Thermal oil systems
What is a thermal fluid system?
A liquid phase thermal fluid system uses a flooded pressure vessel (heater) in which the heat transfer media (fluid) is heated but no vaporization takes place within the vessel.
Why use thermal fluid instead of steam?
Below it the a comparison show us the reason
DESCRIPTION OF THE SYSTEM
Thermal oil heater is a heat carrier (thermal oil) heating plant comprised of a heater, circulation pump, expansion tank, storage tank, DE aerator, piping and a control panel.
1. Heater: thermal oil heater is usually of the tube coil type. It can be heated electrically or fired by oil fuel or gas to raise temperature of the thermal oil.
example of thermal heater
1. Pump: thermal oil which circulates through the system is firstly raised to a temperature of about 300 degree C in the heater. The heated fluid is then circulated through the system and the heat energy is transferred to the heat consuming devices. The heat energy of the thermal oil is then utilized for heating purposes. A temperature drop of approximately 40 degree C in the consumers is usually anticipated. After the heat has been absorbed, the thermal oil is returned to the heater by means of a circulating pump.
2. An expansion tank: is provided in the system to take up expansion of the thermal oil when it is heated. A de-aerator is sometimes installed for removing air in the system. A storage tank, having a capacity to contain all the oil in the system, is also installed. In case of any emergency or repairs, the entire contents in the system can be emptied into this storage tank.
Typical Thermal Fluid Heat System
REQUIREMENTS OF HEAT CARRIERS
The main requirements for an ideal heat carrier are:-
a) High boiling pint.
b) Low solidification temperature.
c) Good thermal stability.
d) Low viscosity.
e) Good heat transfer properties.
f) Low corrosion tendency.
g) Nontoxic and odorless.
Some common thermal fluids include:
1) Dow Dowtherm A, G, RP
2) Monsanto Therminol 55, 59, 66
3) Paratherm NF, HE
4) Petro Canada Calfo AF, Purity FG
Start-Up and Shutdown Procedures
Regardless of system design, size or heat source there are a few basic procedures that should be followed. when starting up or shutting down your heat transfer system. Following these procedures will help maximize your fluids life.
1) A fluid at room temperature may have a viscosity as high 100°Cst but if the system is outdoors and the ambient temperature is below 32°F or 0°C the viscosity could be 1000°Cst or higher.
2) While a fluid with these viscosities is quite easily pumpable it is not yet ‘ready’ for full heat. Your heater whether small or large is designed to apply heat at a set rate in consideration of the fluids flow or velocity.
3) When a fluid hasn’t achieved the ideal viscosity its flow or velocity will not be of that specified and required by the heat source. Basically the fluid will be too thick to allow for efficient flow.
4) If a heater is allowed to fully fire during these periods it will most likely overheat and thermally degrade the fluid. Basically the fluid will move too slow past the heater and absorb too much heat.
5) Therefore when starting up any system it is important to allow for gradual temperature increases until the fluids flow or velocity is with the range required by the boiler.
6) Generally a 20° to 30° incremental increase in the set point will allow for steady even heating without the chance of overheating or thermally degrading the fluid.
When shutting your system down a few basic steps will help ensure that no damage from overheating is inflicted on your thermal fluid.
1) During the course of normal operation your heat source whether a small or a large boiler will be cycling either on and off or from a low fire to a high fire in order to maintain your set temperature. As well within a short period of time the heater piping or vessel will become nearly as hot as the heat source itself. It is also important to remember that your heater is actually hotter than your output temperature.
2) The actual temperature at the impingement point of the heater in most cases will even be higher than that of the recommended maximum fluid temperature for your fluid.
3) If a system is shutdown abruptly without allowing the heat source and adjacent areas to cool when the fluid ceases to flow it will become trapped and subsequently burn’ or thermally degrade Therefore when shutting down any system it is important to simply allow the fluid to cool below 250°F (121°C ) before shutting down the pump.
4) The use of a heat exchanger or leaving your heater blower running will help expedite cooling the fluid temperature to under 250°F (121°C).
Preventing Fires in Thermal Oil Heat-Transfer Systems
As the thermal fluid heating systems include fuel, air and an ignition source, the risk of fire always is present.
However, plants can minimize the risk of fire by strictly observing proper installation, maintenance and operating procedures.
Fire safety in thermal fluid systems depends on three measurements:
• Flash point,
• Fire point, and
• Auto-ignition temperature
The flash point of a fluid is the temperature at which sufficient vapor is generated for the fluid to flash when exposed to an ignition source.
Two common methods of determining a flash point are:
1) The Cleveland Open Cup (COC) test method, which complies with American Society for Testing and Materials (ASTM) D92, uses an open cup partially filled with a fluid sample.
2) The Penske-Martens Closed Cup (PMCC) method, which complies with ASTM D93, uses a container that is closed except for a small opening through which the fluid’s vapor is exposed to a flame.
The fire point is the point at which a fluid generates sufficient vapor to support continued combustion. The fire point typically is 40° F to 100° F hotter than the flash point. The Cleveland Open Cup (COC) test method is used most frequently to find the fire point.
The temperature at which a fluid will ignite without any external source of ignition is the auto-ignition temperature (AIT).
When should a pressurized expansion tank be used?
1) The tank is not the highest point in the piping system.
2) The tank contents can be at a temperature such that exposure of the fluid to the air would cause degradation of the fluid.
3) The fluid is operated above its atmospheric boiling point.
4) The fluid manufacturer recommends the use of an inert blanket.
Expansion Tank – Fluid Level
1) A minimum liquid level must be maintained in the expansion tank to prevent pump cavitation.
2) A liquid level switch or similar device is typically provided and interlocked with the pump and burner to shut them down in the event of a low fluid level condition.
3) The switch should be satisfied before the pump can start.