Medical Gas Professional Healthcare Organization

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Liquid oxygen vaporizers the weak link

  • Thursday, March 26, 2020 7:51 PM
    Message # 8861373

    In this time of increased use of liquid oxygen at health care facilities it is important to pay attention to the ice build up on the vaporizers. The vaporizers at a health care system are the wink link in the system. While they can take short durations of increased flow the potential for over drawing the vaporizer is a real problem of the current pandemic.  When we look at the usage of a typical adult ventilators (thanks Al) they use 318 standard cubic feet per hour (scfh) per unit. If we assume 10 additional units in service (which appears to be extreamly low number according to the news reports) that would be an increased use of 3180 scfh. A vaporizers typical rating is for 8 hours of use at 70 degrees and 70% relative humidity. A vaporizer in continuous use (which these would be) you need to reduce the rating of the vaporizers 30-50%. If you are switching the vaporizers you are closer to the 30% for non-switching you are closer to the 50%. In other words a 10,000 scfh vaporizer would only be good for 5-7 kscfh. The good news is we are coming out of winter so we don’t need to worry about additional derating for freeze periods. If a health care facility is looking to add ventilators you need to consider the ratings of the vaporizers. You can either check with the owner of the bulk system and ask them the rating of the vaporization or you chould look at the data plate on the side of the vaporizer to determine if there is a potential size problem. You can also look at removing the existing ice build up to gain additional surface area to help vaporizer. DO NOT DO THIS BY CHIPPING AT THE ICE OR TRY TO BANG IT OFF WITH BOARDS, HAMMERS. OR ICE PICKS. Talk to the supplier about getting the vaporizer deiced. This should be done CAREFULLY using pressure washers and starting at the top and working your way down. DO NOT START AT THE BOTTOM OF THE VAPORIZER.   

    Please don’t hesitate to contact me if you have any additional questions.

    Stay safe and healthy.

  • Friday, March 27, 2020 7:15 AM
    Reply # 8861933 on 8861373
    Cary Darden (Administrator)

    Thanks for this info Bob, this is great information and something I haven't been considering when doing load calculations for additional ventilators.  Where did you come up with the 318 SCFH number for oxygen flow on ventilators?  The number I have have seen for most adult ventilators has been about 250 SCFH for the max continuous flow (not peak flow). 

    Most ventilators use a blend of oxygen and medical air as well so wouldn't that max flow number be split up between those two gases based on the % O2 that the ventilator is delivering to the patient?  I am not a respiratory therapist and so I'm just going on the information I've been given from several that I've spoken to over the years.  

  • Friday, March 27, 2020 11:01 AM
    Reply # 8862301 on 8861373

    Cary & Bob,

    I also am seeing a weak link in the over extension of the capabilities of the medical air compressor system. The global need for ventilators will undoubtedly be met by what's most readily available. This will require higher demands on the facilities medical air system. We're working on a simple air proportioning unit, that can be manufactured fast and cheap. The ethical delimma is that we might cross the line of current code requirements to react to this extremely time sensitive manner. I would welcome any and all SME's input or suggestions. 

    Scott Jussel

  • Friday, March 27, 2020 1:09 PM
    Reply # 8862648 on 8861373

    Please remember my conversation at MGPHO in regards to Blending oxygen generator with liquid oxygen. 

  • Saturday, March 28, 2020 10:38 AM
    Reply # 8864024 on 8861373
    Al Moon (Administrator)


    On this forums most people, will need additional dialogue / written informational from any item that happen at the annual educational seminar. Perhaps a WHITE PAPER on the subject.


    In My Opinion Only.

    A major review of Chapters # 1 & 12 is in order.

    Working outside the Codes, Standards, Requirements and Laws for Healthcare Compliance, can create chaos and unsafe conditions.

    Best left for times of Marshall Law.   

    Last modified: Saturday, March 28, 2020 4:00 PM | Al Moon (Administrator)
  • Saturday, March 28, 2020 1:17 PM
    Reply # 8864214 on 8861373

    Thanks Bob and all for the informative discussion.

    I believe the assumption of ventilator 02 consumption used above is significantly off, making the estimated bulk 02 consumption calculations above approximately 10 times higher than actual.

    Let’s review the science – calculations are rounded.  The assumption of 318 SCFH per ventilator translates to 150 SLPM flow, using the standard conversion factor of 28.3 liters per SCF. The 150 LPM is close to the peak inspiratory flow (volume of gas moved per unit of time) of a ventilator, but is not at all related to the volume (amount of space a substance occupies or enclosed) of gas used by the ventilator. From my discussion of ventilator flowrate on the forum yesterday, we know a “typical” adult male patient minute volume is approximately 8 LPM, meaning if the patient is receiving 100% 02 (more on this below) the 02 consumed by the ventilator is about the same, 8 LPM. This would compare to approximately 17 SCFH. So, let’s use a 100% safety factor, just for the heck of it, and we get each vent consumption over an hour of 34 SCFH (16 liters consumed per min. ÷ 28.3 x 60 mins per hr.), or slightly less than 10 times the assumed consumption in the above bulk 02 calculations. This makes a huge difference when calculating sizing for bulk 02 systems (including vaporizers) and central med air compressors.


    The ventilator peak inspiratory flowrate duration is usually a fraction of the 1 to 2 seconds of typical inspiratory cycle. Peak flow is an extremely important value used when calculating med gas pipe distribution sizing, but not for actual consumption of gas for sizing central 02 or med air systems.

    Now let’s talk a bit about % oxygen patient usage. Clinicians try to use the lowest inspired oxygen concentration, referred to as factional concentration of inspired oxygen (Fi02), needed to attain an adequate 02 level in the patient’s arterial blood – determined by arterial blood gas analysis, oximeter, and clinical observation. Most patients will not need 100% 02 administration – there are a few exceptions to very high inspired oxygen concentrations and one is advanced pneumonia, which is the way many COVID-19 patients are presenting at emergency departments. Patients in acute care hospitals very rarely receive 21% 02 (all air outlet consumption), so in my view central air compressor sizing should not assume vents use 100% piped med air. Patients intubated or with a tracheostomy, but not on a ventilator, can consume 12 LPM 02 (typical) when receiving 40% 02 by an oxygen nebulizer (aerosol therapy) due to the function and design of this medical device – yea crazy. Additional detail upon request. So, some ambient pressure respiratory care devices can use as much 02 as a ventilator.

    No doubt hospitals will be using more, and in some cases substantially more, bulk 02 for the duration of this COVID-19 pandemic.

    Stay Well

    Last modified: Monday, March 30, 2020 10:41 AM | George Scott

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