In chemical industries we apply insulation to reduce heat losses to the surrounding, which is the cost to product. You can see there are two components for total cost in insulation work. First is the cost of installation of the insulation. And, second is loss of profit due to energy loss from the insulation. Therefore, an optimum insulation will reduce overall cost for the plant whether be it fixed or operating.
Table of Contents
Type of Insulations and Cost Components
Generally two types of Insulation we use in plants. First is Hot Insulation (i.e., glass wool, mineral wool, etc.) and second is Cold Insulation (i.e., polyurethane foam, thermocol, etc.). We can divide the total insulation cost in two parts.
Insulation installation cost
This includes the cost of insulation material & aluminum cladding material and of course insulation cost also.
Variable Cost for the system
It depends on the thickness of insulation, at lower thickness heat loss will be higher and at higher thickness will be low, however insulation installation cost will increase.
So, we decide the insulation thickness after comparing the cost of heat loss v/s cost of insulation installation.
Heat Loss Phenomena
Heat loss to the atmosphere from any surface whether it is hot or cold occur via following mechanisms
Conduction
This heat loss depends on surface temperature, higher the surface temperature higher the heat loss. So, by providing insulation on the surface we can minimize heat loss by this mechanism.
Convection
Due to air flow around the surface heat loss is the form of convective heat loss. Higher the air velocity higher the heat loss. If possible, we can reduce this type of heat loss by providing enclosure or close area around the hot/cold surfaces.
Radiative
At higher temperature around higher than 150 deg C radiative heat transfer becomes prominent part of the heat loss. To eliminate this energy loss provision of aluminium cladding around insulation material is very important. This type of energy loss we observe in fired heaters and boilers prominently.
Heat Loss through Pipe Insulation
It is true as we increase insulation thickness the heat loss will be low, but for pipes and curves insulation it is not always true. Let us consider a pipe with insulation around it as shown in below figure
Here we can see as the insulation thickness increases with that heat transfer area also increases. This increase in surface area will increase the heat loss to the surrounding. In case if area increases more rapidly than the resistance of insulation thickness, heat loss can be higher than the lower insulation thickness. Refer to our case as shown in figure above. The resistance of insulation per linear meter of pipe is as follows
The resistance of air per meter of pipe length is as below, here h is the heat transfer coefficient
To get the maximum heat loss, we can get minimum total resistance (R = Ra + Rb), to get this we take the first derivative and make it equal to zero.
Hence, the maximum energy loss from a pipe will be when the insulation thickness is equal to the ratio of thermal conductivity of the insulation to the heat transfer coefficient of the air. It is desirable to keep the critical radius as small as possible so that the insulation will reduce the heat loss and not an increase in heat loss from a pipe. Or we can say when there is no insulation on the surface there will be maximum heat loss.
Thank you very much for your reading, looking forward for your valuable comments.