How to write technical requirements for EFU dimensions? Data, Testing and Acceptance Key Points

The most common way to write the EFU external dimensions in the technical requirements is 'length × width × height' with three numbers plus a tolerance. This way of writing is barely enough for the purchase of general ceiling FFU, because the ceiling keels are standardized and the sizes only need to match. However, the equipment-side EFU is embedded in the equipment sheet metal, connected to the equipment air duct, and installed close to the wafer path. The overall size is not just a matter of 'can it be put in' - it is directly related to the installation rigidity, sealing fit and air duct alignment. Only the nominal size is written in the technical requirements, and the supplier makes it according to his own understanding. After the goods arrive, it is found that the fixing holes are not aligned, and the air outlet surface is misaligned with the equipment air duct. It is too late to make changes. Aili Industrial's 70mm ultra-thin EFU supports non-standard size customization, with a minimum of 300×300mm and a maximum of 1500×1500mm. The overall dimensions, mounting hole positions and interface positions can be written into the confirmation drawing according to the equipment drawings.

First distinguish three types of dimensions: nominal, fit, and reference.

The overall dimensions of the EFU cannot be confused with the technical requirements. It should be broken down into at least three categories and written clearly.

The nominal size is the nominal length, width, and height of the EFU box, which is used for purchasing orders and model identification. Fitting dimensions are dimensions directly related to equipment installation, including installation hole locations and apertures, air outlet flange dimensions, air inlet locations, and fixed ear spacing. The tolerance requirements for this type of size are the strictest, and any deviation directly affects the assembly. Reference dimensions are dimensions that do not participate in assembly but affect spatial layout, such as the thickness of the thinnest part of the box, the protruding height of the motor, and the location of the junction box. Reference dimensions can give wider tolerances, but must be clearly marked 'reference'.

If the technical requirements only say 'outline dimensions: 600×600×70mm', the supplier may put the tolerance on the length and width, and put the tolerance on the mounting hole spacing at ±3mm. The equipment factory will find that the hole positions do not match when it is assembled. The correct way to write it is to list the matching dimensions separately and indicate 'based on the center line of the mounting hole'.

Key mating surfaces: mounting surface, air outlet surface and fixed points

There are three surfaces of the equipment-side EFU that require special marking of tolerances and geometric requirements.

The mounting surface is the surface that fits the device sheet metal or bracket. The flatness of this surface is more important than the length and width tolerance - if the mounting surface is warped, the box will be pre-deformed after the EFU is fixed, and the filter seal and air duct shape will change accordingly. The technical requirements should be written as 'installation surface flatness ≤ 0.5mm/100mm' instead of just the length and width tolerance.

The air outlet surface is the surface that is connected to the equipment air duct or work area. Differences in side lengths and diagonals of the air outlet are often ignored. If there is rhombus deformation in the box, the diagonal lengths will be inconsistent, a wedge-shaped gap will appear between the air outlet surface and the equipment air duct flange, and the sealing strip will not be tight. The item 'The diagonal length difference between the air outlets ≤ 1mm' should be added to the technical requirements.

The location and number of fixing points need to correspond to the structural ribs of the equipment panel. In addition to the hole spacing, the technical requirements should also indicate 'the distance between the center of the fixing hole and the reference edge of the cabinet' and 'the fixing hole must not fall in the unreinforced area of ​​the cabinet sheet'. Aili Industrial's ultra-thin EFU supports customization of the side air inlet/outlet structure and direct connection of the flange interface to the equipment air duct. The dimensions of these interfaces are marked item by item in the confirmation drawing according to the equipment drawing.

Dimensional inspection: under what conditions and by what method?

It is not enough to write 'test according to drawings' in the technical requirements. The detection results of the EFU external dimensions are affected by the measurement conditions: whether the box is in a free state, whether it has a filter, where the measurement point is selected, and the accuracy of the measuring tool. If these are not specified in the technical requirements, the supplier's factory data and the equipment factory's arrival retest may not match.

It is recommended to stipulate in the technical requirements: the measurement state is that the box is placed freely, without a filter, and no tightening force is applied; the measurement point is the midpoint of the four sides of the box and 50mm from the end point, taking the maximum and minimum values; the measuring tool is a vernier caliper or tape measure with an accuracy of not less than 0.02mm, and the flatness is a knife-edge ruler plus a feeler gauge; record the actual measured length, width, height and diagonal values ​​for each EFU, and attach them to the factory inspection sheet. For bulk purchases, the sampling inspection ratio and judgment rules should also be agreed upon - for example, the first piece is fully inspected, and the batch is inspected at 5%. If any fit size is out of tolerance, the batch will be returned for full inspection.

Key points for acceptance: What to check first after arrival

The order of size acceptance after EFU arrives should be matching size first, then nominal size . The matching size determines whether it can be installed. The nominal size only affects the appearance and space margin. Specific actions: Use a tape measure to check the installation hole distance and fixed ear distance; use a right-angle ruler to check the diagonal line of the air outlet; place the EFU on the equipment installation surface (not fastened) to check the plane fit, and use a feeler gauge to measure the gap; check whether the flange dimensions of the air inlet and air outlet are aligned with the equipment air duct. Any matching dimensions that are out of tolerance should be dealt with before installation, and cannot be forcibly pulled together by fastening bolts.

Aili Industrial organizes the complete machine configuration according to the confirmation drawing when supplying EFU. The overall dimensions, installation interfaces and air duct flange positions are marked one by one in the confirmation drawing. The measured dimensions of the prototype or first unit during verification are compared with the confirmation drawing, and the batch execution is based on the same benchmark. For non-standard size items, the confirmation drawing is the only basis for size acceptance and cannot be replaced by the standard model size in the sample booklet.

Misunderstandings to avoid

It should be avoided to only write the nominal dimensions but not the fit size tolerances. It should be avoided to mix the installation hole distance and the appearance length and width into the same tolerance. It should be avoided to ask the supplier to 'qualify according to the drawing' without agreeing on the measurement status. It should be avoided to only measure the length and width without checking the diagonal and flatness after arrival. It should be avoided to use sample parameters instead of the confirmation drawing as the basis for acceptance. The core of the technical requirements for writing EFU external dimensions is not to 'write all the numbers', but to separately define the three types of dimensions: nominal, fit, and reference, and write the testing conditions and acceptance sequence to an executable level.

Related questions and answers

What is the general tolerance for EFU external dimensions?
The matching dimensions of the equipment-side EFU (mounting hole distance, flange edge) are recommended to be ±0.5-1mm, the nominal length, width and height are ±1-2mm, and the reference dimensions are ±3-5mm. The specific value should be determined based on the processing accuracy and sealing method of the equipment mounting surface. The higher the sealing requirements, the tighter the fit size tolerance.

How to accept non-standard size EFU?
Please refer to the confirmation picture. Tolerances, datum planes and measurement locations for all mating dimensions should be noted on the confirmation drawing. After arrival, first check the mounting hole spacing and flange size, and then check the flatness of the mounting surface and the diagonal line of the air outlet. It is not recommended to use the size chart of standard models for comparison of non-standard EFU.

The EFU height is written as 70mm, but the actual measurement upon arrival is 72mm. Can it be used?
Check the remaining installation space. If the reserved height of the equipment is 80mm, 72mm is still within the range; if the reserved height is 70mm, a deviation of 2mm may cause it to fail to fit in or interfere with the upper structure. In addition to the nominal height, the technical requirements should also indicate the 'total height including the protruding part of the motor' and the 'maximum allowable height' to avoid placing orders based solely on the height of the cabinet.

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Aili Industrial EFU selection and procurement support

Aili Industrial's 70mm ultra-thin EFU supports non-standard size customization, with a minimum size of 300×300mm and a maximum size of 1500×1500mm. The box material can be selected from aluminum alloy, stainless steel or galvanized plate. The side air inlet/outlet structure and flange interface can be customized according to the equipment air duct. The equipment factory can write the overall dimensions, mounting hole positions and interface positions into the confirmation drawing. Aili Industrial organizes prototypes and batches according to the confirmation drawing, and files the dimensional data together with the confirmation drawing.

Learn about Aili Industrial EFU products and customized configurations. Check out the compact models.70mm ultra-thin EFU , the entire machine structure can be viewedEFU equipment fan filter unit , non-standard interface can be viewedCustom EFU . Please submit the equipment installation space dimensions, mating surface requirements and estimated quantity to facilitate confirmation of overall dimensions and interface configuration.