Why do air duct metal expansion joints have to be insulated? -Heat loss is only surface, safety and longevity are the key
Two days ago, I met a friend from a power plant and talked about the metal expansion joint on their desulfurization flue, saying that the insulation layer was scraped and redone, and it was tossed several times. I asked him why he was so serious. He replied: Without insulation, the bellows can't last for two years. This is not an exaggeration.
Many people think that the insulation of the metal expansion joint of the air duct is to save some heat and reduce the electricity bill by a few points. Actually, that's not how the account is calculated. The medium in the air duct, especially high-temperature smoke and steam, has a temperature of three or four hundred degrees at every turn. If thermal insulation is not done, the metal bellows of the expansion joint is directly exposed to the environment. As soon as the temperature difference between the inside and the outside is enlarged, water vapor-acidic condensate will be quickly condensed out of the metal surface. How corrosive is this thing against stainless steel? To put it bluntly, 304 bellows without thermal insulation can have intergranular corrosion within a maintenance cycle, and serious direct perforation and air leakage. Safety? Don't think about it.
Furthermore, the insulation layer can reduce the temperature gradient of the expansion joint itself. The thermal expansion compensation of bellows is calculated according to the uniform temperature of the whole pipe system. If the local temperature fluctuates greatly, the bellows will generate additional stress and the fatigue life will drop linearly. When we matched the metal corrugated expansion joint in the cement industry before, some customers reported that the weld joint of the expansion joint without heat insulation was cracked. After changing the heat insulation structure, there was no problem in operation for three years. Therefore, insulation is not an option, but a life-saving item.
How to pick insulation materials? Rock wool, aluminum silicate wool, aerogel felt, which is more suitable for air duct conditions?
If the material is not selected correctly, insulation is equal to doing it for nothing. There are three mainstream types in the market: rock wool, aluminum silicate wool and aerogel felt. Which is the most appropriate? Scoring conditions.
rock wool: Cheap, temperature resistant to about 650 ℃, but high water absorption. It is okay to use in the dry air duct. Once there is moisture or slight condensation, rock wool will absorb moisture, become heavy and collapse, and the thermal insulation performance will drop from a cliff. In addition to the extreme budget, it is not recommended to be used in the wet working environment of desulfurization flue gas baffle door.
Aluminum silicate wool: Higher temperature resistance, above 1000℃, low thermal conductivity, and stronger acid and alkali resistance than rock wool. In the power station industry, the high-temperature section of corrugated expansion joint, such as boiler outlet and hot air duct, is commonly used as the main insulation layer. The disadvantage is that the price is slightly expensive, and the fibers are easy to fly, so you have to be protected during construction.
Aerogel felt: This has caught fire in recent years. The thermal conductivity at room temperature is only 0.018 W/ (m·K), which is almost 1/5 of that of rock wool. The same thermal insulation effect, the thickness can be reduced by more than half, and it is friendly to the air duct with compact space. We have made an aerogel insulation scheme for the double hinge expansion joint of air-cooled island vacuum pipeline. Customer feedback said that the external surface temperature was reduced by 40℃, and because of its strong hydrophobicity, the condensate problem was basically eliminated. However, the unit price of aerogel is high, and the cost of sharing the whole field can't bear it. Generally, it is only used in parts with a temperature exceeding 300℃ or strict requirements for space.
Practical Suggestions: Rock wool can be used in a dry environment below 300 DEG C; 300-500 DEG C and mild moisture aluminum silicate wool; Above 500℃ or at risk of condensation, bite your teeth and put on aerogel felt. Don't mix, different materials have different shrinkage rates and are easy to delaminate.
Thermal insulation structure design: internal insulation vs external insulation, the cooperation of expansion joint and guide tube should not be forgotten
Thermal insulation structure design is not only wrapped with a layer of cotton. Do you choose internal insulation or external insulation?
External insulationMost commonly, the insulation layer is wrapped outside the expansion joint, which is convenient for construction and can directly see the state of the bellows during maintenance. However, there is a dead hole in external insulation: when the expansion joint absorbs axial displacement, the bellows will expand and contract axially, and if the external insulation layer can't follow, it will be pulled and cracked. Therefore, expansion joints must be set, generally leaving a seam of 20~30mm every 1~1.5 meters, and filling it with soft insulation materials. In addition, the fixing parts (such as hoops and barbed wire) cannot be directly pressed on the bellows, otherwise it is a thermal bridge, and the heat is directly conducted out.
Internal insulationGenerally used near the flue baffle door or high-temperature flue gas pipeline, the insulation layer is laid on the inner wall of the air duct. The advantage is that the external metal shell has a low temperature and is safe; The disadvantage is that the internal insulation is directly faced with high-speed airflow scour, so it has to be protected by anti-scour board. At this timeguide tubeThat would come in handy. The guide cylinder is installed inside the expansion joint, on the one hand, it guides the air flow to reduce the vortex, and on the other hand, it protects the inner insulation layer from being washed away. When we did the matching of single-axis double baffle doors before, we encountered cases where the guide tube design was too short and the insulation layer was washed off. The guide tube should be at least 100mm longer than the expansion joint bellows, and the end of the guide tube should be smooth transition.
Regardless of internal insulation or external insulation, the cooperation between expansion joints and guide tubes must be clearly marked on the drawings in advance, otherwise it is common for on-site rework.
Three details that are prone to rollover during construction and acceptance: lap direction, sealing treatment, and thermal bridge of fixings
No matter how good the insulation is, the construction is rough and all in vain. Eight out of ten items in these three details will roll over.
First: Overlap Direction。 If the insulation layer is laid in multiple layers, the overlap seams of each layer must be staggered, and the stubble must be pressed along the direction of airflow. For example, if the air flow in the air duct flows horizontally, the lapping seam of thermal insulation cotton should be upstream to press downstream, and cannot reverse stubble. Otherwise, the airflow will lift the cotton layer open, exposing the metal bellows over time. When inspecting, take a long ruler in your hand and stab it into the overlap seam. If you can stab it in, it will be unqualified.
Second: Sealing treatment。 There is usually a metal shell (galvanized iron or aluminum) outside the insulation layer, and the seams between the shells must be sealed with sealant. Many construction teams save trouble, directly overlap without sealing glue, or wrap it twice with glass filament cloth. As a result, when it rains or cleaning steam is purged, water seeps in along the gaps, and the insulation layer becomes an "absorbent sponge". When we insulate the desulfurization flue gas baffle door, we require weather-resistant silicone glue to be used for the shell joints, and we should clean and de-oil before gluing. When accepting, we should test with spray water for 24 hours without leakage.
Third: Fixture thermal bridge。 If the nails, pendants and barbed wire that fix the insulation layer directly contact the metal bellows or the outer wall of the air duct, that point is the heat bridge, and the heat will run away directly, and the insulation effect will be discounted by more than 20%. Solution: Add a 5mm thick rubber pad or ceramic fiber pad between the fixing part and the metal surface, and treat the broken bridge. At the time of acceptance, sweep the thermal imager, and the thermal bridge point is clear at a glance.
How to deal with aging of insulation layer? Practical Suggestions for Regular Inspection and Local Replacement
Insulation is not a one-and-done thing. Heat, vibration, moisture will make it age gradually. We recommend a comprehensive inspection at least once a year, focusing on the condition of the insulation in the area of the expansion joint bellows.
Look at the appearance (whether the shell is bulging, corroded or falling off), touch it with your hands (whether the surface temperature is abnormally high), and use a thermal imager (find abnormal temperature gradient). If it is found that the insulation layer is partially damaged, damp and collapsed, it will be partially replaced.
After shoveling off the damaged section, use a wire brush to clean the metal surface and apply anti-rust paint (especially the weld of the bellows). The thickness and density of the new insulation material should be consistent with the original design, and cannot be replaced at will, otherwise the thermal compensation parameters will change. After replacement, seal and fix, and record the replacement location and date. We have encountered customers who used rock wool of different densities during local replacement. As a result, the adjacent areas shrank inconsistent, forming gaps, and the bellows had been corroded by the inspection the following year. Alas, the details are the devil.
For the insulation of the metal expansion joint of the air duct, it takes a little more thought in the early design, and the later operation and maintenance can save a lot of money. Don't wait for air leakage, corrosion and cracking before going back to make up lessons. At that time, the cost will not be the money of insulation.