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Dining Facility

 

AZ DEMA – WAATS Dining Facility and Barracks (DFac) – L4535, Marana, AZ.


Building Description

Constructed in 1997/98 the HVAC system serves the Kitchen and Dining building for the entire WAATS site as well as approximately 100 Dorm rooms used to house the flight training students. Although relatively old, the facility utilizes many energy conservation capabilities available at that time: variable pumping and airflow schemes, air and water-side economizers, and a four-pipe hot water/chilled water distribution system should have made for an efficient HVAC system, but the system has been plagued by poor performance and excessive utility costs. This poor performance manifested itself in unsatisfactory space temperature control and fashioned unhappy tenants.
The HVAC system consists of:

  • The central plant includes two water chillers with associated pumps and cooling towers, and is primary-secondary pumped and piped. The secondary pumps and the cooling tower fans are all equipped with variable frequency drives.
  • There is a hydronic plate/frame heat exchanger to provide “free” cooling during the appropriate ambient conditions.
  • The hot water system is comprised of one boiler and associated pumps and provides space heating used for zone heating.
  • 2 central station air handling units, one of which is VAV and serves the VAV boxes in the dining areas.
  • Room fan coil units in each Dorm room.
  • Kitchen exhaust/makeup air units and associated hoods.
  • Complete direct digitally controlled EMS.

Project Problems/Issues

  • Severe space pressure variations
  • Inability to utilize the existing plate/frame heat exchanger
  • Numerous maintenance related operational deficiencies
  • Questionable design practices
  • Excessive utility costs

Project Overview

As is often the case, the facility’s HVAC system was substantially oversized and as a result required exacting controls capabilities for efficient operation, yet a lack of HVAC system commissioning, EMS deficiencies, inadequate maintenance practices and questionable design criteria has exacerbated the problems. In most cases, a “bigger is better” approach to HVAC systems is counterproductive and results in the system’s using excessive energy and creates poor space conditions. The combined efforts of the OptimissioningSM team rectified this situation and in the process carried out numerous repairs to inoperable or poorly performing system components

Energy-Environment-Economics provided a detailed engineering study and report of the HVAC System’s current (existing) condition and operation including: HVAC system design and DDC system deficiencies, potential energy conservation opportunities with budgeted project costs and economic analysis, and maintenance related operational and efficiency analysis. The result of this study became an approximately $250,000 Energy Conservation Project including Retro-Commissioning and OptimissioningSM - a combination of HVAC system commissioning and optimization - of the facility. The project included the engineering required for the modernization and automation of the chilled water central plant and the air distribution systems as well as the kitchen systems. Also, project coordination services were provided in concert with the commissioning and optimization services provided. Results of the project reflected a 35% reduction in HVAC related energy costs and improved space conditions.

Solutions Provided

Considering the numerous deficiencies discovered during the commissioning process, the absence of satisfied tenants is not surprising. Many fan coil units serving individual Dorm Rooms had inoperable or disconnected control valves, all of the makeup air units intended to provide “fresh” (ventilation air) to the Dorm Building were inoperable, and some air filters had never been replaced since construction of the facility. The chilled water control valve serving air handling unit #3 (Dining Room) was broken and as such continuously delivering approximately 55-degree air that required reheating during most ambient conditions. The outside air damper actuators for air handling units #3, 4 were broken, thereby introducing excessive outside air into the facility. The condenser water isolation valves serving both chillers were inoperable and as such flowed condenser water thru both chillers at all times, causing higher than normal pressures in the operating chiller and possible reducing life expectancy. Obviously, these issues have created unsatisfactory space conditions and have contributed to the excessive utility costs. The lack of maintenance on the DFac was problematic at best and appeared to be ubiquitous.

Nearly all of the exhaust fans serving the Dining Building had airflow problems, due either to quite aged fan belts or non-existent fan belts. The main exhaust fan serving the Kitchen hood was found to be operating backwards and thus delivering approximately 40% of design airflow. The lack of airflow precludes the required velocity to properly entrain cooking grease in the air and exhaust it to the roof, where it is intended to be collected and disposed of, and is a serious fire hazard. Also, the makeup air unit serving the main Kitchen hood had been inoperable for many months that created a negative pressure condition in the Kitchen, leading to excessive conditioned air being exhausted and wasting considerable amounts of energy.

These and many other maintenance related issues were rectified, but there were also several original design decisions made that could be considered questionable. The following verbiage will attempt to clarify the findings and actions taken made during the OptimissioningSM process

    HVAC System Design Considerations:
  1. The condenser water system incorporated a bypass, possibly for head pressure control, which was allowing condenser water to bypass the chillers during mechanical refrigeration mode and causing the chillers to shut down. This bypass valve has been permanently closed.
  2. The cooling towers were not equipped with automatic isolation valves, and as such each flowed approximately ½ of the design water flow during one-cooling tower operation. Reduced flow rates across the cooling tower will significantly accelerate scaling, as is evidenced by the current condition of the cooling towers. Also, reduced flows thru condenser barrels – an ongoing condition due to the failed condenser water isolation valve actuators - will cause head pressure control issues, and will enhance the possibility of scaling of the condenser tubes. The cooling tower fill has been replaced, and the heavy scaling that has been reported in the condenser tubes is being removed.
  3. The outside air quantities scheduled for air handling units #3 and #4 and for the makeup air units serving the Dorm fan coil units are extremely high and, coupled with the lack of temperature controls, would have had a significant contribution to overall energy costs. Of course the lack of maintenance and condition of the Dorm makeup air units (i.e. inoperable) mitigated this from occurring, albeit at a cost of providing inadequate “fresh” air to the Dorm rooms. The units are back in operation and airflow quantities have been reduced as much as possible using current motors and drives, but are still moving approximately 200% too much outside air. Air handling unit #3 has been equipped with a space CO2 control, which will prevent the introduction of unnecessary outside air
  4. The chilled water and condenser water pumps appear to be egregiously oversized, as do the hot water pumps. The secondary chilled water pumps and the hot water pumps are served by variable frequency drives, making this a lesser issue (except for maintaining minimum flow rates thru the hot water boilers). The constant flow condenser water pumps’ manual isolation valves were utilized to prevent over-pumping and overflowing of the cooling towers, an adequate but not particularly efficient means of control.

The chilled water/condenser water system’s OptimissioningSM effort included the replacement of the cooling tower fill, the replacement of the two chiller’s condenser water isolation valves, the addition of two cooling tower isolation valves, the addition of variable frequency drives for the cooling tower fans, and the addition of a flow measuring station in the chilled water system decoupler. The central plant was also re-balanced, and new temperature sensors were installed throughout the system. Controls hardware was added as required, and numerous modifications to the sequences of operations and setpoints were accomplished.

An innovative solution to the lack of chilled water temperature control was also devised using existing control wiring, so the chillers are now capable of resetting their leaving chilled water temperature as required based upon system demand. The reset schedule is currently based upon wet-bulb (WB) ambient temperature, and is indexed to the heat exchanger operation in order to minimize radical temperature swings in the supply chilled water. Having the ability to control and monitor the leaving water temperature setpoint also allowed the system “fail-overs” to be based upon specific criteria and all but eliminated the possibility of operating two chillers (one on partial capacity due to a refrigerant circuit malfunction) when one was capable of meeting the demand.

The addition of the flow meter in the decoupler allowed flow rates to be monitored, and was a valuable tool in the water balance process and establishing the differential pressure setpoint for secondary pump control. This setpoint, originally at 14 PSI, was adjusted down to 8 PSI during OptimissioningSM. Also, the (controls sequence) timing of the lead/lag of the secondary pumps, an issue that prevented one of the pumps from ever running, was corrected and now allows the run times to be equalized and provides redundancy in the event of a pump failure.

Also, as predicted, it appears that one chiller will be adequate to provide cooling for the entire facility on a design cooling day as opposed to both systems being in operation. A major factor in achieving the goal was the reduction of outside air quantities throughout the entire facility, including the Kitchen air handling units and the Dorm makeup air units. This will significantly reduce operating costs and was made possible by changes made throughout the HVAC System, including the air distribution and controls systems as well as the central plant.

Inoperable air distribution system components were identified and replaced with superior technology. Air handling unit #3’s inoperable chilled water control valve was replaced as was the valve at air handling unit #4, using pressure independent type valves. Use of a pressure independent type control valve will improve the chilled water system’s temperature differential (DT), reducing pumping costs and chiller run-times. Our experience with these types of valves has been excellent control capability, industrial quality, and substantially reduced a system differential pressure (DP) requirement that in turn allows other system chilled water valves to control more effectively. The higher system DT also allows the chillers to load more fully, eliminating the need to run multiple chillers at reduced loads, and saving the energy associated with the unnecessary chiller’s operation (cooling tower fans, condenser water pumps and primary chilled water pumps).

The outside air/mixed air/return air damper actuators at both air handling units were replaced, and demand controlled ventilation (CO2 monitoring) was installed for air handling unit #3. Air handling unit #3 is equipped with variable frequency drives on the supply and return fans. Previously, the return fan tracked the supply fan as a percentage of speed. This method is common, but due to differing fan types, their respective operating characteristics and much different operating duties it is never an effective method of control. It was determined that the return air fan was unnecessary except at high (above 50%) outside air quantities introduced during outside air economizer operations, and as such is now disabled below 50% damper position.

It was also apparent that the supply static pressure setpoint was be too high, as evidenced by the supply fan operating at 58 Hz even though the deck temperature was 54 degrees. At a 54-degree deck temperature, you would expect that at least some of the VAV boxes would be satisfied, requiring reduced airflows and that the supply fan in turn would unload. However, the VAV boxes are not currently visible on the Alerton DDC system, so the automatic static pressure reset control scheme is not capable of being used. A new, much lower setpoint of .5” was selected to control supply fan speed. Also, a supply air temperature reset schedule, based upon a linear range of outside air temperatures, was established for the air handling unit and will result in reduced energy costs and more comfortable space conditions.

No space pressure controls existed in the Dining/Kitchen Facility even though it is desirable to ensure that the Kitchen remain negative in relation to the Dining area at all times for odor control. The minimum outside air damper position on air handling unit #3 was 15%, and we discovered at this condition that excess “fresh” air was being delivered at this condition. The addition of ventilation controls (i.e. CO2 sensor) made it possible to allow air handling unit #3’s outside air damper to close (0%) completely and avoid delivering too much (and wasting energy) or too little outside air to the space.

The Test and Balance process determined minimal impact on space pressure relationships with widely varying outside air quantities delivered by air handling unit #3, and the same was discovered of air handling unit #4. Exhaust fans #10, 11, 13 are electrically interlocked with air handling unit #3, and it was determined that a minimum outside air damper position (allowable) of 0% for air handling unit #3 still has sufficient leakage to not effect space pressures. As such, the minimum damper position was set to 0%, reducing outside air loads and conserving energy. The same was true of air handling unit #4. The balance of the exhaust fans and the makeup air unit was excellent, essentially eliminating the need for additional makeup air.

Exhaust hoods #11, 13 are served (provided with) makeup air directly from VAV boxes, an unusual and wasteful energy practice. It is unusual in the sense that VAV boxes by definition vary the amount of airflow delivered to a space, but the exhaust fan serving the hoods are constant volume. It is wasteful because the makeup air is conditioned and then exhausted. Because hood #13 is never used, the supply grilles serving the adjacent spaces were closed, the exhaust fan was disabled, and the airflow to the hood was set to provide space cooling. If the hood is placed back into operation, the current supply air quantity will match the exhaust air quantity.

The outside air quantities for the Dorm makeup air units were also adjusted down as far as possible. The new quantities are still unnecessarily high and consideration should be given to changing the fan sheaves/motors to allow the quantities to be reduced further. However, the current conditions of the Dorm makeup air units, combined with the changes made to the Kitchen outside air quantities, and the changes made to the EMS system, have already proven extremely effective in reducing the load on the chilled water system.

Numerous changes to the operating sequences of the central plant, makeup air units serving the Dorm, and the air handling units serving the Dining facility were made during OptimissioningSM. Notably, an outside air wet-bulb (WB) temperature based supply chilled water temperature reset was developed. Using the supply chilled water reset schedule, the lag chiller is now called only when a deviation from setpoint (from the outside air WB reset schedule) of 15-degrees for 10 minutes is detected. Deviation from setpoint for a period of time is also used to enable the lag cooling tower and the lag secondary chilled water pump, 5-degrees for 20 minutes and 5 PSI for 15 minutes respectively. All of these setpoints and times are user adjustable.

The 100% outside air makeup air units serving the room fan coil units in the Dorm were previously being controlled identically to the room fan coil units: maintaining a “room” (duct) temperature of approximately 75-degrees. Working within the original Honeywell LonWorks database – the makeup air units are not currently controlled by the Alerton system – their control was changed to maintain a setpoint of 60-degrees below 60-degrees outside, 80-degrees above 80-degrees outside, and provide no heating or cooling between 60-80 degrees outside.

The VAV boxes served by air handling unit #3 and serving the Dining area are not currently controlled by the Alerton system. However, the supply air static pressure and the supply air temperature were in “Auto-Reset” mode, implying that the VAV boxes actual demand was controlling air handling unit #3’s operation. As this was impossible, the static pressure control was changed to manual, and a setpoint of .5” established during OptimissioningSM and airside balance. The supply air temperature is now controlled by an outside air temperature reset schedule from 68-degrees to 55-degrees.

The return air fan of air handling unit #3 was controlled as a percentage of supply fan speed (76%). Typically, return air fans are unnecessary devices that are improperly controlled, and this was the case for air handling unit #3. the return air fan has been disabled except when the economizer is allowed (outside air temperature below 70-degrees) and the outside air dampers have opened to at least 50%, at which time the return air fan is operated at 30% of the supply fan speed in order to provide pressure relief from the space. The minimum outside air damper position was changed to 0% when it was determined that sufficient outside air leaked thru closed dampers to maintain acceptable indoor air quality. A new CO2 sensor modulates the dampers to maintain an adjustable indoor CO2 level of 1000 ppm.

Results

Cumulatively, all aspects of the project may have actually increased the actual load served by the central plant; many necessary electrical loads were repaired and placed back into operation, outside air is now being supplied to the Dorm rooms, and many inoperable control valves were repaired so as to satisfy the loads. But when considered comprehensively the OptimissioningSM of the facility has more than offset those additional loads and even lowered the entire facility’s energy consumption by approximately 20+% and demand by approximately 15%.
Considering that only the HVAC system – a single component of the facility’s energy consuming systems – was optimized these are very impressive results. The following charts were created from actual utility data provided by Mr. Seaton, and generated by his Utility Manager software. The project was completed in 2006.

kWhChart TotalkWhChart

 

 

 

 

      Copyright Energy Environment Economics - 2003