•Heat exchanger performance was numerically simulated for extreme winter conditions.•Two freeze protection techniques for the counter-flow heat exchanger were compared.•Threshold temperatures for ...trouble-free heat exchanger operation were determined.
In this paper, a theoretical analysis of heat and mass transfer in a counter-flow heat exchanger used for energy recovery in air handling unit (AHU) under sub-zero operating conditions is presented. The effect of the latent heat of water vapour condensation on heat recovery efficiency was analysed. Two main frost prevention techniques (preheating and bypassing the outdoor airflow) were analysed and compared. The values of critical outdoor air temperatures and outdoor-to-return airflow heat capacity ratios were determined on the basis of parametric analysis of frosting limits for different inlet return airflow conditions. It was found out that bypass method is characterized by lower heat recovery efficiency in comparison with preheating technique for examined range of relative humidity often observed in typical buildings applications.
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•Outdoor air relative humidity affects frost accumulation limit temperatures.•Frost growth does not influence the Number of Transfer Units significantly.•Latent heat flux contributed to the local ...frost density should not be neglected.
This paper tackles the issue of frost formation in non-hygroscopic rotary heat exchangers used for energy recovery from exhaust air under high-speed rotor conditions by means of numerical simulations and experimental approaches. On the basis of some idealized assumptions, a frost growth submodel is presented to predict the behavior of the rotary heat exchanger under frosting conditions. Frost formation is modeled by considering the mass diffusion of water vapor through the frost layer, taking into account supersaturation phenomena. Calculations were carried out using a three-zone model based on the modified ε-NTU method. The local heat transfer coefficient and the NTU calculation method resulting from the influence of the heat exchanger entrance region were also applied. The obtained correlations for the temperature effectiveness agree with the simulation data within uncertainty bounds. The results of the numerical simulations allowed us to determine the outdoor air conditions that initiated the frost accumulation phenomenon inside the thermal wheel for two values of return air relative humidity: RH2i=20% and RH2i=40%. In both cases, the threshold temperature for unsafe operating conditions increases with increasing relative humidity of the outdoor air. Under ‘frost accumulation’ operating conditions, the frost growth rate is approximately five times higher at RH2i=40% than at RH2i=20%. In this regard, the need to implement frost protection techniques increases significantly with an increase in relative humidity of return airflow. Further analysis conducted for the operation of a thermal wheel’s operation under frosting conditions revealed that a latent heat flux contributed to the local frost density should not be neglected in a compact heat exchanger’s model. Interestingly, the operation time of the rotary heat exchanger, and hence the growth of the thickness of the frost layer, has a significant influence on a local heat transfer coefficient α, however, it does not affect the Number of Transfer Units (NTU) visibly.
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•A numerical study of heat and mass transfer in counter-flow exchanger is presented.•The e-NTU model of counter-flow plate heat exchanger is developed.•The most probable variants of year-round ...operation performance are established.•The values of critical outdoor temperatures are determined for frost-free operation.•The frost control strategies using face and bypass dampers are proposed.
An accurate and efficient model based on the modified ε-NTU method was developed for numerical simulations and analysis of coupled heat and mass transfer inside the counter-flow plate heat exchanger under frosting operating conditions. The proposed model was validated with experimental data. The successful comparison between simulated and experimental data indicates that the developed model is capable to predict adequately operating performance of the counter-flow plate heat exchanger under sub-zero outdoor air temperature conditions. Three active heat and mass transfer areas were established in a counter-flow plate heat exchanger equipped with bypass damper and face and bypass dampers. The detail analysis of these particular heat and mass transfer zones creation revealed the most probable variants of year-round operating conditions of the counter-flow heat exchanger. The implementation of the particular variant of heat and mass transfer depends upon the relations of the temperatures in two decisive zones on the return air channel surface (in the “cold” and “hot” zone) and the value of the inlet return airflow dew point temperature. It was established, that the most unfavourable operating conditions at sub-zero outdoor air temperature occur at the value of inlet return air dew point temperature equalled to 0°C. Unfortunately, such value of dew point temperature corresponds to the normal indoor air conditions in winter season. The values of critical outdoor temperatures were determined on the base of parametric frosting limits analysis conducted under different inlet return airflow conditions for different values of heat recovery efficiency of the counter-flow plate heat exchanger at different opening levels of face and bypass dampers. It was established, that the frost tends to take place with increasing temperature effectiveness of the heat exchanger. It was established, that the fully open bypass technique does not provide complete frost protection under sub-zero outdoor air temperature operating conditions.
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In the rooms with dominant sensible heat load, volume airflow depends on many factors incl. pre-established temperature difference between exhaust and supply airflow. As the temperature difference is ...getting higher, airflow volume drops down, consequently, the cost of AHU is reduced. In high industrial halls with air exhaust grids located under the ceiling additional temperature gradient above working zone should be taken into consideration. In this regard, experimental research of the vertical air temperature gradient in high industrial halls were carried out for the case of mixing ventilation system The paper presents the results of air temperature distribution measurements in high technological hall (mechanically ventilated) under significant sensible heat load conditions. The supply airflow was delivered to the hall with the help of the swirl diffusers while exhaust grids were located under the hall ceiling. Basing on the air temperature distribution measurements performed on the seven pre-established levels, air temperature gradient in the area between 2.0 and 7.0 m above the floor was calculated and analysed.
The aim of the given paper is to present the analysis of tourism services by means of time series models and forecasting of evaluation of tourism services. Tourism services are analysed according to ...various parameters: hotel stay price, hotel services quality (such as cleanness of rooms, check-in, information provision etc.), catering quality and medical service quality. The research has been undertaken on the basis of responses of foreign guests of 13 hotels in Kołobrzeg region taken from 1400 questionnaires divided according to age and sex of respondents. Various econometric models were used for the analysis of statistic regularities. First, customers evaluated quality of their stay in hotels. These data were examined during the 2006-2009 time period as a stochastic process. It was found that the processes are nonstationary, that is why the ARIMA (1,1,1) model was used in the study. On the basis of the analyses and prognoses one can deduce that models of time series make it possible to estimate a tendency that occurs for an analysed parameter, however misprediction is quite possible to appear (up to 30 %). Similar results were achieved during the analysis of evaluation of hotel stay quality on the basis of sex of respondents. One should point out that an average value of male evaluation rate is higher than an average value of female evaluation rate. Time series for analysed variables were integrated into level I(1). A certain co-integrational connection was found between the evaluation of hotel stay and number of stays in a given hotel, where tourists that have already visited a given hotel for several times presumably give a high evaluation rate of hotel stay quality. The result of the analysis of long-term relation between hotel stay and duration of check-in is presented in high mutual dependence of hotel evaluation rate on evaluation of check-in duration. A hotel stay price influences hotel stay evaluation in a negative way, but catering and medical services are considered as standard and do not have any particular influence on hotel stay evaluation. The undertaken study shows that methods that take into account time series can be successfully used in analysis of parameters of tourist comfort and in evaluation of hotel services.
Celem niniejszej pracy jest analiza jakości świadczonych usług w zależności od szeregu czynników: ceny pobytu w hotelu, jakości usług hotelarskich (czystość w pokoju, rejestracja pobytu, informowanie klientów itp.), jakości usług gastronomicznych (oferty gastronomiczne), ilości i jakości usług sanatoryjno-rehabilitacyjnych z wykorzystaniem metod szeregów czasowych oraz prognozowania oceny jakości świadczonych usług. Badania były przeprowadzone na podstawie ankietowania gości zagranicznych z 13 hoteli regionu kołobrzeskiego, razem powyżej 1400 ankiet w zależności od wieku i płci ankietowanych. Dla ustalenia prawidłowości statystycznych zastosowano różne modele ekonometryczne. Po pierwsze, ocenianie jakości pobytu w hotelach przez klientów było rozpatrywane w przestrzeni czasowej 2006-2009 jako proces stochastyczny. Ustalono, że procesy są niestacjonarne, dlatego badanie bylo przeprowadzone na podstawie modelu ARIMA(1,1,1). Badania i prognozy pozwalają zrobić wniosek, że modele szeregów czasowych mogą oszacować trend, występujący dla badanego wskaźnika, natomiast błąd prognozowania jest dość wysoki (do 30%). Podobne wyniki były otrzymane rownież podczas badania ocen jakości pobytu w hotelach w zależności od płci badanych. Zaznaczymy, że średnia wartość ocen mężczyzn jest wyższa niż średnia ocen kobiet. Szeregi czasowe dla badanych zmiennych miały stopien zintegrowania I(1). Została ustalona znaczna więź integracyjna między oceną pobytu w hotelu oraz ilością odwiedzin wybranego hotelu, kiedy turyści nie po raz pierwszy odwiedzający hotel przeważnie wystawiają wysokie oceny jakości pobytu w hotelu. Wynikiem badania zależności długookresowej pobytu w hotelu od czasu trwania procedury zameldowania jest bardza wysoka współzależność oceny pobytu od oceny czasu trwania procedury zameldowania. Cena pobytu w hotelu negatywnie wpływa na ocenę pobytu w hotelu, natomiast usługi sanatoryjne i gastronomiczne odbierane są jako standard i nieznacznie wpływają na ocenę pobytu w hotelu. Przeprowadzone badania wskazują, że metody szeregów czasowych mogą być skutecznie zastosowane w badaniu wskaźników turystycznych i ocenie jakości usług hotelarskich.
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Autorzy przedstawiają koncepcję wykorzystania modelowania ekonometrycznego w działalności usługowej, w branży hotelarskiej, do pomocy budowania postaw pro-klienckich, mającego wskazać newralgiczne ...punkty w obsłudze klienta, decydujące o przyszłych wyborach klienta, odnoszących się do podejmowania decyzji o zakupie usług konkretnego przedsiębiorstwa hotelowego.
Autorzy przedstawiają koncepcję wykorzystania modelowania ekonometrycznego w działalności usługowej, w branży hotelarskiej, do pomocy budowania postaw pro-klienckich, mającego wskazać newralgiczne ...punkty w obsłudze klienta, decydujące o przyszłych wyborach klienta, odnoszących się do podejmowania decyzji o zakupie usług konkretnego przedsiębiorstwa hotelowego.
We prove that the spectra (sets of values) of the upper and lower Sergeev frequencies of zeros of a linear difference equation of order higher than one are Suslin sets of the interval
Moreover, we ...prove the inverse theorem for upper frequencies of zeros of second-order equations under the additional assumption that 0 belongs to the spectrum.
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