Process heating is a critical factor in many manufacturing processes and contributes to the overall cost of operation. Insulation helps minimize heat loss in these application, but often, insulation and its role in energy efficiency are not considered during the design phase. Yet after construction, adding or increasing insulation thickness may be difficult. Using a less efficient heating system is like throwing energy dollars out the window. In addition, insufficient insulation can jeopardize worker safety as well as product quality. It also can lead to increased greenhouse gas emissions and damage to temperature-sensitive equipment.
Key aspects of designing an energy-efficient heating system include:
- Knowledge of your heating requirements.
- Examining potential sources of heat loss.
- Selection of insulating material.
- Separate or integrated heating.
- Proper installation techniques.
This article will look each of these aspects more closely.
FIGURE 1. Damaged insulation is a major source of heat loss.
Determining Heating Requirements
Process, industrial or application engineers typically will outline a flow diagram to describe the process requirements. Equipment specific to the manufacturing process is planned along with any material-handling requirements to move material from one area to another. When the process requires heating material, calculations are made to consider:
- The lowest temperature for material introduced.
- The desired temperature increase during that stage.
- How quickly the temperature must rise.
- Environmental conditions around the equipment.
All of these should be used to determine the amount and location of required heating.
FIGURE 2. Insulating jackets can be standard or custom fitted shapes.
Sources of Heat Loss
Any time heat is transferred to a mass other than as intended, it is considered heat loss. Heat loss can result from:
- Conduction. This is the transfer of heat from a hot mass to a mass that is colder. If a pipe is hot and heat transfers to a pipe stand or bracket, that heat transfer to the pipe stand or bracket is considered a heat loss.
- Convection. This is the result of heat from a hot object moving to a cooler area. An example is hot air leaking to atmosphere from an opening in a furnace wall.
- Radiation. This is the transfer of electromagnetic energy through space.
- Evaporation. Additional energy is required to evaporate condensate during cooling or as part of a chemical reaction.
A major source of heat loss is poor insulating practice. This can be insufficient thickness, but more often, it is the result of damaged insulation (figure 1). Important items to look for during insulation inspection are:
- Tears in the outer covering.
- Damaged or missing lagging.
- Loose insulation on the floor.
- Discoloration of insulation or outer coverings.
- Exposed surfaces.
- Areas of increased temperature identified by thermal images.
In some cases, further investigation — for example, removing coverings or the insulation — is needed to identify the underlying problem.
FIGURE 3. Polyimide fiber is a more durable alternative to fiberglass.
Insulating Material Selection
Material selection is a critical component to energy efficiency. Different materials and thicknesses impact efficiency by reducing heat loss. Application temperature, installation, environment and durability are vital considerations. All of these materials can be applied directly over piping or flow-control components, and some can be used in formed insulators with covering material. Fiberglass, aerogel, polyimide fiber and silicone sponge can be used in all-in-one heated insulating jackets. If the insulation will be removed and reinstalled in the same location, the chosen insulation should be durable and easy to handle. Thermal conductivity ratings for a number of materials are shown in table 1.
Heating jackets are insulators with built-in heating wire, the jackets may include covers of different materials. Covers are used to prevent UV degradation, unintentional damage, moisture permeation or environmental wear. (For more about heating jackets, see the sidebar, “Heating Jackets Provide Temperature Uniformity and Ease Installation.”)
Fiberglass mat can be cut and wrapped around pipe, components or vessels. Fiberglass has a low thermal conductivity, and it is suitable for high temperature applications; however, it requires an outer layer.
FIGURE 4. Foam pipe insulators can be used for low temperature applications.
A fiberglass-insulating jacket is made by sandwiching insulation between two pieces of material selected specifically for the application (figure 2). Outdoor applications require moisture-resistant fabrics to prevent water or other liquids from saturating the insulation and decreasing performance (reduced R-value). The jacket also may protect the insulation from abrasion or unintentional tearing.
Calcium silicate is made from limestone and diatomaceous earth. The material is molded into required shapes for pipes, fittings, components or flat sheets. It has high compressive strength and is abuse resistant. Also, some variations are water resistant. It has a low thermal conductivity value; however, installation of calcium silicate insulation may be time-consuming.
Mineral wool is made by spinning the fibers of molten minerals such as slag and ceramics. It can be shaped and stiffened by adding binding materials. Thermal conductivity is lower than fiberglass, and density is higher. The higher density means it is much less likely to fold or slump.
Silica aerogel works well in many industrial applications because it has low thermal conductivity and a profile up to 75 percent thinner than other insulation materials. The material is moderately durable and can be removed and reused. Important properties include its hydrophobicity, meaning it is resistant to moisture. (Moisture can cause corrosion on surfaces under the insulation that may not be noticed under casual inspection.)
Polyimide fiber is a lightweight, reinforced insulating blanket with low thermal conductivity. It is more durable than fiberglass, and it is resistant to acids, hydrocarbons and most solvents. It is drape-able for easy installation and can be used as insulation in component jackets (figure 3).
Silicone sponge can be manufactured in different ways that impact the overall performance; however, application temperatures are lower. One drawback is that silicone sponge often lacks resistance to wear. A silicone jacket may be placed on the outside of the sponge to increase durability. The cost of silicone sponge is often higher than that of fiberglass.
Elastomeric foam is lower cost compared to silicone sponge; however, it has a lower temperature rating (250°F (120°C)). Tubular foam is flexible, even at colder temperatures, and is used in industrial and residential applications. This material is well suited for freeze protection, insulation of hot water pipes and condensation protection (figure 4).
After determining which type of material may be suitable, it is necessary to determine insulation thickness based on the application. The thicker the material layer is, the less the heat loss. Insulations are available in different densities, which will impact the thermal conductivity.
To determine the thickness, consider required surface temperature. If the temperature exceeds 140°F (60°C), it may be necessary to add protective guards to maintain a safe working environment. The cost of guards vs. the cost of thicker insulation should be weighed, and physical barriers that may prevent access for maintenance should be considered.
FIGURE 5. Components such as valves can be significant sources of heat loss and should be insulated.
Proper heater and insulator installation are critical both for energy efficiency and for the quality of the material flowing through the system. Regardless of whether it is fuel gas, heating oil, steam or honey, the process has special requirements for the exiting fluids. So, proper fit and positioning are critical to meeting these requirements.
Self-regulating cable, flexible heating tapes, heat-trace cable and mineral-insulated (MI) cable are potential options for component heating. Select a heater with the required wattage and coverage area based on the expected heat losses for insulation to be used, and follow the manufacturer’s installation instructions. When heating simple shapes, the options can include cost-effective off-the-shelf jackets that are are able to provide good surface coverage as required to heat and maintain temperature.
Insulation should fit securely, without gaps where heat may escape. It should not be the means to secure the heater in place. Cloth jackets have various fastener types and are designed to ease removal and reuse. The jackets are engineered to match the component’s dimensions. A tight and secure fit ensures that the temperature sensor and heating wires are in direct contact with the component. All pipe, fittings and components should be insulated, leaving no gaps between insulators (figure 5).
In conclusion, heat loss can be a major concern for all companies, not just manufacturers; however, heat loss can be reduced by careful system design and inspection. Utilizing these tips will help reduce heat loss, increase system efficiency and save money. Insulation selection and proper installation are key factors in system design and require careful consideration. A qualified heating company can be consulted for application support.
Heating Jackets Provide Temperature Uniformity and Ease Installation
FIGURE 6. A cloth heating jacket with controller provides optimum heat and energy efficiency.
Cloth heating jackets combine form-fitting insulation and the heat required for a process requiring heat. These can be custom engineered for piping, flow-control components and vessels. Things to consider are:
- Can a heating tape, cable or blanket be placed around the components to maintain good surface contact and uniformity?
- Will insulation need to be removed for maintenance and how time consuming is the process?
FIGURE 7. Vacuum chambers can be another challenge for heating and reducing heat loss. The vacuum chamber jacket requires both heat and insulation.
Valves, pumps, flow meters, flanges, strainers and other flow components pose special challenges. Their complex shapes and variable masses make it especially important to properly heat and insulate as much surface area as possible. If a 4” gate valve has hot gas flowing through it, the valve will quickly lose heat due to the mass of the valve body, which acts like a heat sink. The exposed surfaces of the valve present more area for heat dissipation, and direct contact with a heater is more difficult. A custom cloth heating jacket (figure 6) can include the heating wire, insulation, sensor and controller for maximum efficiency.
FIGURE 8. Heaters can be incorporated to achieve better performance. Shown here are embedded heating tapes.
Vacuum chambers can be another challenge for heating and reducing heat loss. The vacuum chamber jacket (figure 7) requires both heat and insulation. Applying heat with individual tapes and maintaining uniformity would be difficult. Sewing heaters into the jacket greatly increases temperature uniformity, eliminates potential gaps and greatly decreases installation time. Figure 8 shows how heaters can be incorporated to achieve better performance.