A well-designed thermal fluid heating system can provide consistent, reliable and safe operation for decades. By selecting the right system with properly sized equipment, it can run at peak performance for its owner, optimizing output and production for a specific process. A properly designed thermal fluid heating system is composed of the following four main components plus at least one user:
- A thermal fluid heater.
- Circulating pump(s).
- Expansion tank.
- Catch tank.
Depending on the level of system complexity, one could also use additional system controls, control valves, secondary loops, heat exchangers or countless other system variations. But every system should include — at a minimum — the core pieces of equipment listed above. Here are a few tips to use as a baseline to help size and select each of these core pieces of equipment in a basic thermal fluid system.
Sizing the Thermal Fluid Heater
The heater needs to be sized for the maximum BTU/hr requirements of the system during peak load. If the customer plans on expanding in the future, the future BTU/hr requirements also should be considered. In most cases, the customer knows what size heater is required. Otherwise, the heat required must be calculated by using the following equation:
Q = M x CP x ΔT
Q is the heat required.
M is the quantity of material being heated.
CP is the specific heat of material
ΔT is the difference between final temperature and initial temperature.
This calculation must include the product being heated as well as the vessel containing the product and any piping that carries the hot fluid to the product. Remember that the vessel must heat up in order to heat its contents. Heat losses also must be taken into consideration, and proper engineering practices must be followed to determine an appropriate safety factor.
Determining the Required Flow Rate
In many cases, the standard flow rate of the heater can be used as the system flow rate. This is always the simplest approach. For heaters with a fixed flow rate requirement, having a system flow rate requirement that is different than the heater flow rate presents some additional design challenges. In these situations, if the user requires a higher flow rate than can be passed through the heater, a heater bypass is required to carry additional flow around the heater. If the user requires less flow than is required for the heater, the heater must still see the required flow rate, so then a system bypass may be required to manage the additional flow around the user. Alternatively, a three-way control valve may be considered to divert the additional flow around the user.
When flow rates have to be calculated, it is important to revert back to the equation to determine how much flow is required to remove the appropriate amount of heat from the thermal fluid flow stream. However, for this calculation, we must rearrange the equation to solve for flow rate. It looks like this:
M = Q / (CP x ΔT)
Q is the heat being transferred to the user from the thermal fluid.
M is the required flow rate of the thermal fluid
CP is the specific heat of the thermal fluid
ΔT is allowable temperature drop for the thermal fluid across the inlet and outlet of the user.
Sizing the Thermal Fluid Circulating Pump
In most closed-loop thermal fluid systems, pump selection is little more than determining the total flow and discharge pressure required. The total flow required of the main circulating pump can be determined by summing the flows of the individual users. The discharge pressure can be determined by summing the pressure drops of all piping, components and users in the flow path between the discharge of the pump and the pump suction connection. These values can be taken to a pump curve to plot and determine the best pump for your system. It is important to remember to convert the discharge pressure required from pounds per square inch (psi) to feet of head, and to correct for the specific gravity of the fluid at operating temperature. From the pump curve, one can determine the net positive suction head (NPSH) required, the specific impeller size required and the efficiency of the pump. One also can get a feel for what motor horsepower is required to drive the pump.
Sizing the Expansion Tank
To properly size the expansion tank, one needs to know the total system volume, the maximum operating temperatures and the specific thermal fluid used in the system. First, determine the total system volume, including the heater, the piping, the user and the initial-fill volume of the expansion tank (the amount of thermal fluid required to meet the minimum level in the tank. Combined, these amount to every single drop of thermal fluid in the system. Thermal fluid manufacturers publish expansion rates for the fluids they manufacture, so this information may be located on the fluid data sheet. Each fluid expands at a different rate, and the expansion is temperature dependent, so it is important to use the numbers for the specific fluid being utilized in the system. Multiplying the total system volume by the expansion rate gives you the volume required for expansion. The tank provided for the system must have enough expansion capacity to handle the volume required for expansion.
Sizing the Catch Tank
A catch tank should be provided as part of each system to safely collect the discharge from the system’s pressure-relief devices, vents, drains and any other fluid escape points. Because there is no national code to dictate the size of a catch tank, there are several schools of thought for sizing the tanks. One is that the tank must be sized for the entire thermal fluid system volume. The idea is that in the event of a relief valve discharge, for example, the worst-case scenario is that the entire system will discharge, and it needs a safe place to be collected. This is certainly the most conservative approach, but many large systems can be thousands of gallons. Such a large tank may be prohibitively expensive or may push the footprint of the system outside of the allotted space.
Another school of thought is that the catch tank should be sized large enough to handle smaller scale events like thermal fluid “burping” out of the pressure-relief valve. Such events are only momentary, and the volume of fluid discharged is often only a fraction of the entire system volume. For such system designs, a 100- or 200-gal catch tank often is suitable, sometimes upsizing to a 300-gal catch tank, depending on the system size. (Fifty-five gallon drums are not recommended.) NFPA 87 also provides some guidance regarding catch tanks.
In conclusion, much more goes in to a properly designed thermal fluid system than the selection of the four main components mentioned already. For instance, if control valves are included, they must be sized appropriately to balance proper control with pressure drop considerations. The piping also must be sized to balance pressure drop and velocity versus heat transfer efficiency or the cost of installing oversized pipes. Other factors such as whether to blanket the system with an inert gas or whether to include secondary loops also must be considered.
Even with the tips provided in this article, there are additional choices to be made. These guidelines only touch on some of the information required to design a proper thermal fluid system, and this article should by no means be considered all-inclusive for what is required to design a system. Each system is unique and must be engineered on a job-by-job basis. But, the tips in this article should provide guidance to begin designing a safe thermal fluid system that will provide peak performance for many years.
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