A major issue in the undergrounding mining is providing the thermal comfort parameters at the work sites and among those the temperature it's the most important. Also, in calculus of underground mine ventilation, the air temperature must be taken into account. The rate of dimensionless heat flux has a major influence in the mining environment by heating or cooling the air which flows through a mine work. The present paper studies the complex problem of heat transfer between air and the surrounding rocks, by establishing a new unsteady dimensionless heat transfer model based on Bi and Fo criteria. The dimensionless heat flux value was computed based on the dimensionless temperature. The equation which defines the dimensionless temperature and therefore the heat transfer between air and rock massif was solved using two different mathematical methods, obtaining the same result. The developed of mathematical methods were validated by conducting a case study based on data obtained from Jiu Valley underground mines. The methods may prove its efficiency in the design calculus for opening mine works where problems of thermal comfort state settlement occur.

The economizer of a steam boiler is an important component that influences its efficiency. The link between steam boiler efficiency and economizer efficiency is analyzed, by computing some of the quality indicators of heat exchangers like effectiveness, thermal efficiency and gross thermal efficiency, for different boiler loads.

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The primary method of producing steam for power generation is by using combustion boilers. The need to increase energy efficiency in the EU is underlined in the Directive 2012/27/EU on energy efficiency. The goal is to achieve 20% savings on the Union's primary energy consumption by 2020 compared to projections [1]. One way to achieve this goal is by increasing efficiency of electrical power production, and as a result less fuel is required to generate each kWh. Efficiency of retrofitted steam generators represents an important part of papers published on boiler efficiency, but just as much papers were analyzing the efficiency of modern ones, built using state of the art technologies. An extensive research on lignite-fired power plants was carried out by Qian Zhu [2], highlighting that the efficiency of lignite-fired is 88.9% for PC (pulverized coal) and USC PC (ultra-supercritical pulverized coal) units while for CFBC (circulating fluidized bed combustion) is 87.0%. 2. PROBLEM PRESENTATION According to NPC [3] report 76% of steam generators produced 30 years ago, are still in USA, as a result of the long life-cycle and expensiveness. The same situation can be found in the rest of the world too. New generation of steam generators are developed with higher efficiency and low-emissions to meet these requirements. As new units became operational, the average efficiency of power plants will improve, rather than by retrofitting older ones, but this is a process which can take a long time. The right time for steam generator replacement can be estimated by assessment of energy performance of boilers, which requires heat balance calculations. Energy auditing in Romania is regulated by the state and supervised by the regulatory authority ANRE. Calculations must be carried out according to the published guide [4], based on algorithms and equations for heat balance calculations of various installations and equipment found in relevant literature [5][6]. 2.1. A brief presentation of steam generator CRG-1666 is a water tube steam generator (Fig. 1) with natural circulation, two Π shaped flue gas paths, a steam drum and 420 t•h-1 steam output at 137 bar pressure and 540 °C temperature. The rated efficiency is 85%. The rated feed water temperature is 230 °C at 158 bar pressure.