Hot Water vs. Electric Heat Tracing
A confectionery plant commissioned an investment study to compare capital costs for the total installed costs and the total operating costs of both hot water heating systems and electrical heat-tracing systems for process temperature maintenance of a chocolate-carrying pipeline.
Traditionally, the decision on which type of heating method used for any given system is typically made early in the project cycle, and with no cost or technical evaluation performed prior to that decision.
Although there are several heating technologies available, the decision to go with one over the other is typically dependent on what was done on the previous project, thereby bypassing a proper technical and cost evaluation of all available heating systems. The end result could be a new pipeline installation with an inadequate or costly heating system.
A case history demonstrates how an investment study, which looks at total installed costs (TIC), total operating costs (TOC), expandability, flexibility and accuracy, can benefit. The study looked at a typical food manufacturing plant to compare capital costs for both hot water heating systems and electrical heat-tracing systems for process temperature maintenance on pipes and vessels. The objective was process temperature maintenance of chocolate-carrying process piping, and the study utilized the investment study factors (total installed costs, total operating costs, expandability, flexibility and accuracy) to make the final recommendation and selection.
Process Liquid: Chocolate. Normally, chocolate is held in storage between 110 and 115°F (43 and 46°C) and will remain in a fluid state down to approximately 88°F (31°C). However, as the temperature of chocolate drops, the fluid viscosity increases rapidly. Too much heat causes damage (burning) and insufficient heat allows the material to become solid; therefore, consistency is vital for process heating.
Chocolate typically is transported (pumped) through fixed piping systems, and double-walled pipe is common. Based upon this, a hot water system may seem like an obvious, low-cost solution for temperature maintenance. However, temperature controls on these lines are usually rudimentary, without the use of temperature control loops.
Hot Water Tracing
Hot water tracing systems are common due to their simplicity and economy. Hot water for process pipe heating is produced from heat exchange utilizing either steam or electricity. As steam is the most common heat source for process heating, most plants do not consider the costs of heating water for pipe tracing.
Hot water can be used for process pipe temperature maintenance for single- or double-walled pipe systems. Single-wall pipe systems use a tracer tube to carry the hot water next to the process pipe. Double-wall pipe systems can have one of two configurations: process fluid within the pipe with hot water around the pipe, or the reverse.
Hot water can run directly through a tracer tube or jacket. But, because it has finite heat capacity and cools continuously as it flows, the pipe temperature varies. Consequently, hot water tracing systems are ineffective at maintaining a narrow temperature range, and they have a practical temperature limit of 210°F (99°C).
Despite these limits, hot water tracing systems offer some advantages. A hot water source is normally present in most plants. The systems have low technology and service requirements for personnel. It is relatively easy to add equipment and pipe to existing systems when process requirements demand, and insulation is optional due to excess heat capacity.
Hot water tracing systems also have several disadvantages. It is difficult for the water system to adjust to system temperature variations. Flow issues can also be a concern, as the overall system flow and pressure drop can be difficult to balance and manage. Accurate temperature control and monitoring present challenges, particularly as more control is managed remotely. It is difficult to manage water systems remotely due to the simplicity of installation. Water systems also require auxiliary equipment -- water pumps, holding tanks, heat exchangers -- that need mechanical and electrical maintenance. Regular maintenance is required to sustain a leak-free system. And even with regular maintenance, there exists the potential for product contamination due to leaks.
Tables 1 and 2 document the total installed costs and total operating costs of two hot-water systems for the chocolate-carrying piping. Table 1 shows the costs of an insulated system while table 2 shows an uninsulated system. These cost comparisons are based upon a 200' total length using 2" stainless steel pipe with 3" stainless steel jacket, including the costs of constructing a hot water system. The hot water system assumes existing hot water capacity.
Electric Heat Tracing
The main component of an electric heat-tracing system is electric heating cables, which are small, flexible and easily installed. They can be controlled to supply only the heat required to maintain the prescribed pipe temperature. Almost any level of monitoring can be installed on an electrical heat-tracing system. Electricity is almost universally available in plants and in a form that is easily used for pipe tracing applications. Electric heat-tracing systems can be used for process temperature maintenance for single-walled piping systems and for heating control equipment such as flow meters, pumps and control valves.
Electric heat-tracing systems are attached directly to the surface of the pipe. The system is designed to provide the amount of heat necessary for the application. A temperature sensor or RTD must be utilized for maintaining narrow temperature ranges. They are especially good at maintaining narrow temperature ranges and have numerous temperature ranges available.
Self-regulating electric heat tracing offers several process advantages. It continuously adjusts its heat output based on the ambient conditions. With the use of a temperature sensor or RTD, the heating cable can adjust its heatup time and maintain a temperature within a narrow range. It is expandable, and installing additional heat-tracing to adjacent pipe or equipment does not affect heating performance (up to a maximum circuit length). Self-regulating electric heat tracing can be controlled to a much narrower temperature range than hot water. Auxiliary equipment is not required, so the total operating costs include only electrical usage for the electric heat tracing and the control system. It allows remote temperature control and monitoring. Its temperature controller can be connected to an Ethernet-capable company intranet system for remote monitoring of the entire system. It allows single-wall pipe specification, unjacketed valves, equipment and pumps to be used, reducing the total installed costs. Finally, because the tracing runs on the outside of the single-wall pipe, there is no potential product contamination.
Table 3 shows the total installed costs and total operating costs for an electric heat tracing heating system for the chocolate-carrying piping. The electrical heat-tracing example is based upon 2" stainless steel pipe, with pipe insulation and controlled by an individual temperature controller. Temperature indication is accomplished by thermocouple.
Table 4 summarizes the costs of the three systems compared.
Ironically, temperature maintenance systems for chocolate-carrying process piping are often the least engineered and yet most important systems in a typical confectionery plant. Experience and existing systems typically govern the decision-making process, to problematic results. This case study has demonstrated that utilizing electric heat tracing for process temperature maintenance can achieve both economic and engineering success.
Sidebar: 10 Tips: Heat Tracing for Industrial Processes
- Consider an investment study, which looks at total installed costs, total operating costs, expandability, flexibility and accuracy, before specifying new process heating systems.
- Because hot water has finite heat capacity and cools continuously as it flows, the pipe temperature varies.
- A hot water source normally is present in most plants, and the systems have low technology and service requirements for personnel.
- Water systems require auxiliary equipment such as water pumps, holding tanks and heat exchangers that need mechanical and electrical maintenance.
- Electric heat tracing systems can be used for process temperature maintenance for single-walled piping systems and for heating control equipment such as flow meters, pumps and control valves.
- A temperature sensor or RTD must be utilized with electric heat tracing to maintain a narrow temperature range.
- Self-regulating electric heat tracing continuously adjusts its heat output based on the ambient conditions.
- Electric heat tracing can be controlled to a much narrower temperature range than hot water.
- With electric heat tracing, the temperature controller can be connected to an Ethernet-capable company intranet system for remote monitoring of the entire system.
- Because electric heat tracing runs on the outside of the single-wall pipe, there is no potential product contamination.