Building Description

L4605 is a 24/7 helicopter maintenance facility with supporting administrative offices, training and conference areas. The maintenance bays are evaporatively cooled, and are heated with 100% outside air natural gas makeup air units. The adjacent office areas are heated/cooled using a 4-pipe chilled water/hot water system. As would be expected of such a facility, the HVAC requirements of the various areas differ significantly.
The HVAC system consists of:

• The chilled water system consists of one air-cooled water chiller and two constant volume chilled water pumps.
• The hot water system is comprised of two hot water boilers and associated pumps and provides space heating used for zone heating.
• 2 central station air handling units, both are variable volume, serve the VAV boxes in the office/training areas. All air handling units used variable frequency drives for capacity control, both are equipped with supply and return fans, and all were equipped with 100% outside air economizers.
• 35 zone VAV/FPVAV boxes with hot water reheat coils.
• 8 evaporative coolers serving the maintenance bays.
• 4 100% outside air natural gas fired makeup air units serving the maintenance bays.
• Complete direct digitally controlled EMS.

Project Problems/Issues

• Lack of original project commissioning
• Inability to maintain space pressure requirements between evaporative cooled areas and air conditioned areas
• Improper application of energy management capabilities
• Excessive utility costs

Project Overview

The HVAC system is equipped with some energy conservation capabilities: variable hot water pumping schemes, variable airflow (VAV) air distribution, and airside economizers with a four-pipe chilled water/hot water distribution systems with should make for a relatively efficient HVAC system. Direct Digital Controls (DDC) applied to the HVAC system should also aid in creating an efficient operating system. The Maintenance Bay(s) and other maintenance areas located outside the Bay(s) are served by evaporative coolers and 100% outside air natural gas fired heaters.

The building occupancy profile is mixed, including offices and classrooms that are air conditioned via chilled water air handling units. There are also offices served by the chilled water air handling units that are encompassed by maintenance areas that are evaporative cooled. These areas are equipped with hot water unit heaters, while the main Maintenance Bay is evaporative cooled and heated via natural gas. Roof mounted exhaust fans in the Bay are used to provide space pressure control by exhausting 100% outside air introduced by the evaporative coolers/makeup air units. Several other exhaust fans are spread throughout the facility, including the shower/locker area. Some areas have specific temperature and humidity requirements such as the ALSE area as follows:

The scope of work for this project was to assure that the HVAC system was installed per the construction documents and was operating per the construction documents. We also developed and implemented remediation measures for existing deficiencies, particularly in the DDC system. The HVAC system was commissioned and numerous optimization measures were provided for the DDC system.

Solutions Provided

The chilled water system does not have any water-side economizer capability due to the installation of air-cooled water chillers. The central plant was enabled on a call from any chilled water valve position above 0%, thus the central plant was in constant operation. Also, the chilled water reset schedule was not enabled, so the system was in operation and making much too cold of water a significant amount of time. Although not usually desirable on a variable volume pumping/air distribution system, a chilled water reset schedule on a constant volume chilled water system will save energy.

A reset schedule based on ambient dry-bulb (DB) temperature was developed and placed into operation. The supply chilled water temperature is reset from 44-degrees at 105-degrees ambient to 56-degrees at 70-degree ambient, and all setpoints were made adjustable. The central plant – i.e. the lead chilled water pump and chiller – is now requested when the supply air temperature is above the setpoint by 10-degrees for a period of 2 minutes. This sequence allows the chilled water pumps to remain disabled as long as possible. The chilled water system is disabled entirely below 60-degrees outside air temperature, and the facility will be provided air conditioning via the 100% outside air economizers located on the air handling units.

It was discovered that the programming of the chilled water pumps’ lead-lag sequence was faulty and was causing both pumps to be in continuous operation even though one is redundant, an obvious waste of energy. The programming was corrected and the pumps now rotate on a weekly schedule. In the event of a lead pump failure the lag pump is automatically started. A current transformer provides status of each pump and alarms are generated in the DDC system graphics in the event of a fail-to-start condition.

In general, the air distribution system was in fair working order. As would be expected in a facility that has adjacent spaces that are either evaporative cooled and air conditioned (refrigerated air), it is difficult to maintain space humidity limits in the refrigerated areas. And because the space temperature that can be achieved from refrigerated air is significantly lower than that of evaporative cooled air, it is common to see doors open between spaces that promotes a mixing of what should be disparate airstreams. Introducing nearly saturated (evaporative cooled) air into the chilled water system via the return air to the chilled water air handling units increases latent loads that are intended to be exhausted from the space and not returned to the air handling units. These latent loads can only be removed by sufficiently cold chilled water that allows the chilled water coil to reach what is known as the apparatus dew point and condense moisture out of the airstream. Of course this requires the chiller to make colder water – consuming more energy – than would be required without the additional latent load.

From the included graphics, note the minimum outside air CFM settings on air handling units 1 and 2 - 1000 CFM and 80 CFM respectively - but the dampers at 20% and the outside air not under control. The outside air ducts are equipped with airflow measuring stations that are much too large to provide accurate control of outside air quantities at such low velocities. Sufficient velocity is necessary for accurate sensing, and at these low flows (and velocities), due to the area of the outside air duct, there is little to no control of outside air quantities. The outside air dampers, sized for 100% outside air, are also incapable of modulation to such low outside air quantities and as such are most likely introducing excessive outside air which then has to be conditioned by the cooling/heating systems, an obviously wasteful practice. Also, the chilled water valve of air handling unit #2 at 0% (closed) but making 55 degree air with a 64 degree setpoint. It was discovered that the signal to control the chilled water valve was 0.5-10VDC instead of the usual 2-10VDC. The output signal was corrected.

Note from the graphics the minimum CFM setting (80 CFM) on air handling unit 2 but the damper at 20% and the airflow measuring station sensing 259 CFM. Air handling unit 1 minimum setting of 1000 CFM was “delivering” 588 CFM but the damper was showing 20% open. It is obvious that the outside air controls are flawed, so during OptimissioningSM it was determined that the outside air dampers could remain closed and deliver sufficient outside air simply via damper leakage when not in economizer operation. Field measurements showed adequate outside air CFM’s at varying fan speeds, and the close proximity of 100% outside air spaces (Maintenance Bays) and accessibility between the spaces makes outside air issues essentially a moot point. Accordingly, the outside air dampers are now modulated only during economizer mode during appropriate – between 45-degrees and 72-degrees - ambient conditions.

Commissioning of the gas fired makeup air units revealed that the controls were incorrectly controlling the individual stages (3/unit) of heat. The gas valves are reverse acting, not direct acting as programmed. Also, the control signal required was 2-10 VDC, not the 4-20 mA signal being used. Dip switch settings were corrected on all gas valves, and a new sequence of operation for staging of the makeup air units and their individual gas burners was devised and implemented. Prior to Commissioning/Optimissioning^SM, the Hanger Bays suffered from severe overheating due to these deficiencies, and the natural gas consumption reflected this condition. New staging sequences were also developed for the evaporative coolers during Commissioning/Optimissioning^SM, and the coolers were Tested and Balanced.

Numerous items regarding the improper control of the air distribution system were discovered and corrected during the commissioning process, and the original design flaws were addressed to the best of our collective abilities. Those flaws included insufficient controls to allow the supply and return fans to track accurately and airflow measuring stations too large to allow for accurate control of outside air flows. Also:

• The return air fans are currently disabled at all times except during outside air economizer mode at an adjustable outside air damper position. However, controls were added to start the return air fan during normal (chilled water) operation should they be required.
• New supply air temperature reset limits – from 67-degrees to 50-degrees – were established. While 50-degrees is unnecessarily low, the presence of certain problematic zones create conditions where colder supply air is “required”. However, the outside air temperature reset schedule used by the chiller to determine chilled water supply temperature will ultimately limit what supply air temperature can be delivered to the zones.
• Referring to the included graphic screen prints, air handling unit #1 is operating at the high – 1.3” – static setpoint even though the “need more air signal” of 28% indicates all zones satisfied. Numerous hours were dedicated to revising the supply air static reset ranges and limits as shown. Air handling unit #2 also shows similar improvement.

Exhaust Fans 192 A-H for Maintenance Bay Space Pressure Control:
The Maintenance Bay(s) makeup the majority of the evaporative cooled area in the building, and they do have a space pressure control capability but it was deemed less than optimum and refined as discussed below. As discussed above, it is imperative to maintain proper space pressure relationships between the evaporative cooled areas and the air conditioned (chilled water cooled) areas of the facility. Prior to revision, all of the exhaust fans were proportionally enabled on a continual rise above space pressure setpoint. For more accurate control, a proportional-integral-derivative (PID) control loop was devised and implemented as follows.

The present space pressure setpoint is 0.03 “water column (w.c.). When the space pressure rises above this value, a PID loop calculates the demand percentage of exhaust air that is required. The pressure sensor is located in the far south part of the hanger where the first exhaust fan (EF) stage starts.

The staging of the evaporative coolers and the heating makeup air units (MUA) serving the Maintenance Bays were also found to be less than ideal, and temperature control complaints were commonplace. Considering the variety of mistakes made when programming the system, especially the makeup air units, this isn’t surprising. All found deficiencies – incorrect control signal type, incorrect control signal value, and incorrect dip switch settings on the gas valves - were corrected, and the control for these units was refined as follows:

Evaporative Cooler and MAU Heat Control:
The original sequence started all evaporative coolers on the same control signal. Combined with an outside air temperature cooling lockout of -6-degrees assured that all evaporative coolers ran at all times unless turned off at the unit disconnect. This has been modified as described below. The four makeup air units essentially acted as two units, meaning that if both units had identical setpoints they would both start simultaneously. And, as described below, all units started in full (100%) gas heat mode, causing uncomfortable space conditions and consuming excess energy.

The outside air temperature heating lockout, meaning that no heating (makeup air units are disabled) is allowed above this temperature, is now 60- degrees adjustable. The outside air temperature cooling lockout, meaning that no evaporative cooling (evaporative coolers are disabled) is allowed below this temperature, is 80-degrees adjustable. The space temperature setpoint is 81-degrees (NOTE: this was the original design criteria) with zero cooling offset and a heating offset of 16-degrees adjustable, which translates to a heating setpoint of 65-degrees and a cooling setpoint of 81-degrees. Neither fans nor the evaporative coolers’ pumping station are enabled while the space temperature is in the deadband (65-degrees to 81-degrees). Also, evaporative coolers #2, 4 that serve interior (not main Hanger Bay) areas are prohibited from enabling the pumping station.

For outside air temperatures above or below the setpoints, a PID loop determines the on-off and gas burner (makeup air units only) staging of the evaporative coolers and makeup air units. Also, adjustable temperature differentials and times were established for the individual makeup air units’ stage up/down control in order to minimize wide variations in space temperatures.

NOTE: Each makeup air unit is equipped with 3-stages of heat. It was discovered during commissioning that the gas valves were of modulating type and required an analog control signal. Some of the Alerton DDC outputs were configured for 4-20mA instead of the required 0-10VDC, and none of them were inverted – as was required - to start out with 10V for minimum gas staging. Furthermore, some of the Maxitrol Selectra gas valve signal conditioners were in an un-configured and unknown state due to improper dip switch settings. The Maxitrol takes the Alerton signal and converts it to a 0 – 20 VDC signal that the gas modulator/regulator valve is controlled with.

All issues were corrected. The control signal was corrected so that the PID percentage between the stages is the analog signal for the lower stage. Therefore, as the PID percentage goes from 17% to 33%, the stage 2 analog value goes from 0% to 100%. Further, the analog output is 0–10 volts DC (VDC), but is inversed due to the fact that the gas valve operates inversely. The gas valve modulator/regulator is full open at a 0 VDC input.

Results

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 2007.