JPH1133326A - Powder separation device and powder separation sorting method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the condensation of SOx or moisture in a gas to be treated from occurring when backwashing a filter element with a pressurized air despite the fact that the gas to be treated containing SOx is supplied to a powder separation tank after lowering the temperature of the gas. SOLUTION: In a backwashing means 18 which recovers the filtration performance of a filter element 13 by causing a pressurized gas to flow back into the filter element 13 inside the powder separation tank 14 for capturing powder contained in a gas, a heating means 29 for heating up the pressurized gas to be sent back to the filter element 13 at a desired temperature, is installed. The pressurized gas for backwashing is previously heated up at a specified temperature using the heating means 29 to be supplied to the filter element 13. Thus it is possible to prevent the condensation of a sulfur oxide contained in a gas to be treated from occurring by the cooling action of the pressurized gas due to adiabatic expansion.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a powder separating apparatus and a powder separating method for separating and recovering powder contained in a gas by using a filter element.
The present invention relates to a powder separation apparatus and a powder separation method suitable for use when a gas to be treated contains sulfur oxides (SO _x ).

[0002]

2. Description of the Related Art Conventionally, as a method for separating and recovering a specific component contained in an ore, a raw ore is heated and calcined in a rotary kiln or the like, then crushed into fine particles by a crusher, and crushed by a crusher. First, coarse particles are separated and collected by sending them to a classifier using compressed air, and those that have become fine particles are further sent to a powder separation device together with gas, There is known a method of separating and collecting using a filter element provided in a powder separating apparatus.

Heretofore, as a powder separating apparatus used for such an application, a powder separating process equipped with a filter element for collecting powder contained in a gas supplied from a gas supply port has been known. A tank, a gas supply means to send the gas containing the powder to the gas supply port, and exhaustion of the gas from the gas discharge port side so that the gas containing the powder is filtered by the filter element and flows to the gas discharge port. And the powder collected on the outer surface of the filter element is blown off to a powder recovery unit provided at the inner bottom of the powder separation processing tank to recover the filtering ability of the filter element. Various types having an element cleaning means have been developed.

[0004] In such a powder separating apparatus, the material, structure and arrangement of the filter element for collecting the powder, or the element cleaning means are used for the purpose of improving the durability and the processing efficiency. Improvements have been repeated.

[0005] As this element cleaning means, clean pressurized air or the like is caused to flow back to the filter element,
A back-washing method is used in which the powder collected on the outer surface of the filter element is blown off to a powder recovery section provided at the inner bottom of the powder separation processing tank to recover the filtering ability of the filter element. It became so.

In this cleaning method, the filtering performance of the filter element can be always maintained in a good state by interrupting the cleaning process at a constant period during the collection process. That is, it is not necessary to stop the powder separation / recovery processing by the powder separation processing tank every time the element is washed, and the processing performance can be improved by continuous operation.

[0007]

As described above, when the ore is pulverized and the powder is separated and recovered from the gas containing the powder, the gas containing the powder includes: It often contains sulfur oxides (SO _x ) contained in natural ores. Then, when the above-described back washing with pressurized air is performed for washing the filter element of the powder separation treatment tank for treating the gas containing SO _x , sulfuric acid adheres to the surface of the filter element, and the sulfuric acid further adheres. The powder adheres to the filter, and the above-mentioned back-washing method or the like cannot cope with the clogging of the filter, or the corrosion of the members around the filter element by sulfuric acid progresses,
In the worst case, the gas to be treated may flow out of the gas outlet without passing through the filter.

[0008] Such inconvenience, pressurized air during backwashing by, the cooling effect of rapid adiabatic expansion of the compressed air to flow back through the filter element, the temperature is SO _x and water gas to be treated near the filter element down of below dew point, it is going to generate a sulfuric these of the SO _x and water is condensed on the surface of the filter. Therefore, conventionally, even if the temperature of the gas to be treated is lowered by the adiabatic expansion of the pressurized gas for back washing flowing backward through the filter element, the temperature of the gas to be treated does not fall below the acid dew point. In addition, the gas to be processed, which is supplied to the powder separation processing tank, is previously heated to a sufficiently high temperature.

[0009] However, in the case where the gas to be treated is heated to a high temperature, a large fuel cost is required for the heat treatment of the gas to be treated, and the operation cost of the apparatus becomes high. Was. In addition, pipes for guiding the gas to be treated heated to a high temperature to the powder separation processing tank, packing used at the joint of the pipes, and the like must be formed of an expensive material having high heat resistance. For this reason, there has been a problem that the apparatus cost increases.

An object of the present invention is to solve the above problems, it is supplied by lowering the temperature of the gas to be treated containing SO _x in the powder separation tank, opposite the filter element by pressurized air During cleaning, sulfuric acid can be prevented from being generated due to dew condensation on the surface of the filter element due to the cooling effect due to rapid adiabatic expansion of the pressurized air flowing backward through the filter element. As a result, the filtering performance of the filter element can be maintained satisfactorily over a long period of time, and the temperature of the gas to be supplied to the powder separation treatment tank can be lowered to save the fuel used for heating the gas to be treated. At the same time as reducing costs, the cost of equipment is reduced by lowering the level of heat resistance of components such as pipes that supply the gas to be treated to the powder separation processing tank. To provide a powder separation device and powder separation method capable of.

[0011]

According to a first aspect of the present invention, there is provided a powder separating apparatus according to the first aspect of the present invention, wherein a gas is supplied between a gas supply port and a gas discharge port from the gas supply port. A powder separation processing tank equipped with a filter element for collecting powder contained in the gas, and a gas supply means to be processed for sending gas containing powder to a gas supply port of the powder separation processing tank; Exhaust means for sucking gas from the gas discharge port side so that gas supplied from the gas supply means into the powder separation processing tank is filtered by the filter element and flows to the gas discharge port. And a powder separation device for separating and collecting powder in a gas supplied by the gas supply means to be processed into a powder recovery unit at an inner bottom portion of the powder separation processing tank, wherein the filter element includes: Periodically reverse the flow of clean pressurized gas, Backwash means for removing the powder adhering to the outer surface of the filter element to a powder recovery unit provided at the inner bottom of the powder separation treatment tank to restore the filtration ability of the filter element, The back washing means is provided with a heating means capable of adjusting the temperature of the pressurized gas flowing back through the filter element to a desired temperature.

According to another aspect of the present invention, the above object is achieved.
The powder separation method according to claim 1 uses the powder separation device according to claim 1, wherein the powder in the gas supplied by the gas to be treated is supplied to the inner bottom of the powder separation treatment tank. A powder separation method for separating and collecting the powder in a powder recovery section, wherein the backwashing means raises the pressure of a pressurized gas to be sent to a filter element to a predetermined temperature in advance by the heating means, The temperature around the filter element of the gas containing the powder fed into the filter element is set to be equal to or higher than the dew point.

According to the powder separation apparatus and the powder separation method described above, when the filter element is backwashed by the pressurized gas, the temperature of the pressurized gas flowing backward through the filter element drops due to abrupt adiabatic expansion, and the pressure is reduced. A cooling function is applied to the processing gas. However, the pressurized gas for backwashing has been heated in advance by the heating means provided in the backwashing means, so even if the temperature drops due to adiabatic expansion, it does not drop to such a low temperature. The acting cooling action is extremely small as compared with the conventional case in which back washing is performed with pressurized air at room temperature without heating. Therefore, even if the temperature of the gas to be treated including SO _x is lowered and supplied to the powder separation treatment tank,
It is possible to avoid a situation in which the temperature of the gas to be treated around the filter element drops below the moisture dew point temperature due to the cooling effect caused by the adiabatic expansion of the pressurized gas for backwashing, thereby preventing the generation of sulfuric acid.

[0014]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to the illustrated embodiments. FIGS. 1 to 3 show an embodiment of a powder separation apparatus for performing a powder separation method according to the present invention. FIG. 1 is a schematic configuration diagram of a powder separation apparatus 1 of one embodiment.
FIG. 2 is an enlarged view of a portion A in the powder separating apparatus 1 shown in FIG. 1, and FIG. 3 is a cross-sectional view taken along line BB of FIG. First,
After describing the configuration of the powder separation device 1 of one embodiment, a powder separation method of the present invention using the powder separation device 1 will be described.

The powder separating apparatus 1 of this embodiment is used for separating and recovering a specific component contained in an ore, and is connected to an ore pulverizing apparatus 3. This powdering apparatus 3 was heated and fired by the rotary kiln 4 for heating and firing the raw ore so as to be easily crushed, a feeder 5 such as a feeder for supplying the raw ore to the rotary kiln 4, and the rotary kiln 4. A crusher 7 for performing a predetermined crushing process on the ore, and receiving the ore particles crushed by the crusher 7 together with a gas such as a combustion gas generated in the rotary kiln 4, and a coarse particle having a predetermined size or more and a coarse particle having a predetermined size or less And a classifier 8 for collecting coarse particles by separating the fine particles into powdery fine particles. The powdery fine particles that cannot be collected by the classifier 8 are collected together with a gas such as a combustion gas of the rotary kiln 4. It is sent to the powder separation device 1. That is, the powdering device 3
In the case of this embodiment, also functions as a gas-to-be-processed supply means for feeding a gas-to-be-processed containing powder to the powder separating apparatus 1.

The powder separating apparatus 1 according to one embodiment of the present invention
The gas supply port 1 is located between the gas supply port 11 and the gas discharge port 12.
A powder separation processing tank 14 equipped with a filter element 13 for collecting powder contained in the gas to be treated supplied from 1;
A powdering device 3 serving as a gas supply means to be supplied to the gas supply port 11 of the above, and the gas to be treated supplied from the powdering device 3 into the powder separation processing tank 14 is filtered by the filter element 13. Exhaust means 16 for sucking gas from the gas discharge port 12 side so as to flow to the gas discharge port 12, and a clean pressurized gas periodically flowing back to the filter element 13 to capture the gas on the outer surface of the filter element 13. A back-washing means for recovering the filtration ability of the filter element by removing the collected powder to a powder recovery unit (hopper) provided at the inner bottom of the powder separation processing tank; The powder in the gas supplied from the powdering device 3 is separated and collected by the powder collecting unit 17 at the inner bottom of the powder separating tank 14.

As shown in FIGS. 2 and 3, the powder separation processing tank 14 of the powder separating apparatus 1 according to the embodiment includes a substantially cylindrical casing 20 having an axis directed vertically, And a separation gas discharge portion 21 occupying the upper end position of the casing 20.
At the same time, the opening 21 a formed in the separation gas discharge part 21 becomes the gas discharge port 12. Each filter element 13 is attached to the separation gas discharge unit 21 so as to close each opening 21a.

In this embodiment, each filter element 13 is formed by a filtering material into a flat box shape with one opening, so that the internal space 13 a communicates with the flow path in the separation gas discharge portion 21. The opening 13b on one end side of the box is connected and fixed to the separation gas discharge unit 21 with the opening 13b facing the opening 21a of the separation gas discharge unit 21. A sealing material such as a packing is attached to the mutual butting surfaces of the filter element 13 and the separation gas discharge part 21 as necessary to prevent a gap from being formed at the butting part.

That is, the powder separation treatment tank 1 of this embodiment
In FIG. 4, as shown in FIG. 3, the plurality of filter elements 13 are arranged so as to hang from the openings 21 a of the separation gas discharge units 21 with the openings 13 b facing upward. Although only one powder separation processing tank 14 of this embodiment is used, a configuration in which the powder separation processing tank 14 is stacked vertically may be employed.
Only the casing 20 of No. 4 is provided with the above-mentioned powder recovery unit 17.

As shown in FIG. 1, the exhaust means 16 includes an exhaust main pipe 23 into which the separated gas discharge sections 21 inserted into the respective powder separation processing tanks 14 join, the exhaust main pipe 23 and the separated gas discharge sections. A fan 24 for applying a suction force to each filter element 13 through the filter element 21 to discharge the filtered gas that has passed through the filter element 13;
And a scrubber 25 that performs a final treatment of the gas discharged from the scrubber 25. The gas treated by the scrubber 25 is discharged to the atmosphere from an exhaust pipe 25a mounted on the scrubber 25.

As shown in FIGS. 1 to 3, the back washing means 18 includes a compressor 27 for pressurizing clean air to a predetermined pressure.
An air dryer 28 for drying air pressurized by the compressor 27;
Heating means (heater) 29 for raising the temperature of the dry pressurized air that has passed through to the desired temperature, a header tank 30 that stores the dry pressurized air at a predetermined temperature that has passed through the heating means 29, and a dry pressurization in the header tank 30. A cleaning gas main pipe 31 for guiding air to the nearest position of the powder separation processing tank 14;
A backwash gas supply pipe 33 for injecting cleaning dry pressurized air supplied from the main pipe 31 for cleaning gas into the internal space 13a and an opening and closing of each backwash gas supply pipe 33. The electromagnetic valve 34 for controlling is provided.

The solenoid valve 34 is controlled by a control device (not shown).
The opening and closing of the solenoid valve 34 is controlled at a fixed cycle, and the pulsed air cleaning in which the dry pressurized air flows back to the filter element 13 at a fixed cycle is realized by the opening and closing control of the electromagnetic valve 34. An injection nozzle for blowing dry pressurized air to the filter element 13 is provided on an outer peripheral portion of the backwash gas supply pipe 33 facing the internal space 13a of the filter element 13. Further, the heating means 29 and the header tank 3
0, the header tank 30, the cleaning gas main pipe 31, and the like are provided with a heat retaining measure so that the temperature of the cleaning air heated by the heating means 29 does not drop carelessly. You.

In the powder separation method according to the present invention, the powder in the gas to be processed, which is supplied from the powdering apparatus 3, is converted into The powder is separated and collected in the powder collecting section 17 at the inner bottom. Specifically, the exhaust unit 16
Is operated, the gas to be treated supplied from the powdering device 3 to each of the powder separation processing tanks 14 is sucked into the filter element 13, and the powder contained in the gas to be treated is removed by the filter element. Collect at 13. The gas to be treated, which has been subjected to the powder separation and taken into the filter element 13, is sent from the main exhaust pipe 23 to the scrubber 25 and released to the atmosphere.

The powder collected in each filter element 13 by the operation of the exhaust means 16 is operated by the back washing means 18 so that the clean and dry pressurized air flows back to the filter element 13 by the back washing method. The powder is removed from the outer surface of the filter element 13 and collected in a powder recovery unit 17 provided at the inner bottom of the lowermost powder separation tank 14. Here, in the present invention, the pressurized gas for backwashing sent to the filter element 13 by the backwashing means 18 is the SO _{x in the} gas to be treated fed from the pulverizing device 3 to the powder separation treatment tank 14. The heating means 29 raises the temperature to a predetermined temperature in advance to such an extent that water and moisture do not condense on the surface of the filter element 13 due to the adiabatic expansion of the pressurized gas for back washing flowing backward through the filter element 13. To the filter element 13.

For example, in the back washing means 18, the dried pressurized air output from the air dryer 28 is usually at room temperature or lower, and conventionally, the pressurized air at room temperature or lower is stored in the header tank 30 as it is to be cleaned. Sometimes, the filter element 13 is jetted at a stroke. However, in the case of this embodiment, the dried pressurized air output from the air dryer 28 is maintained such that the temperature of the pressurized air stored in the header tank 30 is maintained at about 50 to 100 ° C. The heating means 29 heats and raises the temperature. Although it depends on the flow rate from the air dryer 28, for example, the temperature of the pressurized air stored in the header tank 30 is set to 50 to 1
In order to maintain the temperature at about 00 ° C., the heating means 29 is set to an appropriate temperature higher than the air temperature in the header tank 30 (for example,
(120 to 130 ° C.), and heat the pressurized air.

According to the powder separation method described above, when the filter element 13 is backwashed with pressurized air, the temperature of the pressurized air flowing backward through the filter element 13 drops due to rapid adiabatic expansion, and the pressure of the air to be treated is reduced. Acts to cool the gas. However, the pressurized gas for backwashing is
Since the temperature is raised by the heating means 29 provided in the back washing means 18, even if the temperature drops due to adiabatic expansion, the temperature does not drop to such a low temperature, and the cooling action acting on the gas to be treated is at room temperature without heating. It is extremely small as compared with the conventional case where backwashing is performed with pressurized air.

Therefore, even if the temperature of the gas to be treated including SO _x is reduced and supplied to the powder separation treatment tank 14, the filter element 1 is cooled by the adiabatic expansion of the pressurized gas for backwashing.
The sulfuric acid can be prevented from being generated when the temperature of the gas to be treated in the vicinity of 3 falls below the dew point temperature.

[0028]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS According to an experiment conducted by the inventor of the present invention, pressurized air for backwashing was blown to the filter element 13 at room temperature or lower. If the temperature of the gas to be treated is not set to 140 ° C. or more, which is the acid dew point, SO _x and moisture in the gas to be treated condense on the surface of the filter element 13 to generate sulfuric acid by cooling at the time of back washing. , Filter element 13
In this case, there was a problem of corrosion of the surrounding members, and a problem of clogging of the filter that could not be removed by back washing due to the powder mixed with the condensed sulfuric acid, so that long-term continuous operation could not be performed.

However, as described above, when the pressurized air for back washing is blown to the filter element 13 in a state where the temperature is raised, the powder separating apparatus 14
When the temperature of the gas to be supplied to the filter is in the range of 80 to 100 ° C., the inconvenience that SO _x and moisture in the gas to be processed are condensed on the surface of the filter element 13 does not occur, and the temperature is good. In addition, the washing process could be completed smoothly. The above gas to be treated in the experiment, water content 27vol.%, The concentration of the SO _x were of 200 ppm.

That is, the pressurized air for back washing is 50 to 100.
When the temperature of the gas to be processed supplied from the pulverizing device 3 to the powder separation processing tank 14 is raised from 140 ° C. to 80 to 100
To 60 ° C., the heating temperature in the rotary kiln 4 etc. of the powdering device 3 can be reduced to save fuel used for heating the gas to be treated. As a result, the cost of the apparatus can be reduced by lowering the level of heat resistance of the components such as the pipeline and the packing for supplying the gas to be treated to the powder separation tank 14 at the same time. Further, at the time of the back washing, the powder does not adhere to the filter surface due to the generation of sulfuric acid or the like, and the back washing with the pressurized gas removes the powder adhering to the outer surface of the filter element 13. It can be done reliably.

According to the experiment of the present inventor, the powdering device 3
The gas to be processed supplied to the powder separation processing tank 14 from the gas has a powder concentration of 50 to 100 g / m ³ N, a water concentration of 25 to 30 vol.% (Dew point temperature of 66 to 70 ° C.), The gas temperature when flowing into the powder separation processing tank 14 is 80 to 100 °
When the pressure fluctuates within the range of C, when pressurized air for back washing is blown to the filter element 13 at a temperature equal to or lower than room temperature, the back washing causes clogging of the filter due to dew condensation and powder adhesion. The pressure loss due to the filter gradually increases from the initial value of 100 mmH ₂ O, and after one month of continuous operation, exceeds 220 mmH ₂ O, and the load exceeds the suction capacity of the exhaust means 16 provided in the powder separation apparatus 1. Driving became impossible.

However, as described above, when the temperature of the pressurized air for backwashing is previously raised to 70 to 80 ° C. and the heated pressurized air is injected into the filter element 13, Means that the pressure loss due to the filter is initially 100 mm
Even if it rises from H ₂ O, it stops at about 150 mmH ₂ O,
Even after one month of continuous operation, the pressure loss due to the filter did not increase further, and it was confirmed that stable continuous operation was possible for a long period of several months or more.

In the above-described embodiment, the temperature of the pressurized air for back washing stored in the header tank 30 is 50 to 50.
Although the heat treatment by the heating means 29 is controlled so as to be maintained at about 100 ° C., preferably, the temperature of the pressurized air in the header tank 30 is adjusted to the moisture dew point temperature (67 77 ° C. higher than 10 ° C.
It is preferable to keep the temperature in the range of ° C to 87 ° C. According to such a temperature setting, a sufficient effect can be exerted and reliability is improved even if a temperature loss when flowing through the main pipe 31 for the cleaning gas or the gas supply pipe 33 for the backwash is expected.

The pressurized gas used for backwashing the filter is pressurized air in the above embodiment, but an inert gas may be used depending on the composition of the gas to be treated. Further, in the back washing means 18 of the above-described embodiment, the heating means 29 is provided between the header tank 30 and the air dryer 28, but is provided on the main pipe 31 for the washing gas connected to the header tank 30. You may.

The heating means 2 used for the exhaust means 16
As 9, it is also possible to use a gas burner or the like that directly heats the conduit of the main pipe 31 for the cleaning gas, in addition to an electric heater or the like.

Further, the powder separation apparatus and the powder separation method of the present invention can be used in various ways other than the separation and recovery of powder during the ore pulverization process. For example, it goes without saying that the present invention can also be applied to a dust collector and a dust collecting method for separating and collecting dust from exhaust gas containing dust. Further, the structure of the filter element 13 used in the powder separation device according to the present invention, the arrangement of the filter element 13 in the powder separation processing tank 14, and the like are not limited to the above-described embodiment.

[0037]

According to the powder separating apparatus and the powder separating method of the present invention, the pressurized gas for back washing is heated by the heating means provided in advance in the back washing means. Even if the temperature drops, the relative temperature drop with respect to the non-process gas is small, and the cooling action acting on the gas to be processed is extremely small as compared with the conventional case where back washing is performed with pressurized air at room temperature without heating. Therefore, even if the temperature of the gas to be treated including SO _x is lowered and supplied to the powder separation treatment tank, the temperature of the gas to be treated around the filter element is reduced by the cooling effect accompanying the adiabatic expansion of the pressurized gas for backwashing. Can prevent sulfuric acid from being generated below the dew point temperature, that is, by backwashing with a pressurized gas, it is possible to reliably remove powder adhering to the outer surface of the filter element, The filtering performance of the filter element can be maintained well over a long period. In addition, by lowering the temperature of the gas to be treated supplied to the powder separation treatment tank, the operating cost can be reduced by saving the fuel used for heating the gas to be treated, and at the same time, the gas to be treated is reduced by the powder separation treatment tank. The apparatus cost can be reduced by lowering the level of heat resistance of the components such as the pipelines to be fed to the apparatus.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram showing one embodiment of a powder separation device that performs a powder separation method according to the present invention.

FIG. 2 is an enlarged view of the powder separation device shown in FIG.

FIG. 3 is a sectional view taken along line BB of FIG. 2;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Powder separation device 3 Powdering device (processing target gas supply means) 4 Rotary kiln 5 Transport means 7 Crusher 8 Classifier 11 Gas supply port 12 Gas discharge port 13 Filter element 14 Powder separation processing tank 16 Exhaust means 17 Powder recovery unit 18 Backwashing means 20 Casing 21 Separated gas discharge unit 23 Main exhaust pipe 24 Fan 25 Scrubber 27 Compressor 28 Air dryer 29 Heating means 30 Header tank 31 Main pipe for cleaning gas 33 Gas supply pipe for backwashing

Claims (2)

[Claims] 1. A powder separation tank equipped with a filter element between a gas supply port and a gas discharge port for collecting powder contained in gas supplied from the gas supply port, Gas supply means for sending a gas containing a substance to a gas supply port of the powder separation processing tank; and gas supplied from the gas supply means into the powder separation processing tank by the filter element. Exhaust means for sucking gas from the gas outlet side so that the gas flows through the gas outlet, and powder in the gas supplied by the gas supply means to be processed is supplied to the powder separation processing tank. A powder separation device for separating and collecting the powder at the inner bottom portion of the filter element, wherein a clean pressurized gas is periodically flowed back to the filter element to remove powder adhering to the outer surface of the filter element. Equipped at the inner bottom of the powder separation processing tank Backwash means for recovering the filtration ability of the filter element by dropping it to a powder recovery unit; and heating means capable of adjusting the temperature of the pressurized gas flowing back through the filter element to a desired temperature. A powder separator characterized by being equipped. 2. A powder recovery unit using the powder separation device according to claim 1, wherein the powder in the gas supplied by the processing target gas supply unit is provided at an inner bottom of the powder separation processing tank. A powder separation method for separating and collecting the pressurized gas sent to the filter element by the back-washing unit, in a state where the pressurized gas is heated to a predetermined temperature by the heating unit in advance, and is sent into the powder separation processing tank. A powder separation method comprising: setting a temperature near a filter element of a gas containing powder to be equal to or higher than a dew point.