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 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.