CN212717399U

CN212717399U - Silencer of ventilator

Info

Publication number: CN212717399U
Application number: CN202020836344.7U
Authority: CN
Inventors: 刘媛; 徐贝贝; 王毅娜; 董佳欢; 王烨
Original assignee: 711th Research Institute of CSIC
Current assignee: 711th Research Institute of CSIC
Priority date: 2020-05-18
Filing date: 2020-05-18
Publication date: 2021-03-16
Anticipated expiration: 2030-05-18

Abstract

The utility model discloses a ventilation blower silencer for set up in the fan room, the fan room includes the ventilation blower, the shutter, roof and bottom plate, the shutter sets up on the lateral wall of keeping away from the ventilation blower in fan room, the ventilation blower silencer includes that at least three parallel interval sets up between ventilation blower and shutter and by the noise damping piece that the direction of ventilation blower to the shutter extends, the top of noise damping piece is connected with the roof, the bottom of noise damping piece is connected with the bottom plate, the thickness of every noise damping piece is all inequality, interval inequality between per two adjacent noise damping pieces. According to the silencer of the ventilator of the utility model, the silencing sheet is directly arranged between the top plate and the bottom plate of the fan room, which is convenient for installation and disassembly; the thickness of every noise reduction piece and the interval between every two adjacent noise reduction pieces all set up to inequality for the whole of ventilation blower silencer arranges the homogeneity that is favorable to improving fan room flow field distribution, guarantees that ventilation blower silencer has the characteristic of high noise reduction volume and low flow resistance concurrently.

Description

Silencer of ventilator

Technical Field

The utility model relates to a technical field of making an uproar falls in the boats and ships damping particularly, relates to a ventilation blower silencer.

Background

A large amount of air is needed when a diesel engine in a cabin of a ship burns, all mechanical equipment and electrical equipment in the cabin generate a large amount of heat when working, and in order to guarantee the normal operation of the equipment, mechanical ventilation equipment needing large air volume supplies and exhausts air through a cabin ventilation system. When a ship cabin is arranged, a fan room with a high-air-volume fan and a shutter is often arranged as a medium for exchanging with outside air. The large-air-volume cabin ventilator generates large vibration noise in the continuous operation process, and the large vibration noise is transmitted to each stage of deck of the superstructure through an air outlet, a shutter and the like. Noise tests on the fan room show that when the large-air-volume ventilator runs, noise in the fan room has medium-high frequency broadband characteristics and is accompanied by a low-frequency line spectrum corresponding to the axial frequency of the fan.

With the stricter noise control regulations and the increased awareness of environmental protection, the demand for noise is higher and higher. On one hand, the long-time vibration and radiation noise of mechanical equipment can cause fatigue and damage of a ship structure, and damage is caused to the hearing of a crew and even physical functions; on the other hand, for luxurious cruise ships and passenger rolling ships, the comfort requirement of the ships is far beyond that of common ships, and the experience of deck sightseeing personnel is directly influenced by the excessive noise of a fan room. For some engineering ships such as scientific research ships, since scientific research personnel and some instruments and equipment need to work on the deck, the vibration and noise of the equipment directly affect the normal work of the scientific research personnel and the accuracy and reliability of data collected by the scientific research instruments and equipment. Therefore, it is necessary to take effective control measures against the noise of the cabin fan room ventilation equipment during the design phase of the ship.

The adoption of the ventilator muffler is an effective measure for controlling the noise of the ventilator, and has wide application in cabin noise control. The structure type formula single-pole piece formula or doublestage piece formula silencer of present common use ventilator silencer as an organic whole, noise elimination piece thickness and interval are fixed, and noise elimination effect is relatively poor, need improve the noise elimination volume through the length that increases the silencer. However, after the fan room is built, the installation node of the ventilator silencer usually brings great challenges to installation due to the size limitations of the fan room space and the door or the louver, and even causes the situation that the ventilator silencer cannot be installed. Simultaneously, the large-size integral type silencer is mostly fixed installation, can't satisfy the demand that shutter or fan need dismouting silencer when overhauing.

To this end, it is desirable to provide a ventilator muffler that at least partially addresses the above-mentioned problems.

SUMMERY OF THE UTILITY MODEL

In the summary section a series of concepts in a simplified form is introduced, which will be described in further detail in the detailed description section. The summary of the invention is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used as an aid in determining the scope of the claimed subject matter.

In order to solve above-mentioned problem at least partially, the utility model discloses a ventilation blower silencer for set up in the fan room, the fan room includes ventilation blower, shutter, roof and bottom plate, the shutter sets up keeping away from in the fan room on the lateral wall of ventilation blower, its characterized in that, the ventilation blower silencer includes:

the fan comprises a ventilator, a louver and at least three silencing pieces, wherein the at least three silencing pieces are arranged between the ventilator and the louver at intervals in parallel and extend towards the direction of the louver by the ventilator, the tops of the silencing pieces are connected with a top plate, the bottoms of the silencing pieces are connected with a bottom plate, the thickness of each silencing piece is different, and the distance between every two adjacent silencing pieces is different.

According to the utility model, the silencing sheet of the silencer is directly arranged between the top plate and the bottom plate of the fan room, so that the silencer is convenient to mount and dismount, and the mounting problem of the large-size integrated silencer is solved; the thickness of every noise elimination piece all sets up to inequality, and the interval between per two adjacent noise elimination pieces also all sets up to inequality for the whole of ventilation blower silencer arranges the distribution that is favorable to improving flow field in the fan room, improves the homogeneity that the flow field distributes when guaranteeing ventilation blower silencer noise elimination effect, reduces the resistance of ventilation blower silencer.

Preferably, the silencer further comprises connecting square steels, wherein the connecting square steels are arranged at two ends of the silencing sheet along the length direction of the silencing sheet, are connected with the silencing sheet and are used for fixing the silencing sheet.

Therefore, the strength of the silencing sheet can be improved, and the stability and the reliability of the silencer are ensured.

Preferably, the muffler further comprises:

the first base is arranged on the top plate and is connected with the top of the connecting square steel;

and the second base is arranged on the bottom plate and is connected with the bottom of the connecting square steel.

From this, through the connection of connecting square steel and first base and second base, but convenient and fast ground installs the noise damping piece to between the roof and the bottom plate in fan room, reduces the installation degree of difficulty.

Preferably, the acoustic damping sheet comprises a resistive segment comprising a first perforated panel and a sound absorbing material disposed within the first perforated panel.

From this, be favorable to eliminating the medium-high frequency noise in the fan room through resistive segment.

Preferably, the silencing sheet further comprises a resonance section, the resonance section and the resistive section are adjacently arranged side by side, the resonance section comprises a second perforated panel and a partition plate arranged in the second perforated panel, and a resonance cavity is formed between the second perforated panel and the partition plate.

Therefore, the combined action of the resistive section and the resonance section is beneficial to eliminating the noise of medium-high frequency and low frequency in the fan room, so that the silencer of the ventilator achieves the silencing effect of full frequency band.

Preferably, the number of resistive segments and/or resonant segments is at least one.

Therefore, the number of the required resistive sections and/or resonant sections can be selected according to the noise spectrum characteristics of the fan, and the noise elimination effect of the silencer is guaranteed.

Preferably, at least one of the noise reduction sheets of the at least three noise reduction sheets includes a first noise reduction sheet and a second noise reduction sheet, the second noise reduction sheet being disposed above the first noise reduction sheet.

From this, can select the installation form of noise elimination piece according to the on-the-spot installation condition in fan room, be favorable to reducing the installation degree of difficulty.

Preferably, at least one of the noise-reduction sheets is constructed in one piece.

Preferably, the thickness T of the noise-reduction sheet ranges from: t is more than or equal to 50mm and less than or equal to 400 mm;

the range of the distance D between the silencing sheets is as follows: d is more than or equal to 50mm and less than or equal to 400 mm;

the length of the resistive segment, L1, ranges from: l1 is more than or equal to 400mm and less than or equal to 2000 mm;

the length L2 of the resonance section ranges from: l2 is more than or equal to 800mm and less than or equal to 1200 mm.

From this, through the thickness and the interval of reasonable setting noise elimination piece and the length of resistive section and resonance section, can improve the noise cancelling effect of silencer for the noise elimination piece can be applicable to the fan room of different arrangement forms and the fan of different noise characteristics.

Drawings

The following drawings of the embodiments of the present invention are provided as a part of the present invention for understanding the present invention. There are shown in the drawings, embodiments and descriptions thereof, which are used to explain the principles of the invention.

In the drawings:

fig. 1 is a schematic top view of a ventilator muffler according to an embodiment of the present invention; and

fig. 2 is a schematic side view of the silencer of the ventilator according to fig. 1.

Description of reference numerals:

100: the fan room 110: silencer of ventilator

111: the noise-reduction sheet 112: connecting square steel

113: first base 114: second base

120: top plate 130: base plate

123: first side wall 124: second side wall

125: first end wall 126: second end wall

127: reinforcing rib 128: connecting piece

141: resistive segment 142: first perforated panel

151: the resonance section 152: second perforated panel

153: partition plate 154: first resonant cavity

155: second resonant cavity 161: first silencing sheet

162: second noise cancellation sheet 170: ventilation fan

180: shutter

Detailed Description

In the following description, numerous specific details are set forth in order to provide a more thorough understanding of the present invention. It will be apparent, however, to one skilled in the art, that embodiments of the invention may be practiced without one or more of these specific details. In other instances, well-known features have not been described in order to avoid obscuring embodiments of the present invention.

In order to provide a thorough understanding of the present invention, a detailed description will be provided below to illustrate the ventilator silencer of the present invention. It will be apparent that the practice of the invention is not limited to the particular details which are within the skill of those in the ventilator muffler art. The following detailed description of the preferred embodiments of the invention, however, the invention is capable of other embodiments in addition to those detailed.

Ordinal words such as "first" and "second" are used in this application as a reference, and do not have any other meaning (e.g., a particular order, etc.). Also, for example, the term "first component" does not itself imply the presence of "second component", and the term "second component" does not itself imply the presence of "first component". The terms "upper", "lower", "front", "rear", "left", "right" and the like as used herein are for clarity of description only and are not limiting.

Hereinafter, specific embodiments of the present invention will be described in more detail with reference to the accompanying drawings, which illustrate representative embodiments of the present invention and do not limit the present invention.

Referring to fig. 1 and 2, the fan house 100 of the marine nacelle includes a first side wall 123, a second side wall 124 opposite to the first side wall 123, a first end wall 125, a second end wall 126 opposite to the first end wall 125, and a top plate 120 and a bottom plate 130 connected to the top and bottom of the two side walls and the two end walls, respectively. In order to improve the strength of the fan room 100, a reinforcing rib 127 is provided on the top plate 120 of the fan room 100.

The fan room 100 has a ventilator 170 mounted therein, and in the embodiment shown in fig. 1, the ventilator 170 is mounted on the fan room 100 near the first end wall 125 and the second side wall 124. On the side wall of the fan room 100 remote from the ventilator 170, i.e. on the first side wall 123, a louver 180 is mounted. The ventilator 170 in the present embodiment has a normal rotation mode and a reverse rotation mode, and the fan room 100 supplies and exhausts air to and from the ship system through the ventilator 170 and the louver 180.

A direction parallel to the first and

second sidewalls

123 and 124 is defined as a first direction D1 in the present embodiment, and a direction perpendicular to the first and

second sidewalls

123 and 124 is defined as a second direction D2. In order to maximally reduce noise of the fan room 100 when the ventilator 170 is operated, a ventilator silencer 110 is installed between the ventilator 170 and the louver 180. The ventilator muffler 110 includes at least three muffling fins 111 disposed in parallel and spaced apart between the ventilator 170 and the louver 180 in the first direction D1.

Referring to fig. 1, in the illustrated embodiment, the ventilator muffler 110 includes six muffling fins 111, and the six muffling fins 111 are disposed between the ventilator 170 and the louver 180 in parallel spaced relation along the first direction D1 and extend in the direction of the louver 180 (i.e., the second direction D2) by the ventilator 170. Referring to fig. 2, in order to facilitate the installation and removal of the ventilator silencer 110 and solve the installation problem of the large-sized integrated ventilator silencer, the top of the silencing sheet 111 is connected to the top plate 120 of the fan room 100, and the bottom of the silencing sheet 111 is connected to the bottom plate 130 of the fan room 100. That is, the noise-reduction sheet 111 of the ventilator muffler 110 is directly mounted between the top plate 120 and the bottom plate 130 of the fan room 100.

When the noise reduction sheet 111 of the ventilator silencer 110 is specifically designed, firstly, according to the structural arrangement of the fan room 100 and the wind speed of the ventilator 170, the flow field in the fan room 100 is subjected to simulation calculation or theoretical calculation; and then designing the number and the size of the sound-absorbing sheets 111 according to the calculation result and the installation requirement and the resistance loss requirement of the fan room 100.

In order to make the flow field distribution in the fan room 100 more uniform, the distance D between the adjacent muffling fins 111 is smaller in the region where the flow velocity is higher, that is, the region close to the ventilator 170 in the second direction D2, and the distance D between the adjacent muffling fins 111 is larger in the region where the flow velocity is lower, that is, the region away from the ventilator 170. Accordingly, in the region where the flow velocity is high, the thickness T of the silencing piece 111 is small, and in the region where the flow velocity is low (the region away from the ventilator 170 in the second direction D2), the thickness T of the silencing piece 111 is large, so as to ensure that each silencing piece 111 unit has the same flow area ratio, and the flow velocity of the air flow in each flow passage of the silencing piece 111 is uniform. According to the design principle, the thicknesses of the silencing sheets 111 of the ventilator silencer 110 are different, the distances between every two adjacent silencing sheets 111 are different, and the silencing effect and the flow resistance of the ventilator silencer 110 are comprehensively considered, so that the ventilator silencer 110 has the characteristics of high silencing quantity and low flow resistance.

With continued reference to fig. 1, in the illustrated embodiment, the thickness of each sound-suppressing piece 111 gradually increases in a direction away from the ventilator 170 and parallel to the first direction D1 (a direction extending from the first end wall 125 to the second end wall 126), and the spacing between each two adjacent sound-suppressing pieces 111 also gradually increases. The thicknesses T of the six muffling sheets 111 are different, and the range of the thicknesses T may be: t is more than or equal to 50mm and less than or equal to 400 mm. The distance D between every two adjacent silencing sheets 111 is also different, and the range of the distance D between the adjacent silencing sheets 111 may be: d is more than or equal to 50mm and less than or equal to 400 mm. The specific thickness T of the noise reduction sheet 111 and the distance D between adjacent noise reduction sheets 111 are determined by the flow field distribution characteristics of the fan room 100.

According to the ventilator silencer 110, the silencing sheet 111 of the ventilator silencer 110 is directly mounted between the top plate 120 and the bottom plate 130 of the fan room 100, so that the ventilator silencer 110 is convenient to mount and dismount, and the mounting problem of a large-size integrated ventilator silencer is solved. The thickness T of each silencing piece 111 and the distance D between every two adjacent silencing pieces 111 are designed according to the distribution characteristics of the flow field in the fan room 100, so that the thickness T of each silencing piece 111 is different, the distance D between every two adjacent silencing pieces 111 is also different, the overall arrangement of the ventilator silencer 110 is favorable for improving the distribution of the flow field in the fan room 100, the noise reduction effect of the ventilator silencer 110 is guaranteed, the distribution uniformity of the flow field is improved, and the resistance of the ventilator silencer 110 is reduced.

Preferably, the ventilator silencer 110 further includes connecting square bars 112, and the connecting square bars 112 are disposed at both ends of each silencing piece 111 along the length direction of each silencing piece 111, so as to fix the silencing pieces 111 and enhance the strength of the silencing pieces 111. The connecting square steel 112 is connected with the silencing sheet 111 in a detachable mode such as screw connection or clamping connection, so that the silencing sheet 111 can be conveniently mounted and dismounted. Of course, a connection method such as welding or caulking may be adopted as necessary.

Preferably, the ventilator silencer 110 further includes a fixing base for coupling and fixing the silencing sheet 111 between the top plate 120 and the bottom plate 130 of the fan room 100. Referring specifically to fig. 2, a first base 113 is disposed on a top plate 120 of the fan room 100, and the first base 113 may be mounted on the top plate 120 by welding. The second base 114 is disposed on the bottom plate 130 of the fan room 100, and specifically, the second base 114 may be mounted on the bottom plate 130 by embedding or welding. The noise-reduction sheet 111 is installed between the top plate 120 and the bottom plate 130 by coupling the top of the connection square bar 112 with the first base 113 and the bottom of the connection square bar 112 with the second base 114. In order to reduce the difficulty of mounting and dismounting the silencing sheet 111 between the top plate 120 and the bottom plate 130, the connection mode of the connecting square steel 112 and the first base 113 and the second base 114 is preferably a detachable connection mode such as a screw joint or a clamping joint. This also facilitates servicing of the louvers 180 and ventilator 170. Of course, a connection method such as welding or caulking may be adopted as necessary.

Furthermore, the specific structure of the silencing sheet 111 of the ventilator silencer 110 can be determined according to the actual size of the window and door of the fan room 100. The at least one sound-absorbing fin 111 of the ventilator muffler 110 may be formed as a single piece, or the at least one sound-absorbing fin 111 of the ventilator muffler 110 may be formed in a spliced manner.

Referring specifically to fig. 2, in the illustrated embodiment, the spliced-type muffling sheet 111 includes a first muffling sheet 161 and a second muffling sheet 162, and the second muffling sheet 162 is disposed above the first muffling sheet 161 and connected thereto by a connecting member 128. The connecting member 128 may be in the form of a bolt and nut or the like. The height H1 of the first muffling sheet 161 may range from: h1 is more than or equal to 300mm and less than or equal to 1500 mm; the height H2 of the second muffling sheet 162 may range from: h2 is more than or equal to 300mm and less than or equal to 1500 mm. The height of the first muffling sheet 161 and the height of the second muffling sheet 162 can be designed to be the same or different, and particularly, the height of the first muffling sheet 161 or the height of the second muffling sheet 162 can be adjusted according to the installation conditions of the fan room 100 on site. In the embodiment shown in fig. 2, in order to prevent the noise-canceling sheet 111 and the rib 127 of the top plate 120 from interfering with each other when attached, the height of the second noise-canceling sheet 162 corresponding to the position of the rib 127 is adjusted to a height corresponding to the rib 127.

In order to enable the ventilator silencer 110 to achieve the full-band silencing and noise reducing effects, when the ventilator silencer 110 is designed, simulation calculation or theoretical calculation is performed on a sound field in the fan room 100 according to the structural arrangement of the fan room 100 and the noise characteristics of the ventilator 170; and then designing the structural form of the noise cancellation sheet 111 according to the calculation result and the installation requirement and the noise reduction requirement of the fan room 100.

Referring to fig. 1, in the illustrated embodiment, each muffler sheet 111 includes two resistive segments 141 and a resonant segment 151 located intermediate the two resistive segments 141 and disposed side-by-side adjacent to the resistive segments 141.

The resistive segment 141 includes a first perforated panel 142 and a sound absorbing material disposed within the first perforated panel 142 for canceling high and mid-band noise in the fan house 100. Most sound absorbing materials are porous fibrous materials. The length L1 of the resistive segment 141 may range from: l1 is more than or equal to 400mm and less than or equal to 2000 mm.

The resonant section 151 includes a second perforated panel 152 and a partition 153 disposed inside the second perforated panel 152, and a first resonant cavity 154 and a second resonant cavity 155 are formed between the second perforated panel 152 and the partition 153 for canceling low-frequency band noise in the fan room 100. The length L2 of the resonant section 151 may range from: l2 is more than or equal to 800mm and less than or equal to 1200 mm.

It is understood that the number and arrangement of the resistive sections 141 and the resonant sections 151 of the silencing sheet 111 can be determined according to actual needs. For example, the noise canceling sheet 111 may be configured to have only the noise canceling sheet 111 in the form of the resistive segment 141, or may be configured to include both the resistive segment 141 and the resonant segment 151 disposed side by side adjacent to the resistive segment 141, where the number of the resistive segment 141 and/or the resonant segment 151 is at least one, and the specific number and the lengths of the resistive segment 141 and the resonant segment 151 may be determined according to actual needs.

In a specific embodiment, the fan house 100 has a cross-sectional dimension of 2400mm × 2650mm, referring to fig. 1 and 2, i.e., the width of the first side wall 123 is 2400mm and the height of the first end wall 125 is 2650 mm. The ventilator muffler 110 is designed to have 6 muffling sheets 111 in total, each muffling sheet 111 includes a resistive section 141 and a resonant section 151, and the total length of each muffling sheet 111 is 2000mm, wherein the length L1 of the resistive section 141 is 1200mm, and the length L2 of the resonant section 151 is 800 mm. The thickness T of the silencing sheet 111 in the direction extending from the first end wall 125 to the second end wall 126 is 200mm, 240mm, 260mm, 280mm, 300mm and 320mm, respectively, and the distance D between every two adjacent silencing sheets 111 is 110mm, 120mm, 130mm, 145mm and 150mm, respectively. Wherein the distance between the muffling sheet 111 adjacent to the first end wall 125 and the first end wall 125 is 50mm, and the distance between the muffling sheet 111 adjacent to the second end wall 126 and the second end wall 126 is 75 mm. In this particular embodiment, each of the muffling sheets 111 takes the form of a splice, i.e., each of the muffling sheets 111 includes a first muffling sheet 161 and a second muffling sheet 162. Referring to fig. 2, the height H1 of the first muffling sheet 161 is 1100mm, and the height H2 of the second muffling sheet 162 is 1100 mm. In the illustrated embodiment, the height of the second silencing sheet 162 of the right second silencing sheet 111 is adjusted to 800mm in order to avoid interference with the rib 127 of the top plate 120 when the right second silencing sheet 111 is attached.

The ventilator 170 in the fan room 100 is under rated working condition (60000 m)³And h) the noise generated during operation is about 102dB (A) (the frequency range is 20Hz-20kHz), after the ventilator silencer 110 in the embodiment is applied to the fan room 100, the noise generated during the operation of the ventilator 170 under the rated working condition is about 68dB (A) (the frequency range is 20Hz-20kHz), and the resistance of the ventilator silencer 110 is about 60 Pa. Therefore, the noise elimination amount of the ventilator silencer 110 is up to 34dB (A), the noise level of the fan room 100 of the marine engine room is effectively improved, a comfortable working environment and a living environment can be provided for crews, and meanwhile, the influence of fan noise on collected data of scientific research instruments and equipment placed on a deck can be effectively reduced.

Unless defined otherwise, technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. Terms such as "disposed" and the like, as used herein, may refer to one element being directly attached to another element or one element being attached to another element through intervening elements. Features described herein in one embodiment may be applied to another embodiment, either alone or in combination with other features, unless the feature is otherwise inapplicable or otherwise stated in the other embodiment.

The present invention has been described in terms of the above embodiments, but it is to be understood that the above embodiments are for purposes of illustration and description only and are not intended to limit the invention to the described embodiments. It will be appreciated by those skilled in the art that many more modifications and variations are possible in light of the above teaching and are intended to be included within the scope of the invention.

Claims

1. The utility model provides a ventilation blower silencer for set up in the fan room, the fan room includes ventilation blower, shutter, roof and bottom plate, the shutter sets up the fan room keep away from on the lateral wall of ventilation blower, its characterized in that, the ventilation blower silencer includes:

2. The ventilator muffler according to claim 1, further comprising connecting square steels provided at both ends of the muffling sheet in a length direction of the muffling sheet and connected to the muffling sheet for fixing the muffling sheet.

3. A ventilator muffler in accordance with claim 2, further comprising:

4. The ventilator muffler of claim 1, wherein said muffling sheet comprises a resistive segment comprising a first perforated panel and a sound absorbing material disposed within said first perforated panel.

5. The ventilator muffler of claim 4, wherein said muffling sheet further comprises a resonating section, said resonating section being disposed side-by-side adjacent to said resistive section, said resonating section comprising a second perforated panel and a baffle disposed within said second perforated panel, and said second perforated panel and said baffle forming a resonating cavity therebetween.

6. A ventilator muffler according to claim 5, wherein the number of resistive sections and/or resonant sections is at least one.

7. The ventilator muffler of claim 1, wherein at least one of said noise dampening pieces of said at least three noise dampening pieces comprises a first noise dampening piece and a second noise dampening piece, said second noise dampening piece being disposed above said first noise dampening piece.

8. The ventilator muffler of claim 1 wherein at least one of said at least three squealer plates is integrally constructed.

9. A ventilator muffler according to claim 5 or 6,

the range of the thickness T of the silencing sheet is as follows: t is more than or equal to 50mm and less than or equal to 400 mm;