Total number of Professional Mechanical Engineers in California

I started looking for this information but did not find it on the web. Tried calling the Board of Professional Engineers but was put on a very long hold. I went about a different way to see how many professional mechanical engineers are there in California. Started looking for individuals with high license numbers. Established that M-60000 does not exist, but M-30000 exists. Did this iteration a couple of times and found out that the latest Mechanical Engineering license number (or presently the highest number) is M-33534. I got my license (M-27808) in 1991. This tells me that the Board has issued a total of 5726 mechanical engineering licenses in the last 15 years; that comes out to be an average annual yield of approximately 380 Professional Mechanical Engineers licensed by California.

Computer Room HVAC System Design

It works to your advantage when electronic racks in a computer lab are arranged in a fashion where they take air from one end and discharge heated air through the other side--you get a hot aisle cold aisle arrangement. You provide cold (conditioned) air in the cold aisle. The cooling fans of the equipment suck up that air, air gets heated by electrical load dissipated as heat by the equipment, gets discharged from the other end, and ends up in the hot aisle. You should have your return registers in the hot aisle to gather this air and take it back to the air conditioning unit. With this hot aisle cold aisle arrangement you not only resolve the hot-spot issue (hot air does not get mixed with the cold air while traveling back to the air conditioning unit), but you also get better performance out of the air conditioning equipment (as you increase the coil entering air temperature--providing a higher delta T for better heat transfer). A desirable computer room air conditioning system would have bottom supply (through the floor) or top supply (from the ceiling) in the cold aisle, and top return in the hot aisle.


I am currently designing a cooling and ventilation system for a server lab. The lab has six existing Compu-Aire units (CAC-2034). The room has ducted supply but free (non-ducted) return to the units--units have return air grilles in their bottom front panels. Since the owner is interested in having a hot aisle cold aisle arrangement I was thinking of making return air hard connection to each unit. Compu-Aire Engineering folks in Whittier told me the only place to connect return air to those existing units would be in the back of the units (connection to front would block the front control panel, and there is not enough room in the side to take all that return air). They said they can supply accessories needed to convert the free return air units to ducted return units (a solid panel to go in the front, and back panel with filters and flange for return air connection)--the conversion can be done in the field, by the mechanical contractor. The problem is, I don't have room to move the units out of their niches where they are presently butting the wall. So, I am designing for return air ducts that would bring the air from hot aisles and leave it in front of the unit without making a hard connection to the unit. Although this return air scheme would work without a fan, I want to be sure about the air movement through that route and am specifying inline axial fans for these return ducts (my fear being that if there is too much empty space between the racks the air would find the path of least resistance through the cold aisles to come back to the units). Since overhead space is tight I am not combining return air ducts through lateral connections. It would be many return ducts (probably ten) bringing hot air to the units.

Do you have any comments? I would love to hear from you.

Exposed duct
The designer must not have a choice but to run this long exposed duct on roof. Now milk crates provide support to duct's sagging portion.

Rectangular supply and return from a Rooftop unit, down through roof

Using long radius elbows at 90-degree turns is a good idea.

Janitor Friendly Gas Meters
With natural gas prices going up it is a worthwhile exercise to compare the life cycle cost of a Rooftop Gas-Electric unit with that of a heat pump.

small Gas-Electric Rooftop Unit
You can see the outside air intake, electric disconnect switch, and gas connection.

Screen around Roof Mechanical Equipment

Look closely and you'll see it: behind the tree on the right hand side of the picture. It is the same departmental store seen from a distance.

Four way diffuser in the same departmental store

Not a good idea to put the diffuser on top of the duct without providing any volume control damper.

Exposed ductwork in the same would-be departmental store.

Supply and Return Air Ducts from a Packaged Rooftop unit
This picture is from a departmental store. Notice the duct smoke detectors mounted on both supply and return air ducts. Duct smoke detector is needed to shut off the AC unit if smoke is detected in the space. One scheme is to have duct smoke detector on main return duct only; if you have smoke in the room, the sensor would sense the smoke and the AC unit would be turned off. But what if a motor burns in the AC unit? You can say that the smoke would go to the conditioned space but as soon as it is drawn in the return air duct, the smoke would be sensed and the unit would be shut off. The latest trend is to not rely on this indirect route and provide duct smoke detector in the supply air duct too.