Frequently Asked Questions
1. What is a thermal fluid system?
ANSWER: Also known as hot oil (or sometimes Dowtherm) systems, these are industrial heating systems wherein a special heat transfer liquid is recirculated by a pump, through a fired heat exchanger, where its temperature is raised for use in heating various processes.
2. What are some typical applications?
ANSWER: Usually used in the range from 200° to 750° F, typical applications include:
- Heating press platens such as OSB and plywood presses, laminating presses, rubber and plastic molding presses, and circuit board presses.
- Heating calender and drying rolls such as for non-wovens, papermaking felts.
- Heating chemical, petrochemical and other process equipment such as reactors, heat exchangers, dryers, evaporators.
3. What are the advantages and drawbacks of these systems?
ANSWER: The main advantage is that the systems run at a very low pressure, typically just the pressure required to pump the liquid around the system. When designed correctly they reliably and efficiently produce high temperature heat. They are simple to maintain with only the burner and pump needing regular maintenance.
Disadvantages include the fact that the heat transfer fluids used are flammable and have a low viscosity so they are prone to leakage. The fluids can also break down (thermally and by oxidation) in poorly designed systems causing deposits and sludge that hurt the system performance.
4. How does the heater work?
ANSWER: The heater has cylindrical coils through which the recirculating fluid passes. A gas or oil fired burner fires into the center of the coil system heating up the fluid. Typically the fluid temperature rises between 20° and 100° F between the heater inlet and outlet. The TFS heater has a special three-pass flue gas arrangement for high efficiency.
5. What kind of burner can be used?
ANSWER: TFS can supply any of the national brand burners firing gas, #2 through #6 oil, and waste fuels if required. The burner model is selected and the application engineered to the exact needs of the particular heater.
6. What kind of pump can be used?
ANSWER: Up to 600° F inexpensive air-cooled mechanical seal pumps may be supplied. For higher temperatures water cooled or magnetically coupled pumps can be used. In some applications API pumps may be preferred.
7. Are there any special requirements for isolation valves?
ANSWER: TFS recommends the use of valves with a leak-proof and maintenance-free metal bellows sealing arrangement. The initially higher cost over a packed valve is offset by the minimal maintenance requirements.
8. What is the pump strainer for?
ANSWER: This strainer has a coarse mesh screen that stops solids bigger than about 1/8" that could damage the pump. A sidestream filter with a much smaller screen is sometimes used for systems that have small solids resulting from fluid breakdown in circulation.
9. What is the degasser for?
ANSWER: This is a specially designed tank located at the pump suction that helps to separate any air or vapors from the circulating fluid and vents them up to the expansion tank.
10. What is a Temperature Blocking Vessel?
ANSWER: Our systems have a small buffer tank between the hot circulating fluid and the expansion tank which we call a Temperature Blocking Vessel. As the system heats up, the system liquid expansion flows from the top to the bottom of the Temperature Blocking Vessel to the expansion tank. The initial cold contents of the Temperature Blocking Vessel buffer the expansion tank, reducing its temperature rise. After heat up, the expansion tank is effectively isolated from the system by the Temperature Blocking Vessel and it cools down to near ambient temperature.
11. How big does the expansion tank need to be?
ANSWER: All thermal fluid manufacturers publish data for the volume expansion of their heat transfer fluid from ambient to the system operating temperature. We calculate the initial system volume and apply this expansion factor, add an allowance for minimum and maximum tank fill and select the tank size accordingly.
12. What safety controls are in a typical system?
ANSWER: TFS supplies a comprehensive set of safety controls in all of our systems. All of our heaters incorporate continuous flow monitoring with an orifice plate and differential pressure switch. We have redundant fluid outlet temperature alarms, a high stack temperature alarm and an automatic interlock to allow minimum firing only during warm up, when the oil flow may not be fully established. We provide low and (optional) high expansion tank level alarms. The burner is supervised by an electronic flame programmer with typically a UV flame scanner, and the fuel trains come with low and high fuel pressure alarms and approved block and bleed valves.
13. Is a drain tank always required?
ANSWER: Small systems (up to about 1,000 gallons) often are supplied without a drain tank as it is practical to fill directly from drums. With larger systems the drain tank may be a smaller volume than the entire system as it is rarely necessary to drain the entire system at once.