Operating A Hot Oil Heat Transfer Fluid System

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.

  1. First you start your system pump to get the fluid flowing through the system.
  2. Second, after you know you have good flow, you apply the heat. Heat should be applied in 20°F (11°C) to 25°F (14°C) increments until the heat transfer fluid gets to a viscosity of 10 cP (centipoise) or less. The reason for this is to insure that you have turbulent flow through the heater (where the heat transfer fluid can remove just as much heat as the heater can supply to the coil in the heater) and there will be no thermal cracking of the oil. 
  3. These increment steps are done by taking a system from 70°F (21°C) and increasing the heat by 20°F (11°C) and letting the system run until the temperature on the heater reads 90°F (32°C). Once the heater reads that then dial it up another 20°F (11°C).
  4. Once your heat transfer fluid is 10 cP or less, you can dial the heater to your operating temperature.

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:

  1. Turn off the heater first.
  2. Let the pump continue to circulate the heat transfer fluid to remove any residual heat that is in the heater, the process and the pipes. For some systems this may take awhile depending on your system size. The reason for this is to make sure that when the pump gets turned off, the residual heat in the various components of the system does not thermally crack the heat transfer fluid.
  3. Once the temperature has dropped to 200°F (93°C) the residual heat has been removed and you are safe to shut off the pump.

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:

  • Check the temperature of the oil returning to the heater as well as the oil exiting the heater. When your system is running smooth & efficient, the temperature difference should remain constant. If the temperature difference increases, that is an indication that something is changing in your system.
  • When checking the temperatures, you should also check the pressure drop across the pump and through the heater. If there are no changes in pressure drops, everything is fine. If there are changes, then your system is telling you there is a problem.
  • Check the pump out - is it running smooth, making noise, leaking oil or what appears to be smoking? 
  • Check the heater out - walk around it and make sure that the outer case is ok. Check for leaking oil, burning of paint,   nothing appears to be loose or out of place. If there is something that changes, call your heater manufacture immediately. If you have a flow meter, make sure that the flow rate does not change.
  • Check the expansion tank - is the paint burnt off, are there leaks, does it make noise, does it smell, is the oil at the normal level, does the level tube appear to be clogged, is there what appears to be smoking coming from the vent pipe?
  • Walk the piping system - check all elbows and connections for leaks (if there is -- never open up insulation to find the leak   when the system is hot - this is one of the ways you can start a fire - let the system cool down before investigating the leak), any weird odors, all gates or vents or valves appear to be operating properly, is there what appears to be smoke coming from any of the sections, any different noises from normal?
  • Check the filter system - if the pressure drop is the same, everything is ok and the filter does not need replacement. If there is a greater pressure drop, the filter is being filled with particulate in the oil and needs to be replaced.

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.

Draining Procedure - Draining the system of the heat transfer fluid is not very difficult but it is rather messy and time consuming. If you do it right, you can remove almost all the heat transfer fluid from your system so that when you recharge your system with new heat transfer fluid from MultiTherm, your system will operate more efficient. Here are the general steps you need to do to drain the fluid from your system:
  1. Shut your heater or heat source off and let your pump continue to circulate the oil through the system. You need to remove all the residual heat from the system as well as let the oil cool down.
  2. Once the oil is at a safe temperature to drain from your system, shut off the pump and allow fluid to stop circulating.
  3. If you have a nitrogen blanket on the expansion tank, turn that off.
  4. If you have high point vents, you can open them.
  5. Make note of all positions of closed or partially closed valves or gates, once you make note of all their positions you can open them all up, please do not forget the closed leg to the expansion tank.
  6. Attach your hoses to all the low point drains. In most cases, the two low point drains will be at the pump and another at the process/user.
  7. Use a secondary pump (NOT THE SYSTEM PUMP) to pull the fluid out of the system and into either an empty drum, tote or tanker (depending on the size of your system). Make sure that you label the containers as used heat transfer fluid. You do not want to accidentally put a drum of the used fluid in your system.
  8. When you think you have all the oil out of the system, allow the system to sit for 10 to 15 minutes and try pumping again, some times it take some of the more viscous fluid to settle to the low points.

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:

  1. Once you feel that you have removed all the used oil from the system. Use the secondary pump (DO NOT USE YOUR SYSTEM PUMP) to draw new oil from the container and push it into the system.
  2. First pump new fluid into the system from the low point at the process/user. If you leave the drain open at the system pump or high point vents open and fluid starts to come out, that is the indication that you have filled all the piping at the process/user side. You will need to repeat this at all the process/user low points.
  3. If you have high point vents, you will need to close them.
  4. Once all the process/user loops are all full, attach the secondary pump to the drain at the system pump and start pushing fluid into the system. 
  5. Keep an eye on the expansion tank; keep filling the system until the expansion tank is 1/3 full.
  6. Make sure that all drains and vents are closed tight.
  7. Reset all your valves or gates to the same positions that you had during normal operations, except the one to the expansion tank. Leave both of the legs to the expansion tank open and if you have a nitrogen blanket on the tank do not turn that on yet.
  8. Make sure that the pump did not loose its lubrication or prime on the backside of the seal. If you do not do this step, you will burn your seal out and will end up replacing it in the near future. If you are unsure what you need to do, call your pump manufacture for guidance.

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:

  1. Start your system pump. Do not apply heat yet. Allow the fluid to circulate and remove any air pockets in the system. The air pockets will find their way to the expansion tank and thus out of the system. You may hear some funny noises and when they go away, it is safe to say that the air pockets have been removed and system circulation is back to normal. Also, make sure that your expansion tank is open to atmosphere.
  2. If the expansion tank level has fallen below 1/3 full, you will need to pump some more new oil into the system using the secondary pump through the drain at the system pump location.
  3. Once the system is circulating fine and you have the proper level in your expansion tank, it is ok to apply the heat.
  4. Apply heat in 20°F (11°C) to 25°F (14°C) increments until the heat transfer fluid gets 195°F (90°C). Hold this temperature and walk the system to make sure everything is fine.
  5. Increase heat to 200°F (93°C), this is where water starts to boil and thus if for some reason you got water in your system during the draining & recharging procedure you will find out. If the pump starts to cavitate or if you get spitting and spurting of hot oil out of the expansion tank, you got water in your system. If you have water in your system, make sure that the pipe from the expansion tank is pointed into an empty drum or in a safe location that it cannot hurt anyone. Allow this to go on until the pump cavitations and the spitting and spurting from the expansion tank stops. Depending on the amount of water in your system, this could take a long time (hours and even days).
  6. After all the signs that there is no water in your system. Turn the heat up to 220°F (104°C) to make sure there is no water. Run system at this temperature until you feel comfortable that there is no water in the system.
  7. If there is no water and you have a nitrogen system for the expansion tank, this is the time to turn it back on.
  8. Close one of the legs to the expansion tank.
  9. You are now safe to dial the heater to your operating temperature.