This concludes our two-part series on designing medical gas systems. Part 1 appeared in the March 2005 issue.

Issue: 4/05

In order to determine medical gas flows, we use the same probability theory and line of reasoning mentioned in Part 1 in the March PME. First, we must determine the maximum number of patients that are likely to use a particular gas for a given number of patients under consideration. We then multiply this number by the average flow for that outlet/inlet. Note that we said patients-not outlet/inlets-since, in most cases, this is the real determining factor. Often, extra outlets/inlets are requested for convenience only, not necessarily because all these are to be used simultaneously. However, always check with the owner if an unusual number of outlets are requested at a certain location.

If, for example, an operating room is to be capable of treating two patients simultaneously, then for the purpose of gas-flow determination, it may be treated as two rooms, etc. Typical outlet/inlet requirements for rooms may be found in the American Institute of Architects (AIA) "Minimum Construction Guidelines for Healthcare Facilities."

A priori, different areas of a medical facility have different medical gas demands. According to the Theory of Large Numbers (TLN), if we could poll enough patients in these areas-during the busiest time of the day-then we should be able to determine the true probability of patients in that area using a particular gas. Pipe diameters can then be selected based on the calculated flow and certain parameters, including maximum pipe length, gas-specific gravity, regulator outlet pressure and allowable pressure drop. pressure drop.

Fortunately, these probabilities may be found in certain tables as mentioned earlier. These tables, for example, indicate that the oxygen demand for all patient rooms is approximately 10%, whereas for recovery rooms it's approximately 50%. Again, these use factors can only be applied to the entire facility-not portions of it where there are fewer patients, rooms, etc. But, by definition, this percentage must therefore be fairly close to the true probability of a single patient using a particular gas. Therefore, we may substitute these use factors for true probability in Equations 2 and 3.

It has been determined that for medical compressed air, medical vacuum and oxygen, most areas of a typical medical facility can be broken down into four or five basic categories. Each category has two parameters in common: Probability of use and average flow per patient or bed, etc. For each section of gas pipe where flow is being figured, each category is calculated separately and then added together. For example, if there are 18 patient rooms, two special procedure rooms, and three neonatal intensive care beds being served by a section of oxygen piping, each area is calculated separately and then added together. No category is affected by any other category in determining the final flow. For example, reducing the number of patient rooms (or any other category with the same parameters) does not affect the predicted maximum flow for the two procedure rooms and three neonatal intensive care beds.

As an example, let's say that we are to determine the flow for an oxygen pipe serving two operating rooms (ORs), two recovery rooms, and 12 patient rooms. According to Table 1, demand for the two ORs is 80 lpm (liters per minute). Next we determine the demand for the two recovery rooms. To do this, we multiply 20 lpm times the number of rooms calculated in Equation 2 with n = 2 and true probability = 0.45. For this example, we'll use a "z"


Although calculating medical gas flows utilizing the methods described above might at first appear rather cumbersome, with personal computers, programmable calculators and the like, these methods may be incorporated into powerful yet easy-to-use programs.

As medical procedures are revised and more statistical data is made available, some of the probabilities mentioned here may be adjusted from time to time. Indeed, more studies need to be performed to sharpen these numbers. However, the values indicated in Tables 1-4 do appear to work satisfactorily. As for any engineering and design project, it is vital to work closely with hospital and other medical facility personnel to identify unusual demands, such as air-driven ventilators, unusually large WAGD demands, etc. These atypical flows can have a serious impact on medical gas flow calculations.