There are a variety of different types of ducts, pipes, and equipment that operate below ambient temperature. The primary purpose of the insulation in these cases is to prevent surface condensation and reduce energy use. The cost penalties of surface condensation vary from application to application, but having condensation within the insulation can be severe.
For example, in typical indoor commercial heating, ventilating, and air-conditioning (HVAC) applications, condensation can get into electrical equipment, run onto floors creating a safety hazard, or damage building materials. Wet insulation, in turn, performs poorly, possibly leading to condensation on the surface of the jacketing and energy waste. It can also increase the load on mechanical cooling equipment, possibly resulting in a compressor getting overloaded and burning out. In industrial applications, condensation may corrode the carbon steel onto which it drips, increasing maintenance costs. Overall, water condensation will create all sorts of costly operational headaches that the facility owner should make an effort to avoid.
Porous insulations, such as mineral fiber insulation, can be used on below ambient applications as long as they are covered with a low-vapor-permeance jacketing, referred to as a vapor retarder. It is prudent to use these systems on air-handling ductwork down to a moderate temperature of about 50°F. A drawback to this type of insulation system is that, over time, holes or other imperfections may develop in the jacketing and could allow water vapor intrusion that then condenses on the cold surface, resulting in wet insulation. Since such holes can easily be patched with self-adhering tape, periodic inspections should be conducted to identify and repair these spots before condensation problems develop.
For chilled temperatures below 50°F, such as chilled water pipes, and/or for a relative humidity (RH) above 90 percent, a more robust system—either from an overall low water vapor permeance perspective or a wicking capability—is required. A lower permeance jacketing should ideally be a zero-vapor-permeance material (called a vapor barrier) that is extremely well sealed to itself to prevent any water intrusion. These zero-permeance jacketing materials include both sheet plastic and a self-adhering laminate. For wicking capability, the wicking type of fibrous pipe insulation can provide performance free of vapor condensation problems on chilled water lines down to about 40°F in less than extreme humidity conditions. Both types of systems, however, must be well maintained to avoid water condensation problems.
When well maintained, closed cell foams, which have low vapor permeability values, are well suited for below-ambient systems with minimum maintenance problems. The thicker the insulation, the lower the vapor permeance (vapor permeance equals vapor permeability and thickness). In general, these closed cell foams perform well with chilled water systems and high relative humidity. However, all seams and butt joints must be well sealed. Breaks in the seals and gaps in the closed cell foam insulation material will allow water vapor to intrude; this water vapor, in turn, will condense. This condensation may take more time to develop than with a porous, fibrous insulation material, but with a constant high vapor pressure, it will eventually result in water accumulation in the insulation. This will compromise its thermal performance and lead to the types of condensation problems already mentioned.
For outdoor applications where weather protection is required and/or for very high vapor pressure differential indoor conditions, these closed cell insulation materials perform best when combined with a zero-permeance vapor barrier (as opposed to a vapor retarder). In an actual application, if such a vapor barrier jacket is used, it must be sealed completely to prevent water intrusion or water vapor condensation within the insulation. Certain sheet plastic materials and the new self-adhering, laminate jacketing materials—mentioned earlier for high-temperature systems—can simultaneously provide both weather and vapor barrier protection for below-ambient systems.
For extremely high humidity conditions combined with low operating temperature systems, located either indoors or outdoors, a zero-vapor-permeance system with redundancy will perform best. This can be achieved with at least 2 inches of inorganic cellular insulation covered with the appropriate vapor barrier jacketing. The jacketing should have zero vapor permeance, and the 2 inches of inorganic cellular insulation will have near zero vapor permeance. Further, this type of cellular insulation will not absorb water, should it somehow condense against the insulation. Such an insulation system, however, still requires maintenance to prevent moisture condensation over time. The jacketing must be periodically inspected for holes, and the inorganic cellular insulation must be periodically inspected for any physical damage that might reduce its thickness, leading to surface condensation.
Regardless of the insulation type or low-vapor-permeance jacketing, the insulation system for below-ambient applications should be well designed and well maintained. In most cases, if the insulation system is ignored and/or abused, eventually water condensation problems can occur. The penalty to the owner can be water damage to stored materials, building materials, and equipment, as well as mold growth, energy waste, and cooling system overloads.
The Bottom Line
Well-maintained thermal insulation reduces heat loss and saves money. Damaged insulation saves less money, and missing insulation saves no money at all. With crude oil at about $65 per barrel, delivered heating oil at $2.50 per gallon, and delivered natural gas at more than $10 per million Btus, every day spent ignoring damaged or missing insulation is another day of paying the high cost of wasted energy.
Two years ago, crude oil was trading at about $50 per barrel. Since then, crude oil has sold for as much as $77 per barrel, and now gasoline is selling for a record average price of $3.20 per gallon. People keep waiting for energy prices to drop, but even when prices drop for a few months, they jump up again to an even higher level than they were previously. This is no time to waste energy. Thermal insulation maintenance is easy and inexpensive when compared to energy prices. Those in the insulation industry need to help the managers of industrial facilities understand the economics of insulation.
Whether the motivation is to reduce energy use, to prevent CUI, or to maintain boiler temperatures, when it comes to thermal insulation and its maintenance the old saying, “Pay now or pay later,” is appropriate. The cost of not maintaining insulation correctly is too great for facility owners to ignore.
Source:http://www.insulation.org/articles/article.cfm?id=IO070701
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