GB1577707A

GB1577707A - Concentration and hot-air drying plant

Info

Publication number: GB1577707A
Application number: GB13754/78A
Authority: GB
Original assignee: ALFA LAVAL SA; ALFA-LAVAL SA; CIE EVA DRY; CIE EVA-DRY
Current assignee: ALFA LAVAL SA; ALFA-LAVAL SA; CIE EVA DRY; CIE EVA-DRY
Priority date: 1977-04-13
Filing date: 1978-04-07
Publication date: 1980-10-29
Also published as: DK160578A; FR2387427A1; BR7802277A; FR2387427B1; DE2815828A1; CA1087085A; NL7803949A; JPS53134772A

Description

(54) CONCENTRATION AND HOT-AIR DRYING PLANT (71) We, ALFA-LAVAL S.A., of 62-70 rue Van Tourgueneff, F-78380 Bougival, France, and CIE EVA-DRY, of 30 Avenue de L'Amiral Lemonnier, F-78160 Marly Le-Roi, France, both French Corporate Bodies, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: - This invention relates to a concentrating and hot-air drying plant.

A known concentrating and hot-air drying plant comprises a concentration unit equipped with a steam flow path having its outlet connected to a condenser, and a hotair drying unit supplied with hot air through a feed path comprising means for drawing air from the external atmosphere and means for heating the air to a predetermined temperature. Such plants are used in the food, chemical and pharmaceutical industries, in particular for manufacturing pulverulent products such as powdered milk. In the known plants the concentration unit and the hot-air drying unit are sometimes disposed side-by-side but are independent of each other from the point of view of energy supply. As a rule, the liquid treated by the concentration unit is subsequently treated by the hot-air drying unit, possibly after one or a plurality of other intermediate treatment steps between the concentration stage and the drying stage.

However, the concentration unit and the hot-air drying unit may also be used for processing different products.

In general, the hot air stream utilized for drying the treated product is obtained by taking air from the external atmosphere, then heating this air to a well defined temperature depending on the drying method and the specific nature of the product to be treated, by using any suitable heating device. The consumption of heat energy necessary for heating the air depends on the temperature of the external atmospheric air which varies considerably according to the season and to the period of the day. In addition to the fact that the consumption of energy in the heating device may be relatively high during cold seasons and periods, it is also necessary to incorporate in the heating device a regulation system capable of causing the heating power to vary in dependence upon the temperature of the incoming air drawn from the external atmosphere if the hot-air stream delivered to the drying unit is at a well-defined temperature under all conditions.

According to the present invention there is provided a concentrating and hot-air drying plant comprising a steam-heated concentration unit having a steam flow path, a hot-air drying unit, a feed path for supplying air (as drying medium) to the drying unit and including means for heating the air, an indirect heat exchanger disposed in the feed path upstream of the heating means for preheating the air, and means for supplying steam to the heat exchanger from the steam flow path of the concentration unit.

With such a plant the drying unit can be supplied with hot air at a predetermined temperature irrespective of the season or time of day while the heat energy consumption is lower than the average consumption in the known plants. Furthermore it is possible for the heat energy consumption of the heating means of the drying unit to be kept at a constant value irrespective of seasonsal and day time variations in air temperature.

In a specific embodiment, the heat exchanger is connected to an outlet at the downstream end of the steam flow path of the concentration unit. With this arrange ment it is possible to recover at least a fraction of the heat contained in the steam (at about 45"C) issuing from the concentration unit, which heat was heretofore generally consumed and simply lost in the condenser, and use the recovered heat for preheating the air to be supplied to the drying unit. The quantity of heat which must be delivered by the heating means of the drying unit to heat the air to a predetermined temperature is lower than the quantity of heat required in hitherto known plants, and a considerable amount of energy can be saved.

If the air flow is constant and the steam delivered to the heat exchanger has a constant temperature, the heat exchange surface between the air path and steam path of the heat exchanger may have such an area that the temperature of the pre-heated air delivered by the heat exchanger remains substantially constant while the temperature of the inlet air, taken from the external atmosphere, varies with seasons or during the day time. Under these conditions, since the heating means of the drying unit is supplied with pre-heated air at a substantially consant temperature, the temperature regulation means hitherto required in known plants can safely be dispensed with.

In addition, since at least a fraction of the steam delivered from the concentration unit is normally condensed in the heat exchanger, the quantity of steam condensed in a condenser is reduced considerably.

Thus the consumption of cold water in the condenser may also be reduced. Another advantage may be obtained from the ecological point of view where the water necessary for cooling the condenser is pumped from a river and subsequently returned into the same river, since both the amount of heat transferred to or dissipated in the cooling water and the amount of heated cooling water discharged into the river can be reduced in comparison with known plants.

An embodiment of the present invention will now be described in greater detail, by way of example, with reference to the accompanying drawing which shows diagrammatically a concentration and drying plant in accordance with the invention.

The illustrated plant comprises a concentration unit 1 and a hot-air drying unit 2 to which hot air is supplied through a feed path 3.

The concentration unit 1 may include one or more stages, and as shown comprises two stages connected in series. Each stage includes an evaporation column 4a, 4b, for example of the downward-flowing film type, and a cyclone 5a, 5b for separating liquid droplets from the steam issuing from the evaporation columns 4a, 4b.

The steam from the cyclone 5b of the last stage is directed through a conduit 6 to a water-cooled condenser 7 which may be of the mixer or surface type. Concentration units as above described are well known in the art and therefore a detailed description is not deemed necessary herein.

The drying unit 2 illustrated in the drawing is intended more particularly for obtaining pulverulent products by atomizing a liquid in a hot air stream. To this end, the drying unit 2 comprises a cylindroconical atomizing vessel 8 associated with a separator cyclone 9 and an air extraction fan 10. An atomizer 11 mounted in the top wall of vessel 8 is arranged to atomize a concentrated liquid in the hot-air stream supplied through the feed path 3. The feed path 3 comprises a motor-driven blower 12 capable of drawing a constant flow of atmospheric air through an air inlet 13 provided with a suitable filter, and a heater 14 for heating the air drawn through the inlet 13 to a predetermined temperature. The atomizer and hot-air drying units are well known in the art and therefore a detailed description thereof is also not deemed necessary herein.

The plant further compries a heat exchanger 15, for example of finned battery type, having its air path disposed within the feed path 3 upstream of the heating device 14, and, as shown, between the air inlet 13 and the motor-driven blower 12.

The heat exchanger 15 could alternatively be disposed between the blower 12 and the heating device 14. A portion of the steam coming from the last cyclone 5b of the concentration unit 1 is diverted to the steam path 15a of the heat exchanger 15 through a conduit 16 connected to conduit 6 upstream of condenser 7. Thus, a fraction of the steam is condensed in the heat exchanger 15 and its latent heat of condensation is transferred to the incoming air for pre-heating the air.

Preferably, the heat exchange surface area of the heat exchanger 15 is so large (with due consideration for the contant air flow and also the constant temperature of steam supplied through conduit 16,) that the temperature of the air issuing from the heat exchanger 15 remains substantially consant even though the temperature of the atmospheric air supplied through inlet 13 varies for example due to a seasonal and day time variations.

Assume for instance that the plant is in tended for the manufacture of powdered milk. In this case, the steam temperature at the outlet of the last cycle 5b of the con- cent ration unit 1 will be about 45"C. Under these conditions, the air can be heated in the heat exchanger 15 to a constant tem perature of about 35"C. Thus, a constant steam equivalent consumption of less than 1.6 kg of steam per kg of evaporated milk, instead of 1.s kg in conventional plants not equipped with the heat exchanger 15, can be warranted, that is, a heat energy consumption about 10 to 12% lower than the average consumption usually registered in conventional plants.

Instead of taking a portion of the steam issuing from the last cyclone 5b of the concentration unit 1, steam may be taken from an intermediate point in the steam path through concentration unit 1, for example from the top of the last evaporation tower or column 4b, through a branch conduit 16' shown in broken line in the drawing.

The choice of the point at which steam is diverted to the heat exchanger 15 should constitute a compromise between on the one hand the desired air temperature at the outlet of the heat exchanger, which, of course, should be as high as possible, and, on the other hand, the cost of the steam thus drawn and the influence thereof on the efficiency of the concentration unit.

It is also possible to supply steam to the heat exchanger 15 through conduits 6 and 16 and through conduit 16'. In this case, the exchanger 15 may comprise either two separate heat exchanger units having their air paths connected in series, or a single heat exchanger provided with two separate steam paths spaced along its air path.

Since the temperature of the steam supplied through conduit 16' is higher than the temperature of the steam supplied through conduit 16, the steam path 15a of the heat exchanger 15, which is the first steam path occurring along the air path, is connected to the conduit 16, and the steam path 15b of the heat exchanger 15 which is second in said air path is connected to conduit 16'.

Alternatively, instead of branching the steam path 15a, of the heat exchanger 15 off the condneser 7 as described hereinabove, the steam path 15a of the heat exchanger 15 may be connected in series between the outlet of the last cyclone 5b of the concentration unit 1 and the condenser, the latter then being positioned as shown in broken lines at 7' in the drawing.

Under these conditions, all the steam emerging from the last cyclone 5b is directed to the heat exchanger 15 where the whole or part of this steam is condensed, any residual uncondensed steam fraction being directed through a conduit 17 to the condenser 7' to be condensed therein.

As a rule, the air flow through the feed path 3 and consequently in the air path of the heat exchanger 15 is not sufficient to condense all the steam fed from the last cyclone 5b, so that in most cases condenser 7 or 7' will have to be retained, but it may have smaller dimensions in comparison with the size of condensers used in conventional plants not equipped with the heat exchanger 15. The same feed path 3 may be used but for heating a larger air flow in order to supply hot air to several drying units or to one single drying unit and to a duct for directing the hot-air for use in heating the buildings in which the plant is erected. In this case, the air flow may be sufficient to condense all the steam emerging from the last cyclone 5b, and the condenser 7 or 7' may be dispensed with completely.

There are many possible modiflcations to the described plant as readers skilled in the art will realise. For example, while the specification refers in particular to a drying unit adapted to atomize a liquid in a hotair stream, the invention is also applicable to other types of hot-air drying units. Likewise, while the description refers to a concentration unit comprising an evaporation tower of the downward-flowing film type, the invention is also applicable to plants comprising other types of concentrators, provided that the concentrators comprise a heating steam path.

WHAT WE CLAIM IS: - 1. A concentraitng and hot-air drying plant comprising a steam-heated concentration unit having a steam flow path, a hotair drying unit, a feed path for supplying air (as drying medium) to the drying unit and including means for heating the air, an indirect heat exchanger disposed in the feed path upstream of the heating means for preheating the air and means for supplying steam to the heat exchanger from the steam flow path of the concentration unit.

2. A plant according to claim 1, wherein means for supplying steam to the heat exchanger is connected to an outlet at the downstream end of the steam flow path of the concentration unit.

3. A plant according to claim 2, wherein the outlet is connected to a condenser and the means for supplying steam to the heat exchanger is branched off the condenser.

4. A plant according to claim 2, wherein the heat exchanger is connected in series between the outlet and a condenser.

5. A plant according to claim 1, wherein means for supplying steam to the heat ex changer is connected to an intermediate point along the steam flow path of the con centration unit.

6. A plant according to any one of claims 1 to 5, wherein the heat exchanger comprises first and second steam paths positioned in turn along the air feed path, the first steam path of the heat exchanger is connected by steam supplying means to

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. perature of about 35"C. Thus, a constant steam equivalent consumption of less than 1.6 kg of steam per kg of evaporated milk, instead of 1.s kg in conventional plants not equipped with the heat exchanger 15, can be warranted, that is, a heat energy consumption about 10 to 12% lower than the average consumption usually registered in conventional plants. Instead of taking a portion of the steam issuing from the last cyclone 5b of the concentration unit 1, steam may be taken from an intermediate point in the steam path through concentration unit 1, for example from the top of the last evaporation tower or column 4b, through a branch conduit 16' shown in broken line in the drawing. The choice of the point at which steam is diverted to the heat exchanger 15 should constitute a compromise between on the one hand the desired air temperature at the outlet of the heat exchanger, which, of course, should be as high as possible, and, on the other hand, the cost of the steam thus drawn and the influence thereof on the efficiency of the concentration unit. It is also possible to supply steam to the heat exchanger 15 through conduits 6 and 16 and through conduit 16'. In this case, the exchanger 15 may comprise either two separate heat exchanger units having their air paths connected in series, or a single heat exchanger provided with two separate steam paths spaced along its air path. Since the temperature of the steam supplied through conduit 16' is higher than the temperature of the steam supplied through conduit 16, the steam path 15a of the heat exchanger 15, which is the first steam path occurring along the air path, is connected to the conduit 16, and the steam path 15b of the heat exchanger 15 which is second in said air path is connected to conduit 16'. Alternatively, instead of branching the steam path 15a, of the heat exchanger 15 off the condneser 7 as described hereinabove, the steam path 15a of the heat exchanger 15 may be connected in series between the outlet of the last cyclone 5b of the concentration unit 1 and the condenser, the latter then being positioned as shown in broken lines at 7' in the drawing. Under these conditions, all the steam emerging from the last cyclone 5b is directed to the heat exchanger 15 where the whole or part of this steam is condensed, any residual uncondensed steam fraction being directed through a conduit 17 to the condenser 7' to be condensed therein. As a rule, the air flow through the feed path 3 and consequently in the air path of the heat exchanger 15 is not sufficient to condense all the steam fed from the last cyclone 5b, so that in most cases condenser 7 or 7' will have to be retained, but it may have smaller dimensions in comparison with the size of condensers used in conventional plants not equipped with the heat exchanger 15. The same feed path 3 may be used but for heating a larger air flow in order to supply hot air to several drying units or to one single drying unit and to a duct for directing the hot-air for use in heating the buildings in which the plant is erected. In this case, the air flow may be sufficient to condense all the steam emerging from the last cyclone 5b, and the condenser 7 or 7' may be dispensed with completely. There are many possible modiflcations to the described plant as readers skilled in the art will realise. For example, while the specification refers in particular to a drying unit adapted to atomize a liquid in a hotair stream, the invention is also applicable to other types of hot-air drying units. Likewise, while the description refers to a concentration unit comprising an evaporation tower of the downward-flowing film type, the invention is also applicable to plants comprising other types of concentrators, provided that the concentrators comprise a heating steam path. WHAT WE CLAIM IS: -

1. A concentraitng and hot-air drying plant comprising a steam-heated concentration unit having a steam flow path, a hotair drying unit, a feed path for supplying air (as drying medium) to the drying unit and including means for heating the air, an indirect heat exchanger disposed in the feed path upstream of the heating means for preheating the air and means for supplying steam to the heat exchanger from the steam flow path of the concentration unit.

an outlet at the downstream end of the steam flow path of the concentration unit, and the second steam path of the heat exchanger is connected by steam supplying means to an intermediate point along the steam flow path of the concentration unit.

7. A plant according to any one of the preceding claims, wherein the heat exchanger is the finned battery type.

8. A plant according to any one of the preceding claims, wherein the heat exchanger has such a heat exchange surface area that, for a given air flow and given constant temperature of the steam fed to the exchanger, the temperature of the air leaving the heat exchangerd in the feed path remains substantially constant despite any variation in the temperature of atmospheric air supplied to the heat exchanger.

9. A concentrating and hot-air drying plant substantially as herein described with reference to the accompanying drawing.