Thermal Insulation by PUF

Creation of refrigeration (coldness) costs money and energy.  Almost always electricity is a “high density” energy.  Electricity is a “high density” energy form and costs and consumer dearly, and is even more dear to the exchequer or tax payer in addition to being a precious commodity in a developing country.Unbonded/loose rock or glass fibrous materials are energy inefficient in the long run and should not be used, except for loose fill applications in cavities etc.
Air-Conditioned spaces like office/residential buildings and refrigerated systems like piping/ducts carrying cold fluids including brine/chilled water, cooled air, etc., unless properly insulated, gain large amounts of heat, specially in a tropical country like ours, from surroundings.

To maintain process/comfort parameters, the refrigeration equipment therefore has to work overtime to throw out this extra heat, meaning thereby an inflated electricity bill, on an ongoing basis, to the consumer.  Proper and judiciously designed insulation cuts down the heat influx and ensures minimum energy consumption and expenses.

Creation of refrigeration (coldness) costs money and energy.  Almost always electricity is a “high density” energy.  Electricity is a “high density” energy form and costs and consumer dearly, and is even more dear to the exchequer or tax payer in addition to being a precious commodity in a developing country.

Since properly designed insulation eliminates over?working of the refrigeration equipment, it minimises equipment maintenance expenses and down?time, thereby balancing the owner’s economics.  Thermal insulation can make a lower?rated air?conditioning equipment adequate, thereby reducing capital costs.  Financially, thermal insulation is a low?cost system if designed properly and applied well.  It needs no major maintenance.

Insulation system design involves selection of material for insulation extent of insulation, and the application method.  Each of these factors is discussed in turn hereafter.

Insulation Material

Possible alternatives are

  • Resin bonded rockwool
  • Resin bonded glasswool
  • Polyurethane foam
  • Polyisocyanurate foam
  • Phenolic foam
  • Expanded polystyrene foam.

Foamglass is not available in the country and Calcium Silicate Ceramic fibre etc.  are high temperature insulation materials, hence not being discussed.

The technical properties which need to be considered for material selection are :

Thermal Conductivity:  It signifies the ability to allow transmission of heat through the body or the material.  It should be as low as possible for good insulation.

Since properly designed insulation eliminates over?working of the refrigeration equipment, it minimises equipment maintenance expenses and down?time, thereby balancing the owner’s economics.  Thermal insulation can make a lower?rated air?conditioning equipment adequate, thereby reducing capital costs.  Financially, thermal insulation is a low?cost system if designed properly and applied well.  It needs no major maintenance.

Thermal Diffusivity: It is one of the most important properties, since it signifies heat flow through the body of the material in transient or unsteady state condition, which actually almost always exists.  This is because total thermal equilibrium is actually never established, due to variation in ambient temperature, variation in cooling load, input power disturbances and failures, etc.  Thermal diffusivity signifies how fast the cooled temperature would rise whenever the refrigeration effect is disturbed due to change in ambient temperature, cooling load, power supply, etc.  It should be as low as possible.

Stability And Sturdiness: In the physical sense stability and sturdiness are important because the insulation material, once applied, should serve its purpose for years.  The material should not sag, should have adequate sturdiness to retain its shape and properties even when applied in vertical configuration like wall insulation.

Fire Resistance: In the present day context fire resistance is a very important criterion.  A number of devastating fires have been reported in the recent past, from industry and buildings alike.  Major national newspapers reported that insulation had a major role in and casualties  and losses in the recent fires. It is therefore prudent to assess the insulation material in the context of its known behaviour as regards combustibility, ignitability, surface spread of flame, smoke generation toxic gas emission etc.

Unbonded/loose rock or glass fibrous materials are energy?inefficient in the long run and should not be used, except for loose?fill applications in cavities etc.

Extent Of Insulation

Thickness of insulation is designed based on one of the following criteria : prevention of condensation, limit of heat gain, prevention of freezing or limit of temperature gain over specified time duration or length of piping.

The procedure adopted for thickness design based on anti?condensation involves:

  • Determination of design surface temperature based on ambient temperature and relative humidity;
  • Initial assumption of thickness;
  • Calculation of surface temperature over insulation; and
  • If the calculated surface temperature, the thickness has to be increased and step from No 3 have to be repeated.

The parameters that figure in the design calculations are

  • Design ambient temperature
  • Relative humidity
  • Emissivity of insulation system finish
  • Ambient air speed
  • Process temperature
  • Pipe/duct dimensions.

It is obvious that the actual detail of the entire insulation system (i.e. including finish etc.) has to be conceived beforehand, so that the appropriate emissivity factor can be used in the thickness calculation.

The design relative humidity should not be unnecessarily high and the worst ever seen in the respective location.  Selection of design RH should be such that normally it is not expected to be exceeded for any extended period.  It is a normal practice to consider RH of 85 percent for design.  Higher RH would lead to unnecessarily higher calculated insulation thicknesses.  Design surface temperature is usually taken as 0.5C above the dew point at the design ambient temperature and RH, read from psychrometric charts.

Whereas details of actual design calculations is beyond the scope of this article, recommended insulation thicknesses for common standard applications are provided at the end of this paper.

Application

One important component of a cold insulation system is its vapour barrier.  Since the thickness of the insulation material is designed so as to maintain a surface temperature higher than the dew point of the ambient air all through the year, condensation of moisture on the insulation surface is ruled out.  However, differential water vapour pressure stays as such and there is a continuous “pull” on atmospheric water vapour towards the cooled surface, through the insulation material.  Whereas a low vapour  permeability helps insulation material would retard the influx of the atmospheric water retard the influx of the atmospheric water vapour, total elimination of this vapour influx is required to ensure insulation effectiveness in the long run.  A vapour barrier, an impermeable layer on the warm face of the insulation, is an absolute must in all cold insulation systems, to act as a physical barrier between atmospheric water vapour and insulation system, the vapour barrier should not have any puncture, should have all  joints adequately overlapped and sealed.  All adhesives/sealants/mastics should be fire resistant, as otherwise the excellent fire resistance of resin bonded rockwool would be compromised.

It is obvious that the actual detail of the entire insulation system (i.e. including finish etc.) has to be conceived beforehand, so that the appropriate emissivity factor can be used in the thickness calculation.

The final finishing/weather barrier for pipes/ducts should preferably be aluminium sheet cladding, for ease/speed of application, achieving good finish, light weight combined with effectiveness of protection, ease of cleaning etc.  a layer of atleast 20mm thick resin bonded rockwool felt should be wrapped on the vapour barrier, only after which the cladding sheet should be applied and retained with self?tapping screws, with all joints of the cladding sheet properly grooved, lapped screwed and sealed.  The 20mm thick resin bonded rockwool felt layer is meant to protect the vapour barrier from puncturing when the sheet cladding in fixed by screwing.

For indoor building insulation application like wall/ceiling insulation, the vapour barrier has to be stuck to the wall/ceiling face and then only the resin bonded rockwool has to be applied.  If any insulation material other than resin bonded rockwool is used, plaster finish should be applied to compensate for lower fire resistance of such materials.

Overdeck insulation of roof and floor insulation requires some amount of compressive strength for taking foot traffic, which can not be readily obtained with fibrous materials, though high density resin bonded rockwool “fire batts” are used for thermal?fire insulation of floor in many developed countries, while taking care to incorporate suitable ancillary structures for building the required commpressive strength.  Till such practice comes into vogue in our country.  TF or SE (Treated for Fire/Self Extinguishing) quality expanded polystyrene or polyurethane foam are recommened for such applications

The magic of PUF insulation

A new component which is replacing old construction and insulation materials abroad and has now made inroads in India is PUF (pollyurethane foam).The acrononym PUF is being bandied as “a magic” by certain refrigeration and air?conditioning companies.

What exactly is PUF? It`s a foam much like the kitchen sponge but it is a rigid foam unlike the domestic scrubber which is a flexible foam. There are three types of foam:

  • Flexible
  • semi?rigid
  • rigid.

Polyurethane foams were first commercially introduced by Dr Otto Bayer during World War II Since then they have gained popularity the world over. In India too there are manufacturers who make both the flexible and rigid types.

PUF strongly attracted the refrigeration industry because it is an excellent insulator even in  thin sections. Take, for example a 165 litre fridge by using PUF?filled walls one can have a fridge with same internal capacity but with a smaller body . On the other hand, if one chooses to have the same outer size, then with PUF insulation, the inner volume will go upto 180 litres. This is  the reason why all refrigerator companies have replaced their traditional insulation materials with rigid PUF. At first one company used rigid PUF for their refrigerator door, then another company used it for the entire refrigerator body and that’s perhaps how the ad war began ? one company making it appear that it was the pioneer user of the new insulation material while the other claimed to have been using it for years.

Rigid PUF`s are highly crossed linked cellular polymers with a closed cell structure. Each bubble within the material has unbroken walls and restricts gas movement. The air trapped in these bubbles act as insulator. And air i a very good insulator it is the air trapped in the wool fibres that make a sweater to retain the wearers body warmth.

PUFs are produced by reacting hydroxyl compounds and isocyanates in the presence of catalyst, stabiliser, blowing agents etc. Rigid PUF besides being a good insulator has advantages like high mechanical strength, excellent ageing resistance, light weight, buoyancy, easy mouldability, good adhesion, resistance to corrosion and decay. It can be made resistant to fire by adding fire retardents of the chlorophosphate based compounds. The addition of fire retardants makes PUF safer while continuing to retain its insulating properties. PUFs without fire retardants, when burnt give out toxic gases, which are often the cause of death in fires.

The list of applications of rigid PUF is almost endless:

  • You can have PUF insulated building, flat and sloping roofs sealed with bitumen or plastic sheeting;
  • Terrace roofs and parking decks can be insulted using easy to lay foam slabs and panels with high compressive strength.
  • Pitched roofs can be insulated on, between or under the rafters with foam slabs or panels of various types.
  • Ceiling and floors of various types can be insulated with PUF slabs, mouldings and panels.
  • Light weight saw?tooth roofs can be insulated with mouldings and slabs and walls (including precast concrete walls) with sandwich panels.

In fact many sensitive areas like airports, the  roofing is already insulated with rigid PUF. It might sound expensive initially, but the cost will soon be recovered as the insualtion will save a lot energy used for airconditioning. Many industries are going with rigid PUF insulation: It is not only insulation, but has excellent fire resistance.

PUFs have many applications.Refrigerated storage facilities; freezers, walk in coolers, refrigerated and deep freeze warehouses, cooling water circuits and cooling tanks can all be thermally insulated.

Half?pipe sections, full sections and on?site foaming systems can be used to insulate pipes carrying hot and cold liquids in buildings and industrial plants. Industrial water tanks, oil tanks, drying plants and grain silos are other applications where PUF insulation can be used.

The fact that PUF exhibits superior thermal insulating properties and possesses an inherent structural rigidity makes it possible to have excellent air conditioner cabinets with thin, rigid and efficient walls. A substantial savings in production costs can be achieved through automated fabrication techniques. The water heater is another domestic appliance where rigid PUF plays a vital part. PUF due to its uncomparable strength, is used to make outer case of computer units to protect inner parts.

Plastic thermowares have recently witnessed an unprecedented boom. The credit for their phenomenol success goes to rigid PUF which keeps eatables hot (or cold) all day long.Since PUF has no attended known bio hazard even if swallowed and since it is not degradable, it is absolutely for making safe food containers, say a thermoware producer.

Rigid PUF is puncture?proof and light weight stiff and impact resistant. That is, even if forced to sink in water, it will again bounce and float. So, it makes the ideal material for ships, lifebouys, surf boards, b oats etc. Floats produced using rigid PUF have proved to be a boon to the fishing industry.

Low density rigid PUF is used for the pour in place packaging of delicate objects. The expanding foam fills up the void around the product, cushioning it completely against any shock and impact, It can, therefore, economically protect a wide range of products of almost any size, shape and weight. It saves labour and reduces large inventory of packing materials. Being light, weight it also reduces transport cost.

It is already playing a major  role, and in times to come, PUF is going to be widely used in India in a number of fields and we may safely travel by a PUF insulated bus and have thermal and acoustical corrections by PUF.

BY Kaushik Das
Lloyd Insulations

© 2012, AIDEC World. All rights reserved.

2 comments for “Thermal Insulation by PUF

  1. Jitendra Singh
    February 21, 2013 at 8:04 AM

    Informative article. But where do I get a demo of PUF material at work ?
    And is this material available at Jaipur at a reasonable cost ?

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