Hydro Tech: When Variable Flow Fails
Hydronic designs regularly utilize variable flow. Flow rates change to match load, reduce energy and, in almost all cases, work great. This is an example where standard design and material selection oversight almost caused a catastrophic problem, and ultimately variable flow rate proved to be the cause.
A Case StudyThis modern 30-story high-rise attracted multiple tenants with special cooling loads. This property was originally constructed with a penthouse central plant and individual floor AHUs, cooled by chilled water risers. The chilled water system was designed as variable flow to each unit. Data room units were initially added to the chilled water loop. However, after hours, the central plant was very inefficient. Two major renovations requiring an additional 40 tons of 24/7 cooling sent the building in search of a new answer.
The solution? A new heat rejection system was designed, and a second set of pipe risers were installed vertically through the building. These connect to variable flow condenser water pumps and the existing cooling towers, and allow future tenants to connect compressor data units to the condenser water hydronic system. Each unit would be required to have a head pressure water flow control valve, so when cooling wasn’t required, flow rate was reduced and pump energy was saved. This worked great for the first 10 years.
The chief engineer in the building called me to come visit in year twelve. He pointed to the Schedule 40 steel condenser water piping on the 10th floor, and there were two small water stains, in-line, that indicated a possible pipe flaw. We coordinated a shutdown to pull out the affected section of pipe, and we expected that a manufacturing defect was the problem. We were surprised.
The pipe section pulled out was 50% blocked by organic matter, and only the center volume was open. Testing the pipe wall thickness revealed less than 50% of the original in several areas. We searched for the reason, as the building condenser water treatment was impeccable, and water treatment records and tests had showed above normal maintenance. The condenser water system had main fine-screen strainers and 5 Milacron side-stream media filters that were regularly maintained. Nothing pointed to an answer. We decided upon a second shutdown to pull pipe samples in four more locations.
Interestingly enough, the samples from the second floor looked terrible, but the 15th floor, 25th floor, and penthouse samples were as clean as a whistle. Why did this occur only below 15 floors?
It's All About Variable FlowThe answer proved to be variable flow. When the risers were installed, one of the first heavy load tenants was on the second floor. The tenant left in year 10. The remaining connected load up to the 12th floor was minor, but on floor 12, a major tenant load existed. Calculations indicated a flow rate of 1.5 to 2.5 feet per second (fps) above the 12th floor and from zero to 1.0 fps below. This low flow rate of oxygen-rich condenser water allowed anaerobic growth (microbes) on the steel pipe at the lower levels. The increased flow rate above 1.5 to 2.5 fps presented the problem.
We considered several options to correct the problem. One option we considered was to replace the steel riser piping with copper that would be resistant to the anaerobic attack. Most of the branch piping from the risers were already copper. However, this would have required many outages to critical tenant cooling systems. This was not acceptable to the building owner.
Another option was to replace the piping and to install a bypass valve at the bottom of the risers to keep the flow rate above 1.5 fps. This option would have wasted energy and consumed pump capacity that would be needed for future cooling.
The solution we selected was the replacement of the lower 12 floors of pipe risers with copper piping, the installation of a plate-and-plate heat exchanger, and secondary variable flow pump to allow the building risers to be a “closed” loop from the tower water.
Variable flow systems can be utilized to save energy. However, care must be taken when applying variable flow to open condenser water systems. We found that traditional piping materials were not acceptable. All has been well now for the owner and tenants for many years. ES