HVAC Mystery: Falsely Accused
A main air handler located on the eighth floor controls the interior of the building. A second air handler located on the eleventh floor provides high-velocity air to 336 independently controlled convection units. Each one of the convection units also has a hot water supply, cold water supply, and a common return.
The air handler located on the eighth floor is a typical design and uses the hot deck/cold deck arrangement with the help of 50 mixing boxes to provide control to the interior of the building.
The perimeter of the building is a different story. Over the past 50 years, just about every hvac design engineer and service technician that has reviewed this problem has concluded that nothing could be done to correct it. During the winter months (with the main chillers off), providing cooling to the perimeter units was essential to keep about 40% of the offices located on the outside walls from overheating.
When the main chiller is off, the chilled water pump can be used to circulate water through the chilled water coils located just after the mixed-air section of the eighth- and eleventh-floor air handlers. This cooled the water with outside air to be used in the secondary induction loop and, in-turn, provided cooling at the convection units. However this also resulted in a loss of control over the cold deck temperature on the eighth-floor air handler. This caused the interior of the building to be very hot. With this arrangement, the only way to control the building was to manually change over the systems based on which part of the building was the hottest.
If the flow to the eighth-floor chilled water coils was proportioned (V-20) to maintain the cold deck high limit temperature, the secondary induction loop temperature would increase above the maximum limit of 65 degrees F to keep the convection units from overheating.
Over the past 50 years, it was believed that the air handlers and chilled water coils were not sized properly to cool both the secondary induction loop and the eighth-floor air-handler cold deck. Every scenario available was tried to correct this problem.
In the past all of the controls, piping, valves, fans, and pumps were independently controlled and located all over the 140,000-sq-ft, 12-story building. We recently used a local contractor (Ancoma Mechanical) to install a new chiller and to automate the control system. After careful consideration and much research, we elected to install an Automated Logic control package.
Much To See With DDCThe transition from using independent pneumatic controls to a digital supervision control package can only be described as massive. As we installed this new system, we were able to view the entire three-pipe configuration from a single location, and it became very clear that the source of the cooling problem that had affected the building for so many years was caused by something entirely different than anyone had suspected.
Based on the percentage of convection units calling for heat, and the setpoint of the hot water temperature, a resulting pressure imbalance in the three-pipe system bypassed the convection units and back-filled the secondary chilled water loop. This heated the secondary loop to a temperature equal to or greater than the common return. This was hot enough so if a convection unit called for cooling, it actually heated the office.
We then found that by modulating the (V-9A) secondary induction pump bypass valve and the (V-9B) hot water supply valve (based on system pressures), we were able to maintain an equal pressure across the hot and cold water piping and a lower pressure in the common return, using the three-pipe arrangement to our advantage. This stopped the hot water from bypassing into the secondary induction loop causing the system sizing to be adequate to dissipate the heat in the secondary induction loop and allowing the eighth-floor cold deck temperature to be manageable. After the system was working, we immediately received numerous cold complaints because just about everyone on the east and west sides of the building had their thermostats set to 100% cooling.
The new controls ramp discharge setpoints are based on outside air temperature, and that keeps the three-pipe system balanced so we can provide heating and cooling all year.
New Sequence, New SuccessBoth heating and cooling are required to accurately control the building in the summer and the winter months.
The flow configuration starts at the chilled water pumps, with the main chillers off.
The water flows in two directions, first to the eighth-floor air handler chilled water coil through the (V-20) control valve, the (V-20) control valve proportions closed if the eighth-floor air-handler cold deck temperature reaches the high-limit setpoint.
The water is cooled in the chilled water coil as the outside air passes over the coil; after the water is cooled in this coil, it returns to the chilled water pumps to repeat the process.
The water also flows through the eleventh-floor air-handler chilled water coil and is cooled with outside air to 55 degrees.
If the secondary chilled water supply is above its setpoint of 65 degrees, the (V-5) valve proportions with the (V-6) valve to maintain the secondary chilled water supply setpoint. The chilled water then passes through the mixing valves on the convection units where it is mixed with hot water through independent controls to maintain the occupied setpoint.
After the hot and cold water is mixed in the convection units, it flows to the inlet of the secondary induction pumps where (V-9A) is proportioned to maintain an equal pressure in the hot and cold water piping and a slightly lower pressure in the return piping.
Now the mixed water is discharged to either the chilled water return pumps to repeat the process, or discharged through the (V-9B) control valve that is proportioned to maintain an equal pressure in the hot water and cold water piping.
It is important to understand that the two N.O. (locked open) valves are in place to maintain positive circulation as V-5, V-6, V-9A, and V-9B are proportioned to maintain a balanced system. In the past if the outside air temperature was above 55 degrees, the main chillers were started. Now if the outside air temperature is above 50 degrees, the ninth floor chiller valves, V-21, V-22, are automatically opened and the booster pump is started.
This supplemental cooling of the main loop will continue to operate until the building load, based on a secondary loop high limit of 68 degrees, requires the two 250 ton main chillers to start.
The (50-ton) ninth-floor chiller will now handle the building load with an outside air temperature range of 50 degrees to 75 degrees.
As a result of the controls retrofit the staff is now able to automatically shut down both 125-hp boilers for about 3.5 months each year. Because of the common return in the three-pipe system this also unloads the main chillers.
The savings in gas, electric, maintenance, and operating costs are expected to result in a high rate of payback. Based on 10 cents per kW and the average outdoor air temperature for Rochester NY, the savings to RG&E is $770 to $800 dollars each day that the boilers are off. The best result of the job, however, is the highly increased confront level for all the employees at Rochester Gas & Electric. ES
Editor's Note: Some images associated with this article do not effectively translate to this website. If you wish to view them, please refer to the print version.