JPH09192447A - Gas purifier - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively reduce harmful materials to prevent air pollution by mixing liquid in a case with gas to be purified containing harmful materials to form a gas-liquid mixture and separating the harmful material containing liquid and the gas in which a harmful material content is reduced. SOLUTION: Gas to be treated G is caused to flow into an upper cylindrical body 9 with strong force from an annular slot part A between a lower cylindrical body 8 and the upper cylindrical body 9, and at the same time, a proper quantity of liquid S is jetted into the upper cylindrical body 9 from inside the lower cylindrical body 8. The gas G and the liquid S are severely agitated by plural brushes 25 to form a gas-liquid mixture M, and besides, since the lower cylindrical body 8 and the upper cylindrical body 9 are vertical, the liquid S is uniformly jetted, not channeling on the inner surface side of the upper cylindrical body 9, and the gas G surrounds the liquid S. After that, the liquid S is separated from the gas G by a mixing impingement wall body 28 of downward open receiving type with which an upper end 23 of the upper cylindrical body 9 is covered to release the gas G1 from which harmful materials have been removed to the utmost in the air.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gas purification device for purifying a gas containing a harmful substance.

[0002]

2. Description of the Related Art An apparatus for purifying a gas to be purified (for example, an exhaust gas from an automobile or a factory) containing a harmful substance has hitherto been one that uses a filter, a catalyst or the like, or a shower. There have been ones in which the exhaust gas is passed through the shower, and further, there are ones in which the water is blown off by a rotating stirring member to mix the exhaust gas with the water.

[0003]

However, in the case of using a filter or a catalyst, if the amount of exhaust gas exceeds a certain limit, so-called "clogging" occurs, it becomes difficult to purify, and a shower is used. However, the mixing ratio of the exhaust gas and the liquid of the shower is poor, the apparatus becomes large in size, and the equipment cost becomes large, so that it cannot be used for a limited space such as an automobile.

Further, in the case of using a rotating agitator, the moisture is biased to the peripheral wall by the action of centrifugal separation, the exhaust gas and the moisture are less in contact with each other, and the harmful substances in the exhaust gas are efficiently dissolved in the moisture. I couldn't do it.

That is, it is difficult for the conventional device of this type to efficiently reduce or remove a large amount of harmful substances in the combustion exhaust gas in a limited space.
Hazardous substances spread all over the earth, polluting the atmosphere, causing pollution to humans and animals, causing acid rain on the ground, and damaging plants such as forests.

Therefore, it is an object of the present invention to provide a gas purification apparatus that is small in size and can efficiently reduce or remove harmful substances from the gas to be purified to prevent air pollution.

[0007]

In order to achieve the above-mentioned object, a gas purifying apparatus according to the present invention has a case in which a liquid in which a harmful substance is dissolved is stored, and a harmful substance-containing substance to be purified contained in the case. A mixing mechanism for forming a gas-liquid mixture of the gas for use and the liquid in the case, and a harmful substance-containing liquid in which the harmful substance is dissolved in the gas-liquid mixture formed by the mixing mechanism and reduction or removal of the harmful substance And a separating mechanism for separating the separated gas into a separated gas, wherein the case has a vertically long shape, and the mixing mechanism has a lower part immersed in a liquid and an upper part protruding from the upper surface of the liquid. A vertical lower cylinder body, and an upper cylinder body that is externally fitted so as to form an annular space on the outer peripheral side of the upper end of the lower cylinder body and that forms a predetermined gap with the liquid surface of the liquid. It is an inverted bowl type having an upper wall and a peripheral wall, and An intermediate cylinder body in which the wall is fixed and the lower end of the peripheral wall is immersed in a liquid to form a hollow portion with the upper cylinder body, and a gas pipe for feeding the gas to be purified into the intermediate cylinder body, The gas to be purified provided in the lower portion of the lower cylinder and supplied to the hollow portion of the intermediate cylinder is mixed with the liquid ejected from the lower cylinder in the upper cylinder to form a gas-liquid mixture. And a drive source arranged as described above, and the separation mechanism is configured by a lower opening receiving type mixture collision wall body in which the upper end of the upper cylindrical body is covered at a predetermined interval.

Also, a plurality of mixing mechanisms and a plurality of separating mechanisms corresponding to the mixing mechanisms are provided in the case.

Further, a case in which a liquid in which a harmful substance is dissolved is stored, a mixing mechanism for forming a gas-liquid mixture of the harmful substance-containing gas to be purified contained in the case and the liquid in the case, In a gas purification device comprising a separation mechanism for separating a gas-liquid mixture formed by the mixing mechanism into a harmful substance-containing liquid in which a harmful substance is dissolved and a gas in which the harmful substance is reduced or removed, A plurality of mixing mechanisms and a plurality of separating mechanisms corresponding to the mixing mechanisms are provided in a case, and the cases have a horizontally long shape, and the mixing mechanism is configured such that the lower part is immersed in a liquid. A vertical lower cylindrical body whose upper portion projects from the liquid surface of the liquid, and a predetermined vertical distance between the lower cylindrical body and the liquid surface of the liquid, which are externally fitted so as to form an annular space on the outer peripheral side of the upper end of the lower cylindrical body. An upper cylinder that forms a gap, and a top and bottom wall An intermediate cylindrical body in which the upper wall is fixed to the upper cylindrical bodies in a bowl shape and arranged in parallel, and the lower end of the peripheral wall is immersed in a liquid to form a hollow portion between the upper cylindrical body and the upper cylindrical body; The liquid to be purified is jetted upward from the lower cylinder through the liquid supply pipes respectively arranged in the lower part of the lower cylinder, and the gas to be purified is sent from the gas pipe to the hollow portion and the upper cylinder. And a driving source with a screw for mixing, the separation mechanism,
This is configured by a lower opening receiving type mixture collision wall body in which the upper end of the upper cylindrical body is covered with a predetermined gap.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail with reference to the drawings showing embodiments.

FIG. 1 shows an embodiment of a gas purifying apparatus according to the present invention. This apparatus has a case 1 in which a liquid S in which a harmful substance is dissolved is stored, and a harmful substance-containing cover contained in the case 1. A mixing mechanism 2 that forms a gas-liquid mixture M of the cleaning gas G and the liquid S in the case 1, and a harmful substance-containing liquid in which a harmful substance is dissolved in the gas-liquid mixture M and a harmful substance are reduced or removed. A separation mechanism 3 for separating the separated gas G ₁ ;
Is provided. Here, the harmful substances in the gas to be purified G are sulfur dioxide SO ₂ , carbon monoxide CO, hydrocarbons HC, nitrogen oxides NO _x , and the like, and the gas to be purified G is exhaust gas from an automobile. Gas, exhaust gas from various combustors, etc.

The case 1 has a vertically long shape (cylindrical type or rectangular tube type) having an upper wall 4, a peripheral wall 5 and a bottom wall 6, and a gas exhaust port 7 is formed in the upper wall 4. The liquid S is stored in the lower part of the case 1, and the liquid S is
Effective to dissolve the harmful substances, such as water,
It is a neutralizing agent for alkaline liquids.

The mixing mechanism 2 includes a lower cylinder body 8, an upper cylinder body 9 and
The lower cylinder 8 is provided with the intermediate cylinder 10 and the drive source 21.
Has a tapered portion 11 in the vicinity of the middle thereof, and is a vertical cylindrical type in which the lower portion has a larger diameter than the upper portion with the tapered portion 11 as a boundary. The lower portion of the lower cylindrical body 8 is immersed in the liquid S so that the lower end 13 is located near the bottom wall 6 of the case 1 and the upper portion (upper end 14) is projected from the liquid surface 12.

The upper cylinder 9 is a vertical cylinder type, and the lower cylinder 8 is
Are arranged on the same axis. Specifically, the upper cylindrical body 9
The inner diameter of is larger than the outer diameter of the upper end 14 of the lower tubular body 8, and its lower end 15 is fitted onto the outer peripheral side of the upper end 14 of the lower tubular body 8 so as to form an annular space A. A predetermined gap B is formed between the lower end 15 and the liquid surface 12.

The intermediate cylindrical body 10 is an inverted bowl type having an upper wall 16 and a peripheral wall 17, the upper wall 16 is fixed to the upper cylindrical body 9, and the lower end 18 of the peripheral wall 17 is the liquid S. It is immersed inside. As a result, a hollow portion 19 is formed between the intermediate cylinder body 10, the liquid surface 12, the upper cylinder body 9 and the lower cylinder body 8. Further, a gas pipe 20 for feeding the gas G to be purified is connected to the peripheral wall 17 of the intermediate cylinder 10 above the liquid surface 12 so as to penetrate the peripheral wall 5 of the case 1.

In the lower part of the lower cylindrical body 8, the lower cylindrical body 8 is
A drive source 21 for ejecting the liquid S therein is provided. The drive source 21 has a shaft 22 that has the same axis as the lower cylinder body 8 and the upper cylinder body 9 and that is rotated by its drive. The shaft 22 has a length from the position of the lower end 13 of the lower tubular body 8 to slightly above the upper end 23 of the upper tubular body 9, and at the lower end portion thereof, the shaft 22 rotates with the rotation of the liquid. A propeller 24 for feeding S upward is fixed, and a plurality of brushes 25 are attached to the upper portion of the propeller 24 at predetermined intervals. As the drive source 21, a motor, a belt drive device from the outside, a gear drive device, or an air drive device is used.

The propeller 24 rotates with the rotation of the shaft 22, and the liquid S is supplied from the lower opening 26 of the lower cylinder 8 to the lower cylinder 8.
It is for feeding the inside upward and reliably ejecting it from the upper opening 27 into the upper cylindrical body 9. Also, gas pipe 20
The gas to be purified G that has entered the upper cylinder 9 through the hollow portion 19 of the intermediate cylinder 10 and the annular space A and the liquid S that has been ejected into the upper cylinder 9 is in the upper cylinder 9. The gas-liquid mixture M is formed by mixing with the above, and the brushes 25 that rotate with the rotation of the shaft 22 stir the gas-liquid mixture M to mix them more uniformly and reliably. The shaft 22 is rotatably supported by a bearing (not shown). Also, the bearing is liquid S
And the flow of the gas-liquid mixture M is not disturbed.

Next, the separating mechanism 3 has a lower opening receiving type mixture collision wall body 28 such as an inverted hemispherical bowl type or an inverted dish type, and this wall body 28 connects the upper end 23 of the upper cylinder body 9. It is arranged so as to cover at a predetermined interval, that is, the upper end 23 is inserted into the concave portion of the wall body 28. The wall 28 is held by the upper cylinder 9 by a holding frame (not shown).

Although the gas-liquid mixture M formed in the upper cylinder 9 is discharged from the upper opening of the upper cylinder 9, the rotating brush 25 is provided at the tip of the shaft 22. Radially diffused. The diffused gas-liquid mixture M is
Colliding with the concave curved surface 29 of the wall 28 of the separating mechanism 3 (crash),
It is again separated into gas and liquid. At this time, the harmful substance of the gas to be purified G is dissolved in the liquid, and the gas G ₁ does not contain the harmful substance. Then, the purified gas G ₁ is discharged to the outside from the gas exhaust port 7 of the case 1, and the harmful substance-containing liquid is mixed in the liquid S below the case 1.

According to the gas purifying apparatus of the present invention configured as described above, however, the gas G to be purified enters the upper cylindrical body 9 from the annular space A between the lower cylindrical body 8 and the upper cylindrical body 9. At the same time that the liquid G flows vigorously, an appropriate amount of the liquid S is ejected from the lower cylindrical body 8 into the upper cylindrical body 9 by the driving of the driving source 21, and the plurality of brushes 25 vigorously agitate the gas G and the liquid S. As a result, a gas-liquid mixture M in which the gas G and the liquid S are uniformly mixed is generated, and since the lower cylinder 8 and the upper cylinder 9 are provided vertically, the liquid S is The gas G is jetted uniformly to the inner surface side of the tubular body 9 without deviation, and the gas G is sent so as to surround the jetted liquid S.
The contact rate between the liquid S and the liquid S becomes large. Therefore, it is effective in dissolving the harmful substance of the gas to be purified G in the liquid S, and the clean purified gas G ₁ in which the harmful substance is removed as much as possible can be discharged to the outside.

Since the liquid S in the case 1 is gradually reduced in water content by evaporation, a water level maintaining means 31 having an auxiliary tank 30 for the liquid S is connected to the lower part of the case 1 so as to communicate with the inside of the case 1. The water level of the liquid S is kept constant. Also,
Due to long-term use, the liquid S contains a large amount of harmful substances,
Since the purification efficiency of the liquid S decreases, a discharge port 32 for regularly exchanging the liquid S and extracting a precipitate is provided near the bottom wall 6 of the case 1. Although not shown, it is also preferable that the liquid 1 is circulated by a circulation mechanism and filtered by a filter or the like.

Further, it is also preferable to connect a plurality of these devices in series. That is, if the gas discharge port 7 of one device and the gas pipe 20 of another device are connected to each other, the gas from which harmful substances have been reduced or removed by one device can be supplied to another device. With this other device, when a harmful substance remains in the gas, the harmful substance can be further reduced or removed.

Next, FIG. 2 shows another embodiment of the gas purifying apparatus of the present invention. This device comprises a mixing mechanism 2 of the gas purifying device (described above in FIG. 1) and this mixing mechanism 2.
And a plurality of separating mechanisms 3 corresponding to the above are provided in the case 1.

That is, the mixing mechanism 2 (the lower cylinder body 8, the upper cylinder body 9, the intermediate cylinder body 10, the gas pipe 20 and the drive source 21) having the mixture collision wall 28 of the separation mechanism 3 on the upper side is provided. ) Are connected side by side, and specifically, adjacent intermediate cylinders 10 (of the mixing mechanisms 2 and 2), upper walls 16 and 16 to peripheral walls 17,
The 17 are connected to each other by a connecting member 33 at predetermined intervals.
Further, the gas pipe 20 is branched into a plurality of pipes, and each of the intermediate cylinders 10 ...
It is connected to the upper wall 16 ... In addition, also in the case of this device, the water level maintaining means 31 and the discharge port 32 are provided.

Therefore, according to this gas purifying apparatus, the efficiency of reducing or removing harmful substances of the gas G to be purified becomes much more excellent. That is, a large amount of gas G can be turned into clean gas G ₁ in a short time.

FIG. 3 shows another embodiment. In this apparatus, a plurality of mixing mechanisms 2 and a plurality of separating mechanisms 3 corresponding to the mixing mechanisms 2 are provided in a horizontally long case 1. The plurality of mixing mechanisms 2 ... Injects the liquid S upwards from the inside of each lower cylinder 8 via the liquid feed pipes 37 provided at the bottom of each lower cylinder 8. A drive source 34 with a screw 36 for causing the operation is provided. This drive source 34
For this, a motor, an external belt driving device, a gear driving device, or an air driving device is used.

To be more specific, the adjoining mixing mechanisms 2 and 2 having the mixture collision wall 28 of the separating mechanism 3 on the upper side.
Is the upper wall 16, 16 to the peripheral walls 17, 17 of the intermediate tubular body 10, 10.
Are connected to each other with a connecting member 33 at predetermined intervals,
The gas pipe 20 is branched into a plurality of pipes, and the upper wall of each intermediate cylinder 10 ...
It is connected to 16 ... Further, one end of each of the plurality of liquid-introducing pipes 37 is disposed under each lower cylindrical body 8 ... And the other ends are bound to form one cylindrical body 38. Further, a drive source 34 for sending the liquid S from the inside of the tubular body 38 through the pipes 37 to each of the lower tubular bodies 8 is provided on the outer side of the peripheral wall 5 of the case 1 and has a screw 36 at the tip thereof. The shaft 35 of the fixed drive source 34 penetrates the peripheral wall 5 of the case 1 and is inserted into the cylindrical body 38.

In the case of this apparatus, the shaft 22 provided at the axial center positions of the lower cylinder body 8 and the upper cylinder body 9 is pivotally supported by a bearing (not shown). It may be rotatably and pivotally attached to the wall 28.

When the shaft 35 and the screw 36 are rotated by the drive of the drive source 34 such as a motor, the liquid S flows from the cylinder 38 into the pipes 37 and is sent into the lower cylinders 8. And is ejected into the upper cylindrical body 9 ... At the same time, the propellant 24 receives a rotational force by the flowing liquid S to rotate the shaft 22, and the brushes 25 rotate. Then, the gas-liquid mixture M is generated and the gas to be purified G is efficiently purified.

By arranging the pipe 37 under each lower cylinder 8, the lower cylinder 8 can be shortened and the height H of the case 1 can be made small, and the motor etc. The liquid S can be sent to each of the lower cylinders 8 ...

Further, as shown in FIG. 4, the shaft 35 having the drive source 34 and the screw 36 may be provided on the outside of the bottom wall 6 of the case 1 and in the center of the juxtaposed lower cylinders 8 ... . Further, it is also preferable that the intermediate cylinder 10 having the upper wall 16 fixed to a plurality of juxtaposed upper cylinders 9 is integrated into one to enlarge the hollow portion 19 into which the gas G to be purified is sucked. By doing so, the intermediate pipe without branching the gas pipe 20.
It can be connected to only one place of 10.

The amount of the liquid S ejected into the upper cylinder 9 is adjusted by adjusting the rotational drive (rotation speed) of the drive sources 21 and 34. Further, the gas purifying apparatus of the present invention is not limited to the above-mentioned embodiment, and for example, the drive sources 21, 34 are
It may be configured by taking out from a rotary shaft or a rotary gear that is rotated by the power of an automobile, a ship, various combustors, or the like via a belt gear or the like, and the design can be freely changed without departing from the scope of the present invention. Further, as for the wall body 28 for collision with the mixture, if the lower opening receiving type is used, various design changes can be freely made.

[0033]

Since the present invention is configured as described above, the following effects can be obtained.

(According to the gas purifying apparatus of claim 1)
Since the entire device is a compact vertical type, it does not require a large installation space.

(According to the gas purifying apparatus of the first or third aspect), when the gas G to be purified flows into the upper cylindrical body 9 vigorously from the annular space A between the lower cylindrical body 8 and the upper cylindrical body 9. At the same time, a proper amount of the liquid S is ejected from the lower cylindrical body 8 into the upper cylindrical body 9, and the plurality of brushes 25 vigorously agitate the gas G and the liquid S, so that the gas G and the liquid S are uniform. In addition to the generation of the gas-liquid mixture M mixed with the above, since the lower cylinder body 8 and the upper cylinder body 9 are provided vertically, the liquid S is evenly distributed on the inner surface side of the upper cylinder body 9. Is ejected to
Since the gas G is sent so as to surround the ejected liquid S, the contact rate between the gas G and the liquid S becomes large. Therefore, it is effective in dissolving the harmful substance of the gas to be purified G in the liquid S, and the clean gas G ₁ from which the harmful substance is removed as much as possible can be discharged to the outside to prevent air pollution.

(According to the gas purifying apparatus of the second or third aspect), since the entire apparatus is not enlarged and the width is small, a large installation space is not required. For example, a driver's cab such as a truck or a trailer and a luggage compartment. It is effective to install in a narrow space. Further, it is possible to efficiently remove harmful substances in the gas to be purified G. That is, a large amount of gas G to be purified in a short time
Can be processed into a clean gas G ₁ .

(According to the gas purifying apparatus of claim 3)
Since the height dimension H (height of the entire apparatus) of the case 1 can be made small by shortening the length of the lower tubular body 8, it can be mounted in a narrow space under the floor of an automobile, for example.

[Brief description of the drawings]

FIG. 1 is a simplified cross-sectional view showing an embodiment of a gas purifying device of the present invention.

FIG. 2 is a simplified cross-sectional view showing another embodiment.

FIG. 3 is a simplified cross-sectional view showing another embodiment.

FIG. 4 is a simplified cross-sectional view showing a modified example.

[Explanation of symbols]

1 Case 2 Mixing mechanism 3 Separation mechanism 8 Lower cylinder 9 Upper cylinder 10 Intermediate cylinder 12 Liquid level 16 Upper wall 17 Surrounding wall 18 Lower end 19 Hollow part 20 Gas pipe 21 Drive source 23 Upper end 28 Mixer collision wall 34 Drive source 36 Screw A Annular void B Predetermined gap G Gas to be purified G ₁ Gas M Gas-liquid mixture S Liquid

 ─────────────────────────────────────────────────── ─── Continuation of the front page (71) Applicant 594043269 Fumiko Takahashi 3-1-44-909 Minatojima Nakamachi, Chuo-ku, Kobe-shi, Hyogo (71) Applicant 594043270 Takayoshi Ishimine 2577-8 Awawa, Misaki-cho, Sennan-gun, Osaka ( 72) Inventor Nobuo Suzuki 1971, Mikami, Misaki-cho, Sennan-gun, Osaka Prefecture 2

Claims (3)

[Claims] 1. A case 1 in which a liquid S in which a harmful substance is dissolved is stored, and a gas-liquid mixture M of a harmful substance-containing gas G to be purified and a liquid S in the case 1 stored in the case 1. Separation mechanism 3 for separating the mixing mechanism 2 to be formed and the gas-liquid mixture M formed by the mixing mechanism 2 into the harmful substance-containing liquid in which the harmful substance is dissolved and the gas G ₁ in which the harmful substance is reduced or removed. In the gas purifying apparatus including the above, the case 1 has a vertically long shape, and the mixing mechanism 2 has a vertical shape in which the lower part is immersed in the liquid S and the upper part projects from the liquid surface 12 of the liquid S. A lower cylinder 8 and an upper cylinder which is externally fitted to the outer peripheral side of an upper end 14 of the lower cylinder 8 so as to form an annular space A and forms a predetermined gap B between the liquid surface 12 of the liquid S. Body 9 and upper wall 16
An inverted bowl having a peripheral wall 17 and the upper wall 16 fixed to the upper cylindrical body 9 and the lower end 18 of the peripheral wall 17 is the liquid S.
The intermediate tubular body 10 which is immersed in the upper tubular body 9 to form a hollow portion 19 with the upper tubular body 9, the gas pipe 20 for feeding the gas to be purified G into the intermediate tubular body 10, and the lower tubular body 8 The gas to be purified G provided in the lower portion and supplied to the hollow portion 19 of the intermediate cylindrical body 10.
Is arranged in the upper cylinder 9 so as to form a gas-liquid mixture M by mixing with the liquid S ejected from the lower cylinder 8.
And the separating mechanism 3 has an upper end of the upper cylindrical body 9.
A gas purifying device comprising a lower opening receiving type mixture collision wall body (28) covering 23 at a predetermined interval. 2. The gas purifying apparatus according to claim 1, wherein a plurality of mixing mechanisms 2 and a plurality of separating mechanisms 3 corresponding to the mixing mechanisms 2 are provided in the case 1. 3. A case 1 in which a liquid S in which a harmful substance is dissolved is stored, and a gas-liquid mixture M of a gas G to be purified containing a harmful substance contained in the case 1 and a liquid S in the case 1. Separation mechanism 3 for separating the mixing mechanism 2 to be formed and the gas-liquid mixture M formed by the mixing mechanism 2 into the harmful substance-containing liquid in which the harmful substance is dissolved and the gas G ₁ in which the harmful substance is reduced or removed. And a mixing mechanism 2 and a separation mechanism 3 corresponding to the mixing mechanism 2,
A plurality of them are provided in the case 1, and the case 1 has a horizontally long shape, and the mixing mechanism 2 is a vertical structure in which the lower part is immersed in the liquid S and the upper part projects from the liquid surface 12 of the liquid S. A predetermined gap B is formed between the lower cylindrical body 8 and the upper surface 14 of the lower cylindrical body 8 which is fitted on the outer peripheral side of the lower cylindrical body 8 so as to form an annular space A and the liquid surface 12 of the liquid S. An upper cylinder body 9
The upper wall 16 is fixed to each of the upper cylindrical bodies 9 of the inverted bowl shape having the upper wall 16 and the peripheral wall 17 and arranged side by side, and the lower end 18 of the peripheral wall 17 is immersed in the liquid S to form the upper cylinder. The liquid S is moved upward from the lower tubular body 8 through the intermediate tubular body 10 that forms a hollow portion 19 between the body 9 and the liquid feeding pipe 37 that is disposed in the lower portion of the lower tubular body 8. The separation mechanism 3 includes a drive source 34 with a screw 36 that mixes the gas to be purified G that is jetted out and sent from the gas pipe 20 into the hollow portion 19 with the inside of the upper cylinder 9, and the separation mechanism 3 is an upper cylinder. A gas purifying device comprising a lower opening receiving type mixture collision wall body 28 in which an upper end 23 of the body 9 is covered with a predetermined gap.