Key points of air-conditioner construction

3.1 Air hose project

1.It is desirable to install the HEPA at air outlet.
2.Thermal insulation shall be carried out on the air hose by means of 1″-thick PE plate.
3.The air hose juncture shall be sealed via Silicone.
4.It is most desirable to have air hose of 0.8mm at the minimum. The width of air hose shall be between 45 to 120 cm. Iron sheet of 1.0 mm shall be used. If the width is over 120 cm, 1.2mm-thick galvanized sheet iron shall be used.
5.It is advisable to add pad at the joint of air hose flange and fasten it with screws before sealing it with Silicone.
6.Upon connecting the air hose flange, fill and connect the non-insulation gaps with 1″-thick PE plate before sticking the entire flange with PE plate of 20cm wide and 1／4″ thick.
7.It is advisable to control the aspect ratio of air hose to no more than 3.5.
8.After the air hose has been produced properly, clean the interior of the air hose before hoisting. The port needs to be sealed when the construction is suspended.
9.It is necessary to use non-deteriorated soft rubber head for packing.
10.The installation of HEPA shall be carried out by professional manufacturer. It is unwise to finish the installation by ordinary installation workers.
11.The part of connection shall be sealed via Silicone.
12.Interior thermal insulation shall not be carried out so as to avoid pollution.  13.When the insulation board is stuck, apply the super glue on the side of air hose and the side of insulation board. After approx. five to ten minutes, stick and press the sides.
14.When applying the aluminum flexible pipe, avoid excessive bending. The joints shall be sealed and without air leakage.
15.It is advisable to use aluminum flexible pipe along with 1″ insulated or reinforced insulated hose to connect the main air hose and the HEPA case. It is unwise to use the glass wool hose for general air conditioners.
16.It must be careful when using reinforced insulated hose to connect the air main and the HEPA. The outer layer shall not be pierced in order to avoid leakage and pollution.
17.The commissure between insulation board shall be cut flat and then slightly pressed and stuck upon applying the glue. The joint shall be properly stuck with insulated tape of approx. 1″ to 2″ wide and 1/4″ width.
18.Adjustable air hose shall be installed at the parting of air hose.
19.The fresh air port shall be equipped with strainer and (self-regulation) baffle.
20.After drilling holes in the wall, repair shall be made properly via Silicone or soft filler. Besides, hard board shall be added for protection and sealing.
21.Owners shall try to request to make the air hose at the construction site, so as to supervise construction and control quality.

According to conservative estimation, air leakage is 5%~15% with traditional air duct system (Static pressure of about 15~25mmAq), in most cases, however, static pressure of air duct in clean room is ≥75mmAq; therefore, unless construction quality is enhanced, the circumstance of leakage will be very serious.

In general, if air conditioning duct is in poor airtightness, cooling capacity will be decreased at most, however, in the event of leakage from clean room duct, the pressure inside ceiling may be higher than pressure inside clean room, as a result, the air inside ceiling will penetrate into clean room and destroy its cleanliness. Furthermore, during downtime of air handing unit, the air duct will be contaminated due to reverse osmosis of dust. Such case is one of the main causes of substandard cleanliness. Thus, air duct should have to be maintained airtight as possible. Therefore, during construction of air duct, it must be sealed with Silicone.

※Precautions on the use of Silicone sealant:

1. In feeding Silicone sealant, the sealing space should by fully distributed with sealant.
2. After compression fitting of air duct, feeding of Silicone in and out the joint is preferable to confirm safe sealing, construction inside air duct when feeding Silicone sealant is preferred.
3. If flange coupling is used, feeding of Silicone sealant is preferred in addition to application of adhesive lining on both sides.
4. In case of proper construction, Silicone sealant can be applied to air duct with static pressure not exceeding 90mm; in case of higher static pressure, construction by means of fusion splicing is preferred.

3.2 Aspects of Air-conditioner Room

Generally, air-conditioner room is divided into two parts, namely, ① chiller unit room; ② air handing unit room.

Taking into account vibration and noise, chiller unit room is generally kept far from clean room, key points for construction of chiller unit room is the same as that of common air conditioner. Although air handing unit is subject to vibration and noise, in order to reduce the cost of air duct and easy return air, AHU room is generally located close to clean room. For air handing unit (AHU) for the use of clean room, due to high static pressure, large air flow, and low temperature of exhaust air, in addition to basic requirements for common air conditioner, following aspects should be strengthened specially:

① Setup of concrete foundation base and optimal shockproof base
For AHU used in clean room, due to bulky volume and intense vibration, additional concrete foundation base (≥20cm high) is preferred, and shockproof base should be made up of superior material and in sufficient number.

② Differential pressure gauge should be mounted on the filter to understand the proper time for replacement of filter
Prefilter, secondary filter and even HEPA are mounted on AHU used in clean room. Differential pressure gauge should be mounted on various stages of filter. Duly replacement of filter helps save running cost and maintain proper cleanliness.

③Use of waterproof canvas joint and additional insulation
The temperature of exhaust air of AHU used in clean room is 2~3℃ lower than common air conditioner, thus waterproof canvas joint connecting AHU and branch pipe is apt to condensate and accumulate moisture.

④ Make sure sealed pipeline running across all holes of unit room wall and floor
Firstly, it functions for sound insulation; secondly, in the location where return air pipe is not directly connected to AHU, the value of negative pressure is very high in the unit room, large amount of external air will be penetrated from any aperture (Inc. door & window slot), leading to higher AHU load, and impact on stability of temperature and humidity.

⑤ U-trap must be mounted on drain pipe of air handing unit (AHU)
Water trap should be mounted on drainage riser beneath washbasin in washroom and bathroom to prevent ingress of smell and pest from water trap into clean room via drain pipe. However, this is not key point for installation of water trap on AHU drain pipe, and the purpose is to ensure 「unobstructed drainage」.

Insert a sucker into the drink, if you seal the top of sucker with finger, take it out of the drink, the drink is held in the sucker without leakage; if you release your finger, the drink inside the sucker will drain. This is vacuum siphon effect, and the same situation will occur with AHU drainage. In general, negative pressure at AHU drip tray (Fig. 4-1) is -1-~-25mmAq; if no water trap is mounted on drain pipe, drainage is impossible during running of AHU, instead, it drains from the bottom of AHU.

Unfortunately, most air conditioner designers seem to believe 「water trap」 is a common sense and fail to specify the same. I’ve seen the “initiative action” of draining AHU drip tray with the miniature pump for fish tank, and “great work” of using 1m high water trap.

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