Secondary Loops and Control for Thermal Fluid Systems

Thermal fluid systems use an indirect heating method involving the warming of liquid phase heat transfer media like water, glycol, or thermal oil, circulating it to heat energy users in a closed-loop system. Enhancing the capabilities of traditional boilers, thermal fluid heating systems are an optimal choice for their dependability, flexibility, and degree of temperature control. Thermal fluid heating systems achieve this through the use of a primary loop as well as one or more secondary loops.

Secondary loops satisfy a user’s demands by consuming the thermal fluid’s energy, depending on the user’s precise temperature and flow rate requirements. Read on to understand more about secondary loops in thermal fluid systems, their components, and how they achieve heat transfer.

Primary vs. Secondary Loops

The primary loop in a thermal fluid system provides thermal fluid to users while keeping the flow rate and temperature constant, regardless of return temperatures. This loop is typically made up of a heater, one or more circulation pumps, and drain and expansion tanks, with the potential for a flow bypass valve to help provide a heater with continual flow while users are modulating. Any secondary loops fulfill the needs of the process variable by using energy originating from the thermal fluid at the user’s desired flow rate and temperature.

Types of Secondary Controls

The secondary loop in a thermal fluid system manages the energy amount passing into a heat consumer, whatever the system’s consumer type may be, to control a tank’s bulk temperature, process outlet temperature, manage flow rate, and more. These loops fall into two basic categories:

  • Flow control loops. A flow control loop uses thermal fluid at the temperature set by the primary loop and modifies the thermal fluid flow rate using a control valve. How do flow control valves work in conjunction with flow control loops? Installing a control valve helps to adjust the flow rate through the user.
  • Temperature control loops.Conversely, a temperature control loop uses a pump as well as a control valve to manage thermal fluid temperature through the user. The temperature control loop receives thermal fluid at a temperature exceeding that which is optimal for the loop and decreases it until it is under the primary loop temperature.

Flow control loops can function as part of temperature control loops. However, both loops can also be used simultaneously in one system.

Secondary cooling loops, a subsection of temperature control loops, utilize heat exchangers to decrease fluid temperatures. As for how heat exchangers work, these devices transfer heat between liquids, gases, or vapors of dissimilar temperatures while preventing any direct contact between these fluids. Their designs differ in their flow direction, material construction, and method of heat transfer.

There are several types of heat exchangers available depending on the needs of the application, including:

  • Shell-and-tube. These common heat exchangers consist of a group of tubes within a cylindrical shell, each carrying a fluid of a different temperature, and the heat exchange occurs between the two. They are applicable to a variety of fluid types, temperatures, and pressures.
  • Plate-and-frame. These heat exchangers are for low-temperature applications and use multiple plates to separate hot and cold fluids, preventing mixing. They are a scalable option to facilitate future expansion and provide a large surface area for heat transfer.
  • Shell-and-plate. Shell-and-plate heat exchangers incorporate the designs of the shell-and-tube and plate-and-frame varieties. One fluid passes between the plates in a welded plate pack and the other flows through the shell, with ideal heat exchange for high-temperature and -pressure tasks.
  • Spiral. Ideal for protecting against fouling, these heat exchanges utilize two spiral-shaped plates that act as paths for one hot and one cold fluid. They support turbulent flow.
  • Serpentine finned-tube. The fins on the sides of the tube add surface area, thereby increasing the heat exchangers’ capability to heat or cool quickly. Applications with low-density gas typically make use of this heat exchanger variety.

Choose Sigma Thermal

The team at Sigma Thermal will be happy to assist you in finding the right thermal fluid system for your application. With facilities worldwide, we supply customers in an array of industries with design, engineering, and service capabilities for high-performance heating systems. Contact us today to request a quote or learn more about our capabilities for secondary loops in a thermal fluid heating system.