Back To The Future

Engineered Systems, Oct, 1999 by Mike Shoulders

The two-pipe system, an idea discarded decades ago, is becoming the future of school heating and cooling in Indiana. As Confucius said, "Study the past if you would divine the future."

Six years ago, Paul Roberts, director of facilities for the New Albany-Floyd County Schools (New Albany, IN), hired the architect-engineering firm of Veazey, Parrott & Shoulders (VPS, Evansville, IN) to design a two-pipe hvac system for Green Valley Elementary School (New Albany, IN). The conversation went something like this:

VPS -- "We gave up on the two-pipe system and idea 30 years ago."

Roberts -- "I can't afford anything else."

VPS -- "Hmmm. Well, on second thought..."

Wanting the project, the firm realized that age-old problems could become opportunities for creative design. And that's exactly what happened. Innovations from VPS' principal engineer, Thomas H. Durkin, P.E., were coupled with adaptations of existing technology to create a remarkably efficient hvac system -- less expensive to build (or retrofit), and less expensive to operate.

With 61 Indiana schools now having the "new-and-improved" two-pipe system in place, Durkin and the respective school districts are finding out that annual savings are huge, even while adding air conditioning to previously non-cooled facilities. At last count -- and that was after 38 successful installations -- Indiana taxpayers had already saved $10 million in construction costs and $750,000 in annual operating costs thanks to the two-pipe renaissance.

"This two-pipe is redefining conventional wisdom," said Roberts. "Most engineers will tell you two-pipe cannot work, but this system does.

"The question used to be, `Why are you going to two-pipe?' Now the question should be, `Why aren't you going to two-pipe?'"

TWO-PIPE BACKGROUND

An engineer has many system options from which to choose when designing a school hvac system.

The most common is the four-pipe system, which uses hot water supply and return pipes for space heating (pipes 1 and 2), and chilled-water supply and return pipes for space cooling (pipes 3 and 4).

Fifty years ago, an innovative engineer determined that he could save clients money by using the same distribution piping for chilled-water cooling that was used for hot water heating, and changing over from heating to cooling in the spring, then changing back in the fall. This innovation became known as the two-pipe system.

The system died (or at least went into hibernation) because occupant comfort was compromised by numerous problems. The concept of a seasonal changeover didn't work with the cool morning-warm afternoon typical of a spring or fall day.

When the two-pipe system was used, it was mostly on projects facing severe budget limitations.

NINE SOLUTIONS TO NINE OLD PROBLEMS

In reviving the concept for modern use, at least nine specific problems of the former two-pipe system were addressed. Let's take a look at each concern, along with the reaction to it within Durkin's strategy.

Problem 1: Schools may need heat in the morning and cooling in the afternoon.

Solution 1: The obvious answer is to design a system that can change over as quickly and as often as a school might require.

Problem 2: It takes too long (12 to 24 hrs) to change over from heating to cooling, and vice versa.

Solution 2: Now, control operating parameters and changeover can be done in about 20 min.

Problem 3: It's hard to know when to change over.

Solution 3: Modern digital energy management systems (ems) can monitor all spaces up to several times a minute. The computer decides when it is time to change over and proceeds automatically.

Problem 4: Boilers cannot tolerate cold water when changing over; chillers cannot tolerate hot water when changing over.

Solution 4: In today's market, users can and should negotiate with manufacturers to modify equipment to meet the required service demands.

Problem 5: Too many valves (as many as eight) are associated with the changeover process, a scenario that is both hard to coordinate and maintain.

Solution 5: Durkin designed a schematic with only one changeover valve. Moreover, its heat trap innovation allows isolation of the boilers with no moving parts.

Problem 6: Systems waste energy during changeover.

Solution 6: If you minimize the energy in the loop, the changeover becomes that much more cost-effective.

Problem 7: Parts of the building are still cold when others need cooling.

Solution 7: High-performance windows and insulation make the building much less prone to drafts, and temperatures are thus much more uniform. Also, economizers provide "free" cooling.

Problem 8: Systems tend to overheat in winter.

Solution 8: Decrease your heating water supply temperatures significantly. This not only prevents said overheating, it also saves energy.

Problem 9: Some parts of the building will always be heat positive, such as kitchens and computer rooms.

Solution 9: True. Therefore, separate systems should be provided for those areas.

MORE SAVINGS

The revised two-pipe system has a significant positive impact on the cost to build schools.