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Designing an Aesthetic /Medical Laser System Heat Exchanger

When designing a heat exchanger for an aesthetic/medical laser system (laser, IPL, RF, and ultrasound), it is important to take multiple parameters into consideration. In this post, we will provide a short review of some of these main parameters.

  1. Find out the total heat to be removed from the heat source, which could be laser, IPL, RF, or ultrasound. The next step is to add the heat generated by the pump and fan while they operate, to the heat from our heat source, as well as some safety overhead.  This is of critical importance as if  the heat exchanger is designed below that total heat capacity, it would not have stable working conditions for our system over an extended period and the system would eventually shut down due to overheating.
  2. The type of liquid used in the cooling system and heat exchanger is important as well. The type of liquid influences both the materials used to manufacture the heat exchanger as well as the actual thermal performance of the system.
    1. If using deionized water, the heat exchanger must be designed with stainless-steel tubing, which is certified for medical equipment (such as 316 stainless-steel). If deionized water is used with tubing that is not stainless-steel, it will contaminate the system and will lead to system leaks.
    2. In case the liquid is not corrosive, the heat exchanger can be designed with tubes that are not made of stainless-steel. It is important to remember that adding inhibitors to corrosive liquid (such as deionized water) does not completely resolve the issue of corrosiveness and therefore, using non stainless-steel tubing will present the same issues mentioned above, most likely at a slower pace.
  3. The available area for the heat exchanger is an important factor. In order to design the proper heat exchanger, it is important to examine and achieve the proper size while considering the thermodynamic aspects as well as the cost-benefit analysis.  Surface ratio, air flow speed, heat exchange area and additional factors will help us determine the proper size.
  4. Ambient temperature is a factor which will have a significant effect on the performance of the cooling system. When the system is at a stable working condition, any increase or decrease in ambient temperature or the temperature of the air blowing across the heat exchanger, will increase or decrease the total system temperature, respectively.  Another important factor is that the closer the ambient temperature is to the temperature of the liquid exiting the heat source, it will become increasingly more difficult to remove that heat.  Removing 600 watts of heat with an ambient temperature of 24° C is completely different than trying to remove the same amount of heat with an ambient temperature of 30° C.  In many Western countries it is customary to work in an air conditioned environment at about 24° C but in countries such as India, China and others, this is not the norm at all, and the ambient temperature may reach 32° C.  We recommend designing the cooling system for an ambient temperature of 30° C – 32° C.

Once we have the general requirements, we can proceed with the actual heat exchanger design:

  1. Heat exchanger size – The heat exchanger size must be able to remove the required amount of heat, using the provided liquid and the ambient temperature selected.  The design should take into consideration the proper flow of air and liquid as well as the pressure drop for both.
  2. Pump selection – When selecting a pump, few parameters must be considered. One of the most important is for the pump to be able to provide the flow rate required by the heat source (diode, lamp, RF or ultrasound).  In addition, the pump must be constructed of materials that are suitable to work with the liquid we are using.  Another important parameter to be considered is that the pump should be able to handle the pressure drop created by the heat exchanger, when working at the required flow rate, as well as the aggregate pressure drop caused by other parts of the cooling system, such as piping, connectors and more.  We must check and verify, using the data provided by the pump manufacturer, that we will be able to use the pump and get the required flow rate while handling the total aggregated pressure drop we calculated for our system.  The heat exchanger must be optimized to get the maximum flow rate with the minimum pressure drop.  It is common to design the heat exchanger to work with a pressure drop range of 0.2 kPa – 0.3 kPa.  Working with pressure drop outside this range can negatively affect the heat exchanger’s thermal performance and reduce the life expectancy of the pump.  The length and radius of the piping in the heat exchanger, which were selected to be able to remove the desired amount of heat with the specified flow rate, will determine the pressure drop caused by the heat exchanger.  We can address this issue by designing the heat exchanger with multiple circuits.
  3. Fan selection – There are several parameters to be considered when selecting the fan.
    1. Amount of air required – This is derived from the total amount of heat to be removed and the required performance of the cooling system.
    2. Maximum pressure drop allowed – We must find the point on the fan’s graph where we will be provided with the proper airflow vs. the pressure drop created on the heat exchanger’s surface. When examining the air flow, we must take into consideration the air speed across the heat exchanger’s surface.  With the pressure drop, this is influenced by both air speed across the heat exchanger’s surface and the actual surface size, depth, and the distances between the fins.
    3. Required noise level
    4. Liquid working temperature – In the case that we are designing a system with liquid temperature above 50° C, we must verify the fan is designed to work in these conditions.

An important thing to remember is that when while still in the design stage, we must work to get to the optimal point that takes into consideration the surface area for heat transfer, heat exchanger surface area, air flow speed and minimal pressure drop.   It is possible to optimize our system by adjusting the heat transfer area and the distances between the fins.  In addition, if we already have the specifications of the pump, we can proceed with the thermal calculations for our selected working point and decide on the specific fan based on the thermal calculation results for the air flow and pressure drop.

Best practices for heat exchanger design and installation:

  1. It is crucial to verify that all of the components are synchronized with the heat exchanger and that the cooling system is at the proper working point after considering all working and thermal parameters.
  2. In the case of deionized water with heat exchanger tubing that is made of stainless steel, a certain standard of welding is required, which is done with Argon (inert gas) to protect the welding environment.
  3. If the heat exchanger is made of stainless steel, it must be properly cleaned at the end of the manufacturing process. This includes removal of any oil deposits (via melting) and any other remnants of the welding process.  Finally, it is important to complete a passivation process to seal all the surfaces of the heat exchanger.  The higher the working temperature of the heat exchanger is, the longer and more extensive the passivation process will be.
  4. It is highly recommended to install the fan in a sealed-off enclosure so we can maximize the air flow and have uniformed flow across the heat exchanger. It is recommended to install the fan at a minimal distance of 25 millimeters from the edge of the heat exchanger.  With this distance, the air that is coming out of the fan, has time to “straighten”, and the flow is uniformed.  Another important thing to remember is that the fan should be installed for air suction.  There are several significant advantages of this process:
    1. Better performance from the heat exchanger as a bigger volume of air is passing through it.
    2. The noise level will be much lower.
    3. Guaranteed air flow using air from outside the system, which is at the ambient temperature the system is desgined for and NOT the air inside the system, which is warmer than the ambient temperature. If we do use the air inside the system and have it flow through our heat exchanger, we must check the difference between the inside and outside air temperature and make sure we can still remove the required amount of heat.  If we have no choice but to use air from within the system, we must verify there are enough ventilation openings that will allow for maximum fresh air, that is within the ambient temperature, to enter the system.  In addition, we must verify the air flow to and from the heat exchanger is not obstructed at any point.
  5. Heat exchanger installation is equally as important:
    1. We recommend installing the heat exchanger at the edge of the system and the fan in suction mode inside the, so it will be possible to maximize the suction of outside air through the heat exchanger.
    2. It is extremely important to install the heat exchanger with the fluid openings (both in and out) towards the bottom so we can completely empty the heat exchanger from ALL fluids, if needed. During transit, the system can be in extremely cold temperature (as low as -20° C).  If the system is in such extreme temperatures for several days, and any fluid was left inside the heat exchanger, the fluid will freeze and may crack the tubing which will cause a leak when the system is turned on.

It is recommended to perform the thermal calculation first.  We recommend using several working scenarios, including extreme ones, to come up with the best result.  Using the best thermal calculation and only then moving to the mechanical one, will allow us to design the optimal heat exchanger, with the best (gold) cost-benefit ratio.

Designing a heat exchanger is a complicated process, which will have a direct effect on the performance of the entire system.  We must take into consideration many factors and variables and if we consider all of them, we will end up with an effective and reliable system that is utilizing the optimal heat exchanger while considering the cost-benefit analysis (while maintaining the system’s reliability and effectiveness).

 

At Thermogym, our experts are prepared and ready to help with all of your heat exchanger needs. More information is available on our website.