchangers was developed and produced. The experimental line enables examinations of
the thermal characteristics of heat exchangers with spiral tubes in single phase and two
phase flow, with or without the presence of air as a non-condensing gas. As part of the
measuring line, a system for venting non-condensable gases after the condensation process
and a system that ensures that only the condensation process takes place on the complete
heat transfer surface are integrated. With certain modifications, it is possible to use the same
experimental line for examination of thermal behavior in other types of heat exchangers
and to use other non-condensable gases. As such, it represents valuable equipment for
further experimental research in the field of heat transfer.
2. Experimental methods of water vapor condensation analysis in the presence of NCG inside
spiral tubes have been developed. Measurement procedures and methodology, analysis of
results and comparisons with other conventional heat exchanger surfaces are described in
detail. Furthermore, the methodology of developing a model of forced convection around
spiral tubes is presented.
3. Based on the obtained measurement data, the negative effects of NCG presence on the water
vapor condensation in spiral tubes were quantified. Negative effects are defined through the
negative effect parameter (ε), which represents the ratio of the mean heat flux during
condensation in the presence of NCG and the mean heat flux during condensation without
the presence of NCG. It has been shown that with the increase of air mass fraction in the
mixture with water vapor, the values of parameter of negative effects decrease, that is, that
the negative effects of the presence of non-condensable gases are increasing.
4. Based on the obtained measurement data, in-tube heat transfer coefficients were calculated.
Their dependence on the mass fraction of air as a NCG during the condensation of water
vapor in spiral tubes is shown. With the increase of air mass fraction in the mixture with
water vapor, the values of the heat transfer coefficients decrease. A significant decrease
occurs in cases of air mass fraction in vapour up to 5 %. With a further increase in the air
mass fraction, the value of the heat transfer coefficients continues to decrease, but with a
lower intensity. Furthermore, the dependence of heat transfer coefficients on water vapor
mass fluxes is presented. For approximately the same air mass fractions and temperature
potentials, the values of the heat transfer coefficients increase with an increase of water
vapor mass fluxes.
5. The quantified negative effects of the a NCG presence on the water vapor condensation in
spiral tubes were compared with the corresponding negative effects in horizontal and
vertical tubes, as other conventional geometries of heat transfer surfaces, with the addition
of calculated combined standard measurement uncertainties. Comparable negative effects
of NCG presence on the water vapor condensation in horizontal and vertical tubes are
calculated according to models from literature. The comparison confirms the hypothesis
that the negative effects of NCG presence during the water vapor condensation in spiral
tubes are smaller compared to the same in other conventional heat transfer surfaces, with
the following remarks:
- The analysis carried out included a comparison of the negative effects of NCG
presence during the condensation of water vapor in spiral tubes with the same
negative effects in horizontal and vertical tubes, as conventional heat exchanger
surfaces.
- The analysis of the results explicitly confirms that the negative effect of NCG
presence on the water vapor condensation in spiral tubes is smaller compared to the
same in horizontal tubes.
- The analysis shows that the negative effect of the NCG presence on the water vapor
condensation in spiral tubes is smaller compared to the same in vertical tubes, which
is shown with non-negligible certainty for air mass fractions greater than 9.3 %. For
smaller air mass fractions, there are indications that the conclusions could be the
same, but it is definitely advisable to conduct additional research.
6. Based on the obtained measurement data in single phase flow, a calculation model of forced
convection of water around bundle of spiral tubes is developed. |
