The preformances of air cooled heat exchangers with water spraying

Ваздушни хладњаци се у термотехници и процесној техници често примењују у условима када су температуре амбијенталног ваздуха јако високе и знатно више од температура усвојених у прорачунима, па није могуће одвести прорачунску количину топлоте. Потребна топлотна снага се у таквим случајевима може достићи ако се околни ваздух орошава (влажи) распршивањем воде на улазу у апарат, јер се тиме снижава температура ваздуха, па се температурна разлика повећава, као и размењена количина топлоте. Појава слоја влаге на размењивачу топлоте кључно мења топлотне и струјне перформансе оваквих апарата. Детаљнија анализа расположиве литературе показала је да радови у којима се истражује проблематика размене топлоте и супстанције на површини оребреног размењивача топлоте са напрскавањем воде нису бројни, те се доводи у питање могућност реалне примене података доступних у литератури за потребе инжењерских прорачуна при експлоатацији оваквих апарата, односно коју методологију прорачуна коефицијената прелаза топлоте и влаге препоручити као релевантну у одређеним условима. У овој докторској дисертацији испитивани су различити режими рада ваздушних хладњака са оребреним цевима при њиховом напрскавању водом на улазу у апарат. Експериментална истраживања рада предметних размењивача топлоте су спроведена на самостално пројектованој инсталацији, која је направљена и монтирана у оквиру топлотне подстанице Машинског факултета у Београду. Развој физичког и прорачунског модела, заснованог на истовременом транспорту топлоте и супстанције, битан је за одређивање и прорачун релевантних параметара рада размењивача топлоте. Применом теорије сличности, уз одговарајуће статистичке методе, одређене су меродавне критеријумске зависности за израчунавање пада притиска, као и коефицијената прелаза топлоте и супстанције, тј. одређени су изрази погодни за инжењерску праксу. Ова докторска дисертација представља наставак истраживања проблема проналажења поуздане прорачунске процедуре за одређивање коефицијената прелаза топлоте, супстанције и трења за широк опсег Рејнолдсових бројева и различите геометријске карактеристике цевних размењивача топлоте са оребрењем који се испитују на Машинском факултету у Београду.In HVAC&R and process engineering, air coolers are often used in the conditions when ambient air temperatures are very high and significantly higher than the temperatures accepted in the calculations, and hence it is not possible to remove the calculated amount of heat. In these cases, the required heat can be reached if the surrounding air is humidified by spraying water at the entrance to the apparatus, because this lowers the air temperature, so the temperature difference increases, as well as the amount of exchanged heat. The appearance of a layer of moisture on the heat exchanger crucially changes the thermal performance and flow pattern of such apparatus. A more detailed analysis of the available literature shows that the papers which investigate the problem of heat and substance exchange on the surface of the finned heat exchanger with water spraying are not numerous. Hence, the possibility of real application of data available in the literature is questionable when it comes to engineering calculations in exploitation of these apparatus, as well as to recommending relevant methodology for calculating the coefficients of heat and mass transfer in certain conditions. This doctoral dissertation examines different operating regimes of air coolers with finned tubes during their spraying with water at the entrance to the apparatus. Experimental research on the operation of these heat exchangers was conducted on a self-designed installation, which was made and installed within the heating substation of the Faculty of Mechanical Engineering in Belgrade. The deployment of a physical and computational model, based on the simultaneous transport of heat and substance, is important for determining and calculating the relevant operating parameters of heat exchangers. Applying the theory of similarity, along with the appropriate statistical methods, the relevant criteria equations for the calculation of pressure drop were determined, as well as the coefficients of heat and mass transfer, i.e. certain equations suitable for engineering practice. This doctoral dissertation is a continuation of research on the problem of finding a reliable calculation procedure for determining the coefficients of heat and mass transfer and the coefficient of friction for a wide range of Reynolds numbers and different geometric characteristics of tubular heat exchangers with fins that are tested at the Faculty of Mechanical Engineering in Belgrade

MAINTENANCE AND RELIABILITY OF INDUSTRIAL REFRIGERATION SYSTEMS

The objective of this paper is emphasizing the importance of maintaining the cooling systems from the perspective of exploitation safety. The comparative analysis of the natural and synthetic refrigerants is made. An overview of the location of possible operation problems is given. There are cited the most common mistakes of industrial cooling installations that hinder maintenance and lead to incidents followed by leaking in the exploitation conditions. The accent is put on parts of refrigeration installations at high pressure in specific periods of operating (evaporator defrosting). Special attention should be paid on monitoring of leaking in liquid lines (especially with over-flooded systems). Finally, the routes and suggestions for better maintenance and safety improvements are given

GROUND SOURCE HEAT PUMP OPERATION ANALYSIS

The paper analyzes the influence of different parameters on the effective Coefficient of Performance (COP) of a ground source heat pump. Heat pump located in the Laboratory for Thermal Science Engineering at the Faculty of Mechanical Engineering in Belgrade is adopted for model. Since the change in soil temperature more or less depends on the change in air temperature, the average monthly air temperatures for the considered heating seasons 2014 ÷ 2018 were used in the calculation. The model enables the analysis of various parameters on performance of heat pump, such as the change of the soil temperature, as well as the change of the return water temperature from the heating system

TESTING OF THE INFLUENCE OF THE SPRAY COOLING SYSTEM ON THE AIR COOLER EFFICIENCY

On the hitherto installed chillers/rooftop units/dry coolers and air coolers, a significant improvement in cooling was noticed in the summer mode when the pipe bundle is sprayed with water. Tests have been performed on systems that produce water mist (micron-sized water droplets), which are formed by spraying water under high pressure through the nozzles/nozzle openings. When a drop of micron-sized water comes in contact with warm air, it evaporates very quickly, absorbing a certain amount of heat, which results in cooling the air. This improves the operation of the cooler in critical periods when the ambient air temperature is very high. The aim of the test is to determine the improvement of the cooler operation under operating conditions with the application of the described system in relation to the operating mode without water spraying (dry operating mode)

TESTING OF AIR HEATERS WITH PLATE FINNED TUBES WITH SPRAY COOLING

This paper deals with the procedure for measuring the relevant parameters of plate-finned tube heat exchangers operating as an air heaters. Besides the description of the experimental installation, the most important geometric characteristics of these appliances are given. In order to determine the relevant parameters for dry and wet regimes (for cases with and without spray cooling), experimental measurements of the heat and hydraulic performance of the heat exchangers were made. Water flowed through the pipes as a warmer fluid. During the experiments, the flow rates and temperatures of the working fluids were varied within the limits defined by the operating parameters of the installation itself, sufficient to allow the obtained results to be generalized. The experimental data were processed by appropriate statistical methods; the aim was to determine equations suitable for engineering practice. The coefficients of the air-side heat and mass transfer and friction factor are defined on the complex geometry of this type of heat exchangers under realistic operating conditions, as a function of the relevant similarity criteria

COMPARATIVE ANALYSIS OF ELECTRIC ENERGY CONSUMPTION OF CASCADE SYSTEM R134a/CO2 WITH SINGLE-STAGE R404A AND TWO-STAGE CO2 INSTALLATION

The paper analyzes the electric energy consumption of three different refrigeration installations: cascade refrigeration system with R134a in the high-temperature circuit and CO2 in the low-temperature circuit, single-stage refrigeration system operating with R404A, and two-stage transcritical CO2 system. The indirect impact of the refrigeration system on global warming through electric energy consumption was examined. Thermodynamic cycles of these installations have been described and models have been developed to analyze the electric energy consumption required to drive the compressor as the largest consumer, for the cooling capacity of the evaporator 5,7 kW at evaporation temperature -30 °C for meteorological 2017, in the city of Belgrade. As a basis for comparative analysis, the existing cascade refrigeration system, which is located in the Laboratory for Thermal Science at the Faculty of Mechanical Engineering in Belgrade, was selected

COMPARATIVE ANALYSIS OF ELECTRIC ENERGY CONSUMPTION OF CASCADE SYSTEM R134a/CO2 WITH SINGLE-STAGE R404A AND TWO-STAGE CO2 INSTALLATION

The paper analyzes the electric energy consumption of three different refrigeration installations: cascade refrigeration system with R134a in the high-temperature circuit and CO2 in the low-temperature circuit, single-stage refrigeration system operating with R404A, and two-stage transcritical CO2 system. The indirect impact of the refrigeration system on global warming through electric energy consumption was examined. Thermodynamic cycles of these installations have been described and models have been developed to analyze the electric energy consumption required to drive the compressor as the largest consumer, for the cooling capacity of the evaporator 5,7 kW at evaporation temperature -30 °C for meteorological 2017, in the city of Belgrade. As a basis for comparative analysis, the existing cascade refrigeration system, which is located in the Laboratory for Thermal Science at the Faculty of Mechanical Engineering in Belgrade, was selected

Thermoeconomic analysis of spiral heat exchanger with constant wall temperature

Thermoeconomic analysis of spiral heat exchanger is conducted. Different geometrical parameters, such as outer diameter, plate height, passage gap, etc. are used and varied in a wide range. Detailed thermal and total costs analyses were performed for two spiral heat exchanger with different process fluids (water and thermal oil) with temperature changes, while the wall temperature was kept constant (condensation). The results were shown graphically. It is determined that optimum values of number of entropy generation units correspond to minimum total annual cost. The optimal solution could be found in the recommended range of geometric sizes for defined inlet and outlet temperatures and process fluid-flow rate

EFFECTS OF PURGING NON-CONDENSABLE GASES FROM AMMONIA REFRIGERATION SYSTEM

This paper analyzes the operational characteristics of the Tuzla brewery refrigeration system after the installation of the Danfoss IPS8 air purging device. In order for an analysis such as this to be conducted correctly, one must understand where in the brewing industry the need for refrigeration occurs, how non-condensable gases are accumulated inside of the refrigeration system, and how the separation of non-condensable gases is done. After the theoretical part of the analysis, in which the thermodynamic basics of the ammonia refrigeration cycle is explained, an analysis of annual savings was conducted. The results of the analysis are presented in the form of annual savings in electricity consumption, for different operating conditions of the specific refrigeration system with and without a purger. At the end of this analysis, the amount of released non-condensable gases were also calculated

Koralacije za određivanje nominalnog pritiska u funkciji od temperature i pritiska za prirubnice od ugljeničnog čelika

Prirubnice od ugljeničnog čelika predstavljaju sastavni deo cevovoda i opreme u različitim granama industrije. Njihova klasifikacija je u standardima izvršena prema nominalnom pritisku (PN). Radi određivanja PN za prirubnice od ugljeničnog čelika potrebno je poznavati materijal prirubnice, kao i radne uslove: pritisak i temperaturu. Za potrebe inženjerske prakse, u radu su date korelacije između pritiska, temperature i nominalnog pritiska, koje su dobijene obradom podataka, datih u standardu SRPS EN 1092 – Prirubnice i njihovi spojevi – kružne prirubnice za cevi, armature fazonske komade i pribor, koje nose oznaku PN. Ocena kvaliteta korelacija izvršena je pomoću statističkih pokazatelja: korelacionog odnosa i srednjeg kvadratnog odstupanja