Designing a thermal fluid system: Both simple and complex

Managing heat flow in a thermal fluid system is critical to providing precise temperature control for process fluids and to protect the equipment involved.

In a system with a fired heater, improper flow could cause the heating coils inside the heater to be damaged or cause rapid degradation of the thermal fluid. To prevent damaging the system, it must be designed to maintain constant flow through the heater when in operation. Some of those designs are illustrated in the accompanying diagrams.

A process fluid could be any type of fluid that requires heat in the manufacturing process including chemical compounds, liquid asphalt, liquid polymer, wax, waste sludge, cooking oil, crude oil, LNG and so forth.

In process heating, you can either heat the process fluid directly or indirectly with a fired heater or an electric heater. Indirect heating uses a thermal fluid (also called heat transfer fluid) such as oil, synthetic oil and water glycol. The thermal fluid is heated by the heater which then circulates through heating coils, an exchanger, or a jacketed vessel to heat the process fluid. The indirect method is a closed loop system, meaning the thermal fluid going out of the heater will return and continuously cycle through the loop.

There are several factors to consider when designing a heating system for proper flow and heat transfer. With vessels containing the process fluids referred to as "users," the following are a few of the factors for consideration.

First, how many users will require heat? You can have a single heater that can provide heat for multiple users. The heater will need to be sized to account for heat loss across all users and the flow rate based on the system design.

Next, will the users run at the same time? If the users are running at the same time the heater will need to be larger whereas if you are only running one user at a time the heater can be sized to accommodate the user that needs the largest duty.

Also, do the users need the same thermal fluid temperature? A system can be designed to accommodate users that all need the same thermal fluid temperature or multiple temperatures at the same time.

Do any of the processes need to cool? Some process fluids may need to cool after being heated. This can be done naturally over time or more quickly with an additional exchanger.

In what timeframe do the processes need to happen? Some users may require a constant source of heat while others may only need heat for a relatively short period of time. And some users may need to be heated up quickly. The quicker the heat up, the larger the heater.

Thermal fluid loops are controlled using a series of valves (typically modulating valves), temperature elements, and pressure sensors to control the flow and maintain precise temperatures. The valves are typically automated and work in concert with the heater controls to make sure the proper temperatures are achieved for each user. When the users reach the desired temperature, bypass piping and valves are used to make sure the heater has proper flow going through it, while also maintaining the temperature of the users.

When one heater is used to heat multiple users, the users will either be in series or in parallel. When in series, a single pump is used to maintain flow over all users and the heater. The flow, measured in gpm (gallons per minute), is the same gpm across all users and the heater. In parallel, a single pump or multiple pumps are used to distribute flow to each user line. An additional circulation pump is sometimes used to maintain flow in the heater. The advantage of a parallel design is the thermal fluid approaches each user at the same temperature, compared to a series design there is heat loss across each user and the downstream users will have a lower approach temperature. You can have a combination of parallel and series systems.

Single line systems

Single line thermal fluid systems are the most basic type of systems, but they can still be quite versatile. Typically, a single circulation pump is used to move heat transfer fluid through the entire system. This type of system can have a single user or multiple users. A single user can have a single heating zone (Figure 1) or multiple zones (Figure 2). With multiple zones, different volumes of product can be heated without heating the entire tank.

Single line multiple users

When multiple users are heated in a single circuit, they can be in parallel (Figure 3) or in series (Figure 4) using a common circulation pump. When the units are in parallel, the thermal fluid approaches the users at a uniform temperature. In this case the circulation pump has a flow rate of at least the combination of all users. When the users are in series, the temperature of the approaching thermal fluid varies based on the heat absorbed by all previous users. This leads to a pump with a lower flow rate and a higher differential pressure.

Figure 5

Multi-line

A multi-line system (Figure 5) is used when there is a single heater and multiple users and you want each user group to have separate thermal fluid loops. A loop (aka circuit) can have multiple users in series or parallel. A single circulation pump can be used to heat the thermal fluid in the heater and side/takeoff pumps are used to circulate thermal fluid through each independent loop. If the required temperature of the thermal fluid is considerably lower than the maximum allowable bulk temperature of the fluid, then the line can pull fluid downstream of their returns. This requires a hotter thermal fluid temperature exiting the heater that will reach the desired temperature for the users after it blends with the cooler return fluid. This allows for a smaller heater circulation pump. If the required temperature is too close to the maximum allowable bulk temperature, then the loops must pull upstream of their returns. This requires the heater circulation rate to equal more than the sum of each loop. An example of this is when you have different sized tanks that heat on independent schedules and require the thermal fluid at the same temperature, but at different flow rates.

Figure 6

Warm oil loop

Warm oil loops (Figure 6) are used when users need to heat with different temperatures of fluid. The main heating loop is the hottest temperature. The less hot loop has its own circulation pump and blends in hotter fluid to maintain a temperature set point. An example of this is when you need to quickly heat a large volume of process fluid in a tank with thermal fluid and maintain a set temperature in the process lines. The process lines are maintained at a set temperature with an insulated oil jacket of the same temperature. This allows for mitigation of line temperature loss without the possibility of overheating the fluid in the pipe that you can see with electric heat trace.

Temperature control loop

Figure 7

A temperature control loop (Figure 7) is very similar to a warm oil loop with the addition need to cool the process. The thermal fluid loop is used to heat the temperature control loop and an exchanger is used to cool the loop. An example of this is heating a process in a jacketed vessel for mixing and/or a reaction and then cooling the process to offload into a temperature sensitive container.

Designing a thermal fluid system can be fairly simple or quite complex. A good design will be efficient, durable, safe, and cost effective. The experts at Astec can help you decide the best type of system for your application, design and build it, and provide support and service.

For more information, call 423-821-5200 or visit us online at astecindustries.com.