Industrial applications of the air direct-contact , gravel , ground heat exchanger

The paper describes the analysis of possibility of using the air direct-contact, gravel, ground heat exchanger (Polish acronym BGWCiM), patented at the Wroclaw University of Science and Technology to prepare air for conditioning rooms in the industry. Indicated the industry sectors where the application may be the most beneficial.


Reasons for raising the issue
The urgent need to save energy obliges to more widespread use of solutions enhancing generation of natural energy which exists in nature and is easily accessible.One of methods to use it is the air direct-contact, gravel, ground heat ex-changer (Polish acronym BGWCiM), tested thoroughly at the Wroclaw University of Science and Technology and the advantages of which will be presented.Industry is one of the most important sectors of a national economy.It is worth mentioning that together with agriculture and construction, industry consumes nearly half of all energy produced.Heavy industry has got an important share.In the processing industry, most of the energy is consumed by the food, chemical, mineral, steel and paper industries [7].Greater efficiency in the use of energy in the economy, and especially in the industrial sector, has got a significant impact on production costs, profits of enterprises, as well as competitiveness of their products on the world market.It also contributes to faster development of enterprises.

The air direct-contact, gravel, ground heat exchanger
For many years, research has been carried out at the Institute of Air Conditioning and District Heating of the Wroclaw University of Science and Technology on efficient capture of natural heat (and also cooling) from a small depth of ground, in the air direct-contact, gravel, ground heat exchanger for ventilation and air conditioning purposes [1][2][3].
In the exchanger, the outside air is led horizontally through a 3-5 m long accumulation bed (fig. 1) [1].Following the contact of the air flowing between the filling of the bed, its temperature is brought closer to the filling's temperature, which during peak load results in heating of the outside air in winter from -18°C to about 0°C, and cooling it in summer from +30°C to about +20°C.Experience has shown that the accumulation bed should be made of washed gravel, gravel or rock grit having a hydraulic diameter of 15-40 mm but without fractions smaller than 3 mm.Stones having a hydraulic diameter of 80-100 mm constitutes a distribution layer.Thermal/humidity insulation should be made of 10 cm polystyrene protected with two layers of double foil.The entire area should be covered with about a half-meter layer of soil in order to enable planting of shrubs or lawn cultivation.
The heat exchanger (distribution and accumulation beds) should not exceed 5 m length and 2.5 m height.Studies have shown that continuous operation of a 5 m long bed during a summer day results in an optimal shift of a heat wave in the filling.As a consequence, lowest temperatures of the air leaving the exchanger are achieved at the highest outdoor temperatures [1,2].
The nominal air velocity, depending on operating mode, may range from 0.05 to 0.20 m/s.Such values result from the heat and mass exchange conditions be-tween the air and the filling, as well as from resistance of the airflow across the exchanger.
A sprinkler system can be performed under the layer of thermal/humidity insulation.Based on a study [1], it was found that sprinkler pipes having a nominal diameter of ø 25 mm, perforated with ø 2 mm holes made at an angle of 90° at intervals of 10 cm are the cheapest and best for this purpose.The optimal distance between pipes is 0.5 m.The sprinkler system enables possible periodic disinfection and washing of the bed.It also facilitates intensification of heat and mass exchange processes in the accumulation filling, especially in the context of increasing the relative humidity of the air which leaves it.The research conducted by the Sanitary-Epidemiology Station in Wroclaw on an exchanger which has been working for many years without sprinkling did not show any microorganisms harmful to human health in the air discharged from the exchanger.On contrary -it was concluded that the number of particles from air pollution following the exchanger got reduced to half of the value contained in atmospheric air [2].Thus, the exchanger is additionally a specific kind of filter.These results are also confirmed by publications concerning other exchangers of this type operating in Poland.The experiments [2] of hydrating of the bed during its operation show that it does not cause a significant decrease in the air temperature.It only, due to moisturising, significantly increases relative humidity to values near to saturation, i.e. φ = 100%.Additionally, during the course of the study, it was noted that similar parameters are obtained by humidifying only the inlet section of the filling [2].This allows for possible savings when constructing systems where it is important for some operating periods to achieve higher relative air humidity at the outlet.In addition, what is important, hydraulics tests of the filling performed during sprinkling of the inlet section only, as well as of the entire filling, did not show increase in resistance of the airflow across the bed.Fig. 2. Practical chart to determine the pressure loss in the bed according to [3].In fig. 3 average values of air temperature after passing through BGWCiM and average times of occurrence of given outdoor temperature data for Wroclaw [2] were presented.They were indicated and statistically calculated on the basis of many years of measurements.This chart can be used in designing and provide the basis for performance of any analyses connected with the full-year operation of the device.
As a result of air contact with the ground in BGWCiM, the soil gets drained during periods of hot summer, moisturised in winter and cleansed.The above is achieved only by negligible energy inputs necessary to overcome the airflow resistance through the bed.

Required air parameters
Each type of production, depending on the technology used, requires very specific microclimate conditions (table 1) to produce a given product of possibly the highest quality.What must not be overlooked is also the human aspect.In order to achieve high productivity, the best possible conditions must also be created in the work area.It often happens that the production requirements and the comfort conditions of the people differ.It is when the technology is more important.Ventilation in an industrial plant constitutes a service component rather than a core business.However, production would be impossible without the optimal conditions of the microclimate.An important issue for the ventilation system to be economically viable.Therefore, it must provide appropriate parameters for both the technological process and people, ensuring minimal investment and operating costs [4].Moreover, in this drawing the so-called climatic curve is indicated, which shows average outdoor air parameters of Poland's geographical area and typical parameters of thermal comfort.
When referring to the air parameters following BGWCiM to the air parameters required in production processes (room temperature and air conditioning temperature), it can be seen that the use of the exchanger for air preheating will not always be cost effective.During the cold season, except for processes requiring very low humidity in the room (dry rooms), preheating and humidification will be almost always beneficial.This differs in the case of the transition and warm periods, when too much humidification of the air in the ground heat exchanger, usually close to the saturation parameters, will cause the need to dry it, which will be associated with additional costs which can exceed the savings resulting from the temperature drop.
Beneficial effects of using BGWCiM should be expected in air treatment systems for air conditioning of rooms with relatively high temperature and high relative humidity (e.g.textile industry, tobacco industry, etc.).

Conclusion
The decision to use BGWCiM to pre-prepare the air in the process of air conditioning of production rooms should be each time preceded by an in-depth analysis of the full-year operation of the system with a full assessment of possible effects.Restrictive requirements for microclimate parameters in many industries will not always be achievable with BGWCiM alone or with BGWCiM serving to pre-prepare the air.Wherever the analysis confirms the suitability of its use, savings of up to 30% per year of energy consumed can to be expected, compared to BGWCiM-free system while reducing power and size of other air treatment devices (e.g.heaters, radiators, heat sources, chillers, humidifiers, etc.) [2].

Fig. 2
Fig.2shows a practical, usable during designing, experimentally determined dependency between air flow velocity (v) and pressure loss (dp) of the accumulation bed for different hydraulic diameters used in BGWCiM.

3 E3SFig. 3 .
Fig.3.Average air temperature values following the ground exchanger and a number of hours of occurrence of given outdoor temperatures during a year for A type of heat exchanger, 5 m length, 0.14 m/s air velocity, operated year-round continuously[2,3].

Table 1 .
continued… In fig. 4 there are areas presented which show the nature of the variability of the outdoor air and the air following BGWCiM, recorded in the climatic zone of Poland.