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
- 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.
- 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
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
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/OptimissioningSM,
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/OptimissioningSM,
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.
- 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
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
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.
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.