Now is the time to further clarify and explain these frequently used terms. 

Okay, let’s be frank. Maybe we goofed.
When the NFPA Technical Committee for Piping Systems took up the question of using compressed air as an alternative to nitrogen for driving surgical tools, the proposal used the name “Instrument Air” (IAir). When the proposal was accepted, so was the name. However, “Instrument Air” seems to be used by people for all sorts of compressed air piping systems, and even in a hospital it's not unheard of to have a system with that name with no relation at all to surgery. Maybe that name wasn't the most clever choice.

Still, it's done, so it's important to understand exactly what NFPA 99 means by Instrument Air.  It's not the same as HVAC instrument air, or laboratory air, or shop air, or medical air - but it may have characteristics similar to those. In some cases, it may even take over roles those other systems perform today. It's an optional, never mandatory, alternative to piped nitrogen.

When one looks at the rules in the standard for IAir, this becomes very obvious. IAir is piped at 185 psi, plus it has to be very dry (-40¡) and filtered to meet the Instrument Society of America standard to ensure the air is clean and oil-free, just like nitrogen. IAir has broader applications but near equal cleanliness and dryness and offers us an opportunity to extend a convenience to our client's staff while reducing overall costs.

Before we dive in, there's another NFPA concept that we need to examine. It's the “Medical Support Gas” that encompasses both NFPA 99-style Instrument Air and Nitrogen. Medical Support Gases are never used for breathing, but are used for other, more utilitarian functions that occur regularly around the hospital - like driving surgical tools or blowing off glassware and catheters, moving and braking booms in the OR and ICU.

The NFPA standard has not taken the Support Gas concept all that far as yet, but the fact that it's defined at all underscores one of the fundamentals about IAir: It is meant to be an alternative to nitrogen. Ordinarily, a facility would have no reason to install both since either may be used as the "medical support gas" for that facility, and they may largely be used interchangeably.

Figure 1. Source Options and Requirements for Instrument Air

Pay Me Now or Pay Me Later

Given that IAir is simply mimicking nitrogen, and most people are more familiar with nitrogen, why ever choose IAir? For the average facility or design engineer, the decision will mostly revolve around money. In the simplest terms, IAir always costs less on a volume basis - that is, one cubic foot of IAir is always less expensive than one cubic foot of nitrogen. This ironclad rule is simply the result of how nitrogen is produced. First, you have to compress air (which is about 78% nitrogen). Then the nitrogen is separated from the compressed air, processed, bottled and trucked to your site. With IAir, you're eliminating all but the first step, so, of course, it is less expensive.

However, the equipment to handle nitrogen on site is less complex, and almost certainly less expensive initially. Sometimes it can even be leased with the lease cost written into the gas price, so the equipment has no up front cost at all. On the other hand, the compressor system to produce IAir has a significant initial cost.

IAir will almost always pay for itself over time, but how quickly will depend entirely on how much you use. The return is greater if you use a lot of gas and you compare with nitrogen drawn from cylinders. The return is less if you use very little gas and you compare it to nitrogen drawn from liquid containers or a bulk storage tank. (Liquid, of course, has its own economic traps, which are outside the scope of this article, but do need to be clearly understood by anyone planning a liquid installation.)

Another consideration is labor. If cylinder nitrogen is used, someone has to change the cylinders and hustle the cylinders and the empties back and forth. All that's gone with IAir, albeit there is some maintenance required for the compressor, which will offset the savings.

Figure 2. Typical Arrangement for Instrument Air Piping

Pearls Beyond Price

While cost is the most important determinant for most users, there are some others worth considering when contemplating this decision.

A consideration not to be ignored is that IAir is the better choice environmentally. It takes a lot of energy to make a cubic foot of nitrogen and a lot of diesel to haul it to your loading dock. An IAir compressor uses less energy and, of course, requires no ongoing transportation of gas, containers and empties.

Anyone who has designed a nitrogen system for a hospital client will recall that nitrogen outlets are almost always limited to the OR, and sometimes only to the major ORs at that. However, NFPA permits the use of medical support gases for any non-respired, medically related pneumatic application. So one could use medical support gases in central sterile supply to clear out catheters, in the morgue and in labs.

In essence, one could use medical support gases for any of a huge variety of applications where a compressed gas would be a convenience, most or even all of them outside the O.R. I joke that with this definition one could even use it to inflate tires - but only for use on ambulances!

The economics of IAir reinforce this point: The return on your investment is best if you can use the IAir widely. If you limit the IAir to only those applications where traditionally you would use nitrogen, your return will naturally be smaller.

Why don't we see nitrogen piped everywhere? Of course, one reason is that the cost of nitrogen discourages widespread use, but there is also an occupational hazard. Releasing large quantities of nitrogen into a work space will dilute the available oxygen and can lead to asphyxiation if carried to extremes. Using IAir reduces the cost and eliminates the occupational hazard, freeing these systems for use over the full spectrum of applications convenient for the staff.

NFPA has defined what an IAir source must look like and how it must perform. A quick read through those requirements will immediately remind one of IAir's origins as an equivalent to nitrogen. The standard distribution pressure is 185 psi (1.3 mPa), so the compressor must be capable of 200 psi (1.4 mPa) minimum. This pressure, plus the fact that the system is never to be used for any respiratory purposes, means that an oil-lubricated compressor is the most economical choice.

To assure the necessary cleanliness, filters are required that will remove any oil and oil vapor. Because of the very demanding tool application, the air must also be quite dry - drier than medical air - at a -40¡ dew point.

The relatively high pressure means that most "off the shelf" components are not suitable and one must be careful to ensure that all the components are rated for the pressure. Receivers, dryers, drains and filters are all examples of components that must be specified correctly for the pressure, and ASME coded where appropriate.

IAir sources usually have a configuration very familiar to anyone who is familiar with medical air. Redundant components (arranged in bypasses to permit service), lag alarm and dew point monitoring are all part of a full flown system. However, because IAir is not life support, there is a unique, less complicated configuration permitted for these sources, which can reduce the initial cost considerably. This configuration allows for a single compressor backed up by cylinders.  You can, of course, also implement an IAir system using a standard manifold.

At the other end, where the user will make their connections, the variety of uses for IAir presents a challenge. Many of the applications for a compressed gas are not able to accept the 185 psig pressure provided for the tools, and this pressure must be reduced. The simplest solution may be to provide an IAir control panel, which allows the user to adjust the pressure locally.

Of course, NFPA also permits the piping of more than one pressure pipeline from a single source, provided the system is correctly controlled and labeled. The cost of the separate piping must be weighed against the cost of the local control(s) to determine which option is best (see Figure 1).

IAir and nitrogen pipelines are essentially identical in design, layout, valving, sizing, etc. There are, of course, some differences in the requirements for local and master alarms, but area alarms are identical (see Figure 2).

A Thing of Beauty is a Joy Forever...

Which raises a question: If the systems are so similar (essentially interchangeable in application), and I can save money using IAir, can I convert my existing nitrogen system over? The answer is “yes, but.” It's quite straightforward to do the conversion until you come to the outlets.

There is a distinct outlet for IAir that is different from and non-interchangeable with the nitrogen outlet, and, of course, the in-wall or in-ceiling portion of the outlet requires something done to enable it to accept only the IAir connection. Some outlet types may be converted readily, but others will simply never be convertible and would have to be ripped out.

Does it matter if you run air through a nitrogen connection? In practical terms, since no one breathes this gas, there is probably very little risk, but it is a clear violation of the standard and of the intended use for the outlets. However, the opposite is not true: Running nitrogen through an IAir outlet does create a potential occupational hazard (described above).

IAir is never mandatory, so one cannot say for instance, "I want some air to blow out catheters; therefore I must install instrument air". There are many such applications that could be served with instrument air but are so basic that all that elaboration is simply not necessary. Blowing catheters out is a typical case. If you have an IAir system anyway, it's a great application. But if you don't have one, you're not required to install one.

But beware - if you install an instrument air outlet or call the system "instrument air", you will be required to put in a complete IAir source. It's better to use what's needed and no more. Perhaps it would be enough to call the system "equipment air" and run it through a lab-style gas cock from a simple little compressor below the countertop. IAir can be a great convenience, but it should never be insisted on when something less expensive will suffice.  

Instrument Air NFPA 99-style is an open door to providing an essential medical utility in a new and perhaps better way. It's a great opportunity to save our client some money and offer them some additional convenience - provided we understand the inherent tradeoffs involved.