Letters

Reconsidering Centralized UPS Strategies Regarding "Don't Pardon the Interruption," (CSE 04/02 p.40, and also in a recent CSE e-newsletter), I have to agree with what the author, Timothy Koch, said, but I feel caution must be expressed on what was not said in the story. The reliability/availability of power is never any better than any single path circuit in the system.

09/01/2002


Reconsidering Centralized UPS Strategies

Regarding "Don't Pardon the Interruption," ( CSE 04/02 p.40, and also in a recent CSE e-newsletter) , I have to agree with what the author, Timothy Koch, said, but I feel caution must be expressed on what was not said in the story.

The reliability/availability of power is never any better than any single path circuit in the system. Reliability/availability is a function of reliability, mean time between failures (MTBF), and mean time to repair (MTTR). Availability is MTBF/(MTTF+ MTTR). Therefore a large-scale UPS unit with a MTBF of 15,000 hours, with an expected MTTR of 5 hours, has an availability of 99.966. But if power is distributed by a single circuit distribution panel, with a reliability of only 99, the overall reliability can be no greater than 99. A small, under-the-desk UPS with an MTBF of 8,000 hours and a MTTR of 5 minutes (replace from spares), has an availability of 99.999.

The closer the reliable power source, and the closer the UPS is placed to the critical load, the more reliable and available the power will be. A multiple-fed utility power system is the most reliable to be found, but the delivery network is subject to a myriad of failure modes, even when redundant feeds are used. When a central UPS with multiple redundancies is installed, and then a single path distribution system is used within the facility, the reliability/avail- ability of the total is no better than the single path distribution system—typically not much better than 99%. To invest in a 99.9999 system and then distribute power using single path circuits is false reliability and is ludicrous. For workstations, the most reliable source of power is a small, under-the-desk UPS, or—if the workstation is so critical that it has redundant power supplies—use two small UPS units under the desk. The reliability of a redundant central UPS can be improved by using a redundant distribution system providing redundant paths to critical equipment that have redundant power supplies. Providing static transfer switching from the redundant UPS units to the redundant distribution paths may improve reliability if the static transfer switches have sufficient reliability. Without a proper maintenance program, even a system of the most reliable critical components will eventually fail unexpectedly and catastrophically.

A distributed UPS system can be placed closer to the critical loads than a central UPS. However, with a subsystem interdependent system, all the subsystems of the entire interdependent system should be powered from a single UPS unit sized for the interdependent system. If any subsystem is to fail for want of power, the entire system is incapacitated and the MTBF of several small UPS units, in general, is less than the MTBF of a larger UPS unit, even though the MTTR may be much faster.

Reliability is always affected by the maintenance program and if the maintenance program is lacking, reliability will suffer.

Gary Schilling, Lockheed-Martin Technical Operations, Springfield, Va.

Ponder These Post-9/11 Electrical Design Considerations

I am an electrical engineer specializing in power systems and have truly never worked in the building design area, but it seems to me some revisions to the National Electrical Code should be considered for adoption as we contemplate the anniversary of Sept. 11. Some of these suggestions may already exist, but here, anyway, are some points to consider:

1) Rather than a single utility feeder, all skyscrapers should be serviced by two or three service connections with entry over two or three different floors. This might create grounding problems, but that's something that can be resolved.

2) Generators or standby power systems also should be located on a floor other than that which serves as the entrance for the main electrical service. Generators should also be served by different breaker boxes.

3) The electrical service entrance room should not be readily observable to non-building staff personnel.

4) Service entrance breaker boxes, etc., should be housed in fire-proof casings.

5) Emergency lighting should be powered independently from the building's major utility source, or even from it's major backup power source.

6) Consider employing some sort of chemically treated material for staircase and emergency egress lighting.

7) From a non-electrical standpoint, multiple HVAC plants on different floors—not just roofs or basements—should be considered.

8) Finally, in the building structure itself, steel wires should be installed between walls, beside the I-beam columns, to support the structure of the walls.

Virendra K. Kaushik, P.E., Westerville, Ohio

No Tolerance for Below-Standard Shop Drawings

Regarding "Putting Your Foot Down on Specs" (Editor's Viewpoint, Specifier's Guide, 06/02, p. 3), and the question as to whether specifiers should enforce original specifications, or kowtow to contractors so as to not rock the boat, I believe shop drawing approval should be taken very seriously.

The contractor is obligated to provide the quality products that have been specified. The engineer is also under obligation, in this case to look after the interest of the owner. Therefore, one cannot accept an inferior product substitution. So, in principle, the shop drawing phase should ensure that the owner gets what was competitively bid.

Ray Moses, Moses and Associates, Beverly Hills, Calif.

Targeted Treatment

Reading "Targeted Treatment Systems: More and Why," ( CSE 06/02, p. 52), got me thinking about sewage treatment in small towns and a need for change.

Consider this scenario: Put a piece of beefsteak out in the sun and rain—protected from bird and beast—and in time it will disappear. Put a bowl of sewage out in the sun, and it will similarly disappear. But a lot of beefsteak or a lot of sewage left in similar conditions will stink, as much of the decomposition will take place anaerobically.

This is indeed the case in many large sewage treatment operations, but fortunately, this decomposition happens in a closed container, known as a digester. This process is then followed up with oxidation treatment.

That's the good news. The bad news is that many small towns, even today, rely on open tank systems. The practice evolved from a scheme proposed many years ago by the Chicago Pump Company. The manufacturer's belief was that if enough oxygen was shot into a tank, the sewage would disappear without smelling. In principal, it was an excellent suggestion. The problem, however, was that a great deal of power is necessary to run the compressors to provide the necessary volume of air. And in practice, the operating cost of this extended aeration has proved too much for many communities.

As a result, many communities aerate only the liquid and haul the settled waste for disposal elsewhere.

Today's advancements in sustainable technology, however, can breathe new life, so to speak, into the Chicago Pump design: use a windmill.

Consider the economics. If the 20-year power cost is set at $40,000 windmill, the cost per year will be $2,000—less than $200 per month.

Additionally, the windmill could be financed as part of the treatment process, making power cost of little importance.

In the long term, waste treatment designers involved with such existing systems should:

1) Evaluate the design of the biochemical oxygen demand.

2) Evaluate the tank's holding tie to provide the required complete oxidation without odor,

3) Size and shape the holding tank to provide the required oxygen-sewage contact.

4) Establish how many air compressors are needed and what their power requirements will be.

5) Evaluate the need for a temperature-protection shed cover.

Additionally, the electrical systems should be wired so that the windmill will store power in batteries, making the compressor run continually. A power utility hookup can provide emergency service.

In the past, a lagoon has been the best sewage treatment for many, if not most small towns. Today, the sewage treatment engineer is obligated to consider complete oxidation extended aeration.

Norman Van Sickle, Silver City, Iowa

Correction : In the table on p. 34 of the "Giants Report" (CSE 02/08), which noted the number of engineers employed by M/E/P discipline at a sampling of our top 100 M/E firms, the number of HVAC engineers for Burns and McDonnell, Kansas City, Mo., was mistakenly reported as zero. Actually, the 1,700-person firm has more than 180 mechanical engineers on staff.





No comments