Operating a Hot Oil Heat Transfer Fluid System is not too hard when you know the basics.
Operating a hot oil heat transfer fluid system can be easy if you understand the components of a system, how to start-up & shutdown the system properly and general operational procedures. Some other things that you need to know are how to drain your hot oil system, how to recharge the system and the proper start-up procedure after recharging with new heat transfer fluid.
System components and their function - The system is made up of a pump that pushes the heat transfer fluid though an insulated piping system to a heater to the processing equipment. There is an expansion tank on the system to allow for the expansion of the hot oil as it gets heated up and contraction when the hot oil is cooled down. To remove particulate from the system, some systems have in-line or side stream (preferred method) filtration units. The in-line filters 100% of the flow, where the side stream takes 10% or less of the system flow rate.
There are two types of pumps used in a hot oil system. There is a gear pump (not as commonly used) and centrifugal pump (the preferred type of pump because it can allow higher flow rates to insure that there is turbulent flow through the heater). The pump is like our heart. It keeps the heat transfer fluid flowing and if it ever stops, there will be major problems with the system. For us, if our heart stops, it means death. So we need to do everything in our power to keep that pump pushing the heat transfer fluid through the system when the heat is on! So, hooking your pump up to a battery/generator backup system would be a smart thing to do to make sure you are covered during an electrical outage.
There are many types of heat transfer fluids. I prefer the non-hazardous, non-toxic, organic petroleum based heat transfer fluid. Most systems of any size run very efficient with this type of fluid. Some benefits are that disposal is the same as used motor oil or hydraulic oil and that the oil protects the system from rusting on the inside. There are synthetic heat transfer fluids that require you to meet EPA standards and regulations for operation and disposal. To me, that is just way too much work when the end result with organic fluids provides the user with a safe and efficient alternative. However, there are certain applications that only a synthetic product will do. I like to think that the heat transfer fluid in a hot oil system is similar to our blood in our circulator system. Healthy blood makes us live longer and run more efficient and a heat transfer fluid is no different in a hot oil system. So why wouldn't someone put something in their system that would promote a safe and health system?Piping is like our veins in our system. It carries the fluid from one location to another in the most direct path. There is rigid piping where welding them together is best because the viscosity of the fluid is so thin at elevated temperatures that it can find it's past threads and seeps out of the system. There are also flexible pipes and hoses that can be used in a system. These are perfectly fine, but just know that if there is not a perfect seal on the connections that you will get some seeping of the oil. For organic fluids, there are only three materials you cannot use - cooper, aluminum and brass. These materials are oxidation catalysts and that is one of the ways to break down an organic heat transfer fluid. I recommend staying with steel or stainless steel for all your piping needs.
Like everything else, there are many types of heaters. Heaters are classified by the amount of BTU produced per hour and the fuel that is used to generate the heat such as electric, gas, oil and wood. You can contact a heater OEM for specific recommendations to meet your application needs. Some systems are smaller in size and come with a pump, heater and expansion tank so that all you have to do is hook your hoses up to them from your application and off you go. Other systems are not that easy. They all have separate components that are purchased and installed when the piping is run. I like to think of systems according to the amount of heat transfer fluid required in the system. Most of the smaller heaters described above can handle a volume of 20 to 500 gallons. Installed systems could range from 300 to 60,000 gallon system.
There are many different processes or applications for hot oil systems. These processes are better known as the users. Some applications include heating dies, cooling dies, heating molds, cooling molds, heating reactors, heating vats, heating process type machines like presses, heating rollers, heating storage tanks, and the list goes on and on. The main thing to know is that each application has it's own specific requirements and each system is designed to meet those requirements. The smaller systems are relatively simple, but the larger systems can become very complicated and over time companies continue to add onto them. The larger systems require design work from engineering or consulting firms and can take many months to design as well as install.
The final component of a hot oil system is the expansion tank. This tank is critical to the operation of the system. Its main purpose is to allow some place for the heat transfer fluid when heated to expand into and when cooled a source to draw fluid from to keep the system full. It is also a built in reserve tank for your system just in case you have a leak it keeps your system full. That is why it is important to keep an eye on the level in your expansion tank daily. If the level drops from its normal position, that means you have developed a leak somewhere. Some general rules of thumb are to fill the expansion tank up 1/3 full when the system is cold. When the system is running hot, it should be 2/3 to ¾ full. There are usually two pipes or legs that run to an expansion tank and when running, one leg needs to be closed to prevent thermal currents from running into the expansion tank and heating the fluid in that tank up. I like to see the temperature of the expansion tank to be less then 140°F (60°C). The reason for this is to prevent oxidation with the air inside the tank. If you have no choice and need to have your expansion tank run hotter then 140°F (60°C), then what you need to do is install a nitrogen blanket on the head of the tank to remove any oxygen molecules and prevent oxidation.
The one thing to note from this section is that you need to define your application requirements first and then determine which system design best fits your needs. However, for every hot oil heat transfer fluid system you will need all the above components.
The Start Up Procedure - This procedure is very simple but I estimate that less than 20% of all hot oil system operators are doing this. So let me inform you the proper start up procedure for a hot oil system.
For example, if you are running MultiTherm PG-1 in your system. You will reach 10 cP at 132°F (55.5°C). So from ambient to 132°F you may have to do 2 or 3 increments before dialing the heater to your 340°F operating temperature.
Things to watch for are the level in the expansion tank should rise as the heat transfer fluid expands. If it does not, you may have a line clogged. Also, at the 200°F (93°C) temperature, you may occur some pump cavitations. If you do, that means you have water or some light end molecules that are low boilers in you system.
The Shutdown Procedure - This procedure is more than just going to the heater and shutting the pump and heater off at the same time. If you do your shut down this way, the residual heat that is left in the heater can exceed the film temperature of the oil and thermal crack the oil that is not moving through the pipe. What we would suggest you do as a shutdown procedure is:
We recommend that you have your pump on some type of auxiliary power source so that when your facility looses power for what ever reason (and there are many) or something happens to the main power source, the pump continues to run and push the fluid through the system. What I have found is that this type of thing is a major reason that the heat transfer fluid in your system starts to breakdown. By-products of thermal cracking are a heavy end molecule (made up of 90-95% carbon) and a light end molecule or low boiler.
Standard Operating Procedures - Once you get the heat transfer fluid system up and running, it should run pretty much on its own. But here are some daily things to keep an eye on when operating your system:
These types of things do not require much time. If you train your operations team, everyone can do this on a normal working basis and when they see something out of the normal, they can report it to the right person to take action and identify the problem before it gets to the point of an unscheduled down time that costs the company money and you a headache to try to fix the problem as quick as you can.
Now your system has been drained of the heat transfer fluid. If you drain it as hot as you can, you will be able to remove almost all the sludge and particulate in your system. This procedure does not remove any of the caked on or carbonized material in your system. If you are looking to do that, you should consider either a process system cleaner or flushing fluid from MultiTherm. Call for details.
Recharging Procedure - Recharging the system with either new heat transfer fluid or flushing fluid is just draining the system in reverse and pumping the fluid into the system. Make sure you have enough new heat transfer fluid to fill your system. There is nothing more aggravating than not having enough fluid when recharging the system. If this happens to you, do not settle for using some of the used fluid or even something that is not acceptable to mix with the new oil. Call MultiTherm for recommendations if you are caught in this situation. Follow this procedure to recharge your system:
Starting the System after Recharging Procedure - Starting the system after a recharge is much different than starting your system after shutting it down. Below is the procedure to restart your system after a recharge: