The energy sources used are limited in magnitude and those will be exhausted by the end of 21st
century even if the present rate of demand is continued. These energy resources can be used for a longer time if proper conservation methods are used and available resources are properly managed. In general energy can be conserved by avoiding the waste. Therefore to reduce the wastage of energy in form of heat, furnaces, and pipes carrying fluids at elevated temperature and turbines are insulated.
What is Thermal Insulation?
: Insulation is defined as a material or combination of materials, which retard the flow of heat. The materials can be adapted to any size, shape or surface. A variety of finishes are used to protect the insulation from mechanical and environmental damage, and to enhance appearance.
Where is Thermal Insulation Installed?: Thermal insulations are materials that insulate the components of industrial
processes. In industrial facilities, such as power plants, refineries, and paper mills, thermal insulations are installed to control heat gain or heat loss on process piping and equipment, steam and condensate distribution systems, boilers, smoke stacks, bag houses and precipitators, and storage tanks.
Functions of Insulation:
Insulation is used to perform one or more of the following functions:
- Reduce heat loss or heat gain to achieve energy conservation.
- Protect the environment through the reduction of CO2, NOx and greenhouse gases.
- Control surface temperatures for personnel and equipment protection.
- Control the temperature of industrial processes.
- Prevent or reduce condensation on surfaces.
- Increase operating efficiency of heating/ventilation/cooling, plumbing, steam, process and power systems.
- Prevent or reduce damage to equipment from exposure to fire or corrosive atmospheres.
- Assist systems in meeting criteria in food and pharmaceutical plants.
- Reduce noise from mechanical systems
Benefits of insulation:
Understanding Heat Flow/Heat Transfer
- Energy savings: Substantial quantities of heat energy are wasted daily in industrial plants nationwide because of uninsulated undermaintained or underinsulated heated or cooled surfaces. Properly designed and installed insulation systems will immediately reduce the need for energy. Benefits to industry include enormous cost savings, improved productivity and enhanced environmental quality.
- Process Control: By reducing heat loss or gain, insulation can help maintain process temperature to a predetermined value or within a predetermined range. The insulation thickness must be sufficient to limit the heat transfer in a dynamic system or limit the temperature change, with time, in a static system. The need to provide time for owners to take remedial action in emergency situations in the event of loss of electrical power, or heat sources is a major reason for this
action in static systems.
- Condensation Control: Specifying sufficient insulation thickness with an effective vapor retarder system is the most effective means of providing a system for controlling condensation on the membrane surface and within the insulation system on cold piping, ducts, chillers and roof drains. Sufficient
insulation thickness is needed to keep the surface temperature of the membrane above the highest possible design dew point temperature of the ambient air so condensation does not form on the surface. The effective vapor retarder system is needed to restrict moisture migration into the system through the facing, joints, seams, penetrations, hangers, and supports. By controlling condensation, the system designer may control the potential for Degrading system service life and performance. Corrosion of pipes, valves and fittings caused by water collected and contained within insulation system.
- Personnel Protection: Thermal insulation is one of the most effective means of protecting workers from second and third degree burns resulting from skin contact for more than 5 seconds with surfaces of hot piping and equipment operating at temperatures above 55o. Insulation reduces the surface temperature of piping or equipment to a safer level, resulting in increased worker safety and the avoidance of worker downtime due to injury.
- Fire Protection: Used in combination with other materials, insulation helps provide fire protection in:
- Fire stop systems designed to provide an effective barrier against the spread of flame, smoke, and gases at penetrations of fire resistance rated assemblies by ducts, pipes, and cable.
- Grease- and air-duct fireproofing.
- Electrical and communications conduit and cable protection.
- Sound Attenuation: Insulation materials can be used in the design of an assembly having a high sound transmission loss to be installed between the source and the surrounding area. Sometimes, insulations with high sound absorption characteristics may be used on the source side of an enclosure to help lower the exposure to people to noise in areas immediately around the noise source by absorption and thereby contribute to the reduction of the noise level on the other
side of the enclosure.
- Aesthetics: Most insulation systems in commercial construction are not generally visible. The common exceptions to this are found in equipment rooms where the heating equipment, cooling equipment, and the associated piping are visible to the personnel who work or otherwise must access these areas. It is common practice to require a finished and neat appearance for insulation surfaces that are visible within the building envelope. These surfaces may also be painted or covered for a more acceptable appearance in the case of hospitals, schools, supermarkets, restaurants and even in industrial facilities in food processing, and computer component manufacturing where visible to the occupants.
- Greenhouse Gas Reduction: Thermal insulation for mechanical systems provides immediate reductions in CO2, NOx and greenhouse gas emissions to the outdoor environment in flue or stack emissions by reducing fuel consumption required at the combustion sites because less heat is gained or lost by the system.
: In order to understand how insulation works, it is important to understand the concept of heat flow or heat transfer. In general, heat always flows from warmer to cooler surfaces. This flow does not stop until the temperature in the two surfaces is equal. Heat is 'transferred' by three different means: conduction, convection and radiation. Insulation reduces the transference of heat.
How Insulation Works
- Conduction: Conduction is direct heat flow through solids. It results from the physical contact of one object with another. Heat is transmitted by molecular motion. Molecules transmit their energy to adjoining molecules of lesser heat content, whose motion is thereby increased. For example, when people first sit down on cold metal chairs, they instantly feel the discomfort that comes from the contact of a warm body with a cold chair as body heat is quickly transferred from the skin, and through clothing, to the chair by conduction.
- Convection: Convection is the flow of heat (forced and natural) within a fluid. A fluid is a substance that may be either a gas or a liquid. The movement of a heat- carrying fluid occurs either by natural convection or by forced convection as in the case of a forced-air furnace. For example, people usually detect a draft when standing close to a single glazed window in the winter. Air within the room tends to stratify so that the air near the ceiling is warmer because it has become less dense when heated and so it rises. This is natural convection. That warm air loses heat to the vertical window because heat flows from hot to cold. This air becomes cooler and denser, so it begins to sink. This is the draft felt by people and is another example of natural convection. Warm air entering a room from a supply duct is an example of forced convection.
- Radiation: Radiation is a process by which heat flows from a higher temperature body to lower temperature body by means of electromagnetic energy transfer. The intensity of emission depends on the temperature and nature of the body surface. The heat transfer by radiation becomes more significant as the temperature of the object rises. Any hot body emits radiation in form of heat, which can be received by an other solid body in the path of the heat radiation. The earth receives all its energy from the sun by radiation. Radiation energy transfer plays an important role in high temperature applications such as metal melting and processing, kilns, ceramics curing and solar heating.
: The basic requirement for thermal insulation is to provide a significant resistance path to the flow of heat through the insulation material. To accomplish this, the insulation material must reduce the rate of heat transfer by conduction, convection, radiation, or any combination of these mechanisms. This provides information on three general types of insulation:
- Mass insulation with air or another gas with thermal properties similar to air within the interstices inside the material. Many cellular insulations, and all fibrous and granular insulations are of this type.
- Mass insulation with low conductivity gas within the interstices inside the material. Some closed cellular insulations are of this type.
- Reflective insulation bounding one or both sides of an air space. Many insulation facings such as FSK (foil/scrim/kraft) are of this type.
Selecting an Insulation
- Mass Insulation: For mass insulation types, the most important physical property is thermal conductivity. Materials with low thermal conductivity allow less heat to be transferred per unit time, per unit temperature difference per inch of thickness. All other items being the same, materials with lower thermal conductivities are better insulators. Commercially available mass insulations have thermal conductivities at 75°F mean temperature less than 0.5 Btu in/(hr, S.F., °F).
- Reflective Insulation: For reflective insulation types, the important physical property is low surface emittance. Surfaces with low emittance have high reflectance. Reflective insulations have emittance values in the range of 0.04 to 0.1.
: The owner, engineer, general contractor, insulation contractor and insulation and accessories manufacturers must communicate with each other from the very beginning of a project in order to minimize problems in the stages of design, specification preparation, construction, operations, and maintenance. Open and frank discussion between all parties is
critical to helping the engineer establish the proper design criteria, define pipe and duct dimensions, select the insulation materials types and
thickness, facings or jackets, and define the installation procedures to be followed. Ambiguities and omissions diminish when communications is encouraged.
Such a discourse, along with knowledge of the most important insulation criteria as detailed below, will aid the engineer in calculating the
thickness required for the intended service.
- Characteristics of Insulation: Insulations have different properties and limitations depending upon the service, location, and required longevity of the application. These are taken into account by engineers when considering the insulation needs of an industrial or commercial application. The insulating
material used for steam pipes should possess the following properties. It should have high insulating efficiency. The maximum heat loss from insulated pipe should
not exceed 1 kcal per hour per square M per oC.
- Insulating efficiency = (bare surface loss – insulated loss) / bare surface loss.
- It should have high mechanical strength so that vibrations and knocks will not adversely affect it.
- It should not be affected by moisture.
- It should not cause corrosion of pipes if chemically decomposed.
- The material should be easily applied or removed.
- It should be able to withstand the temperature to which it will be subjected.
- It should be stable and resist deterioration over the working life of the pipe.
- It should be easily moulded and applied.
- It should not overload the pipe by its dead weight.
- It should not be too costly.
The materials most commonly used for steam pipe insulations are asbestos, magnesia, cork,
hair felt, wool felt, rock wool and diatomaceous earths. Most commercial insulations are either built up from corrugated asbestos paper or laminated asbestos paper artificially roughened to produce air spaces or are
molded or felted with asbestos.
A very common and effective insulation for temperature up to 400o
C is the
molded 85% magnesia (85% carbonate of magnesia and 15% binder). The insulation used for higher temperature should have inner layer of some special high temperature insulation as high temp. Decomposes the inner layer of magnesia. High temperature breaks down the magnesia into asbestos
fiber and magnesium oxide and, therefore destroys cohesion between the pipe and insulation.
A layer of glass silk before giving the layer of magnesia is generally used for pipe insulation when the temperature is above 500o
C. Glass silk has an advantage of cleanliness, is noninflammable and can withstand vibrations and rough handling without losing its form or insulating efficiency. The packing density is varied between 100 to 150 kg per cubic
meter as per requirements.
Generally steam pipes are lagged to a thickness of 8.5 cm with plastic magnesia, which is reinforced with
galvanized wire netting and covered with 1.5 cm thick hard setting of nonconducting material. The lagging finally covered by a wrapping of canvas and two coats of selected paints are given to the surface.
The small steam pipes should have the same insulation thickness as large ones as the heat loss per sq. Meter on a small pipe is higher than large pipe.
The insulation manufacturers publish `insulation efficiency` data for different thickness of their different grades of insulation. Such tables generally provide the efficiency data against two variables as pipe size and temperature difference.
The amount of insulation to be applied is an economic problem like many other power plant design problems. The cost of insulation must be weighed against the saving of heat energy obtained due to insulation.
Definitions related to insulations:
- Thermal Resistance (R) (C m2 h/Cal): The quantity determined by the temperature difference, at steady state, between two defined surfaces of a material or construction that induces a unit heat flow rate through a unit area. A resistance associated with a material shall be specified as a material R. A resistance associated with a system or construction shall be specified as a system R.
- Apparent Thermal Conductivity (ka) (Kcal m/h m2 C): A thermal conductivity assigned to a material that exhibits thermal transmission by several modes of heat transfer resulting in property variation with specimen thickness or surface emittance.
- Thermal Conductivity (k) (K-cal m /h m2 C): The time rate of steady state heat flow through a unit area of a homogenous material induced by a unit temperature gradient in a direction perpendicular to that unit area. Materials with lower k factors are better insulators.
- Density (lb/f3) (kg/m3): This is the weight of a specific volume of material measured in pounds per cubic foot (kilograms per cubic meter).
- Surface Burning Characteristics: These are comparative measurements of flame spread and smoke development with that of select red oak and inorganic cement board. Results of this test may be used as elements of a fire-risk assessment, which takes into, account all of the factors, which are pertinent to an assessment of the fire hazard or fire risk of a particular end use.
- Compressive Resistance: This is a measure of the material to resist deformation (reduction in thickness) under a compressive load. It is important when external loads are applied to an insulation installation. Two examples are deforming the insulation on a pipe at a Clevis type hanger due to the combined weight of the pipe and its contents between the hangers and....Resistance of an insulation to compress on an outdoor rectangular duct due to heavy mechanical loads from external sources such as wind, snow, or occasional foot traffic.
- Thermal Expansion/Contraction and Dimensional Stability: Insulation systems are installed under ambient conditions that may differ from service conditions. When the operating conditions are imposed, metal surfaces may expand or contract differently from the insulation and finish applied. This can create openings and parallel heat flow and moisture flow paths that can degrade system performance. Long term satisfactory service requires that the insulating materials, closure materials, facings, coating, and accessories withstand the rigors of temperature, vibration, abuse, and ambient conditions without adverse loss of dimensions.
- Water Vapor Permeability: This is the time rate of water vapor transmission through unit area of flat material of unit thickness induced by unit vapor pressure difference between two specific surfaces, under specified temperature and humidity conditions. It is important when insulation systems will be operating with service temperatures below the ambient air. Materials and systems with low water vapor permeability are needed in this service.
- Cleanability: Ability of a material to be washed or otherwise cleaned to maintain its appearance.
- Temperature Resistance: Ability of a material to perform its intended function after being subjected to high and low temperatures, which the material might be expected to encounter during normal use.
- Weather Resistance: Ability of a material to be exposed for prolonged periods of time to the outdoors without significant loss of mechanical properties.
- Abuse Resistance: Ability of a material to be exposed for prolonged periods of time to normal physical abuse without significant deformation or punctures.
- Ambient Temperature: The dry bulb temperature of surrounding air when shielded from any sources of incident radiation.
- Corrosion Resistance: Ability of a material to be exposed for prolonged periods of time to a corrosive environment without significant onset of corrosion and the consequential loss of mechanical properties.
- Fire Resistance/Endurance: Capability of an insulation assembly exposed for a defined period of exposure to heat and flame (fire) with only a limited and measurable loss of mechanical properties. Fire endurance is not a comparative surface-burning characteristic for insulation materials.
- Fungal Growth Resistance: Ability of a material to be exposed continuously to damp conditions without the growth of mildew or mold.