back to Chillers

 

The liquid nitrogen chiller has the following components:

-Heat Exchanger (HEX): 5" x 12", 100 plates, 10" thick.  Encased in black polystyrene.

-Solenoid (green with red dot): controls the inflow of liquid nitrogen (LN2)

-Patient HEX:  The smaller device that the large coolant tubing is connected to at the upper left. Transfers heat away from the perfusate.

-Pump (at bottom): Moves coolant through the heat exchanger

-Coolant reservoir: Not shown.  2L.  Allows for expansion and contraction of coolant.

-Liquid Nitrogen (LN2): enters from the left of the solenoid.  LN2 hose not attached in this photo.

-Exhaust: The small clear tubing exiting from the bottom of the heat exchanger. 

-Thermocouples: The thermocouple on the exhaust port is really the only useful one.  It lets the you detect if coolant freezes.

 

 

50/50 Ethylene Glycol (EG) / Water mixture is used as the coolant.  The freezing point of this mixture is -37 deg C, but it can become gummy long before reaching this low temperature.  The heat exchanger is large to help prevent obstruction during aggressive cooling.  Duratherm XLT-120, with a freezing point of -84 deg C, is a future possibility if we want to push the temperature down with more control, but it degrades all plastic, so it adds complexity.  We are also exploring moving to 60/40 EG/water for a lower freezing point of -53 deg C, but with viscosity issues.

 

The chiller has quite a bit of coolant inside its circuit, especially inside the HEX.  This coolant is flowing very rapidly, about the same as a fully open garden hose with good pressure.  The solenoid is sized fairly small to provide appropriate cooling rates that match the HEX.  The chiller is optimized to be resistant to external fluctuations rather than to respond quickly to set point changes.  It will maintain the arterial setpoint within 0.25 deg C for hours at time, regardless of changes in the flow rate or the venous temperature.  Since the automation is tied to the arterial temperature rather than the coolant temperature, the temperature of the coolant will gradually rise if less cooling power is needed.

 

Software

The thermocouples and solenoid are connected to Perfusion Monitor Software that automatically pulses LN2 through the HEX.  Click the Start button at the top to make the Chiller Automation section visible

 

 

Check the radio button to turn chiller automation ON.  As it explains on the screen, the automation is based on the blue bug on the red arterial temperature graph.  This bug indicates the target arterial temperature, and the software will quickly drive it to that temperature without any user input. 

 

It takes time to reach any new target set point, so the chiller automation should be turned on approximately 15 minutes before use so that the coolant can be gradually lowered to the correct temperature ahead of time.  During this initial cooldown, when the arterial line is not flowing, change the Objective at the bottom to Coolant, because this is the only available temperature feedback.

 

Modes

Warming

This mode simply supplies no LN2 to the chiller.  It allows the coolant to warm gradually.

Steady PID

The temperature will be under full computer automation.  The "Time on" for the liquid nitrogen (LN2) pulses is usually set to about 1/2 second.  Don't change it while automation is on, or it will disturb the automation algorithm.  The "Time off" time is determined by the slider which is under the control of the automation.  The slider will be bouncing around to keep the coolant temperature constant.  The PID button is discussed below.

Cooling

A steady pulse of LN2 will be used to cool.  Both the "Time off" slider and the "Time on" value are under your direct control.  Start out very aggressive:

   5 seconds on

   1 second off 

Closely monitor the HEX nitrogen exhaust temperature, and once it gets down to about -5, adjust the times to be less aggressive.  As the coolant temperature approaches zero, set the times to about:

   1 second on

   1 second off

If the HEX nitrogen exhaust gets as low as -20, the coolant is probably starting to freeze in areas.  If it gets down to about -35, it's probably frozen solid in many areas.  This will also cause the unfrozen coolant to plateau at a high temperature like 8 deg C.  If this happens, it will take a long time to thaw, maybe 20 minutes.

 

PID

You shouldn't normally have to change the PID settings.  PID stands for Proportional-Integral-Derivative.  It's a common algorithm used for automation control.  In the chiller automation section, click the PID button.

 

 

Because the automation is designed for maintaining temperature and resisting change, the proportional (P) variable is set high. This causes the pulse T off slider to bounce around very responsively with every little change in arterial temperature.  This is the desired behavior, and could probably even be more aggressive.  The integral (I) is set to cause the midpoint of the fluctuations to gently creep.  If you find that it's not creeping quickly enough, you can grab the slider and move it a few notches to what you think the midpoint should be, and it will carry on from there.  Derivative (D) should remain zero.

 

The main reason why you might need to change the PID settings is due to different characteristics of the hardware.  For example, maybe the LN2 tank doesn't have a pressure builder, or maybe the patient HEX is sized differently than we anticipated.  As we continue testing with different hardware, we will provide more guidelines for PID settings.

 

This chiller is robust and reliable, but in case of failure of some part of the system, there are a backup Dry Ice Chiller and Water Ice Chiller, neither of which require a computer.