Medical Centers Keep Cool With Variable Speed

MAKING MODERN LIVING POSSIBLE
Case Story
Medical Centers Keep Cool
With Variable Speed Compressors
As the oldest inland city in South Carolina dating back to King George III,
Annexair’s solution was to use a recuperative dehumidification design in
Camden is proud of its stately homes, steeplechase racing and southern
the makeup air-handling systems. Each unit employed a Danfoss VSH
resort hospitality. It’s also proudly celebrating the 100-year anniversary of
variable-speed compressor to supply a cooling coil used in conjunction
its hospital -- today known as KershawHealth Medical Center. This modern,
with a fixed plate heat exchanger to recycle thermal energy.
multi-story, 215-bed facility provides general medical and surgical care for
65,000 residents in Kershaw County and surrounding communities. It’s also
“There are several ways to meet requirements for 100 percent outdoor air
keeping up with the times by using energy-efficient, dedicated outdoor air
without paying a high price for electricity,” says Palucci. “Typically, a unit
systems employing Danfoss VSH variable-speed compressors.
like this will use a fixed-speed compressor and slow fan speed to increase
time through the cooling section to pull more moisture from the supply air.
“Today’s hospitals are built to comply with ASHRAE Standard 62-2007,
But that makes the air too cold. Then you’ve got to use hot gas reheat to
which sets the minimum ventilation requirements for acceptable indoor air
raise the sensible temperature, which is extremely inefficient.”
quality,” notes Anthony Palucci, director of sales for Annexair, the system
manufacturer. “Our goal was to provide a product that could not only meet
Palucci proposed a dedicated outdoor unit package that takes advantage
the outside air requirements, but also provide energy savings.”
of a fixed plate heat exchanger to provide free pre-cooling and free-reheat
functions to handle dehumidification at full load. The variable-speed
The ASHRAE standard requires operating rooms to have 25 air changes per
compressor provides the balance of cooling needed at part loads.
hour. At least five of those air changes must use 100 percent outside air to
maintain positive pressure to keep unhealthy air from leaking into the
This design takes advantage of Camden’s maritime climate,” says Palucci.
surgical suite. Other hospital areas require two changes per hour to control
“In the summer, humidity is high and temperatures average 78 degrees
odors and provide ventilation for comfort. But using a traditional
Fahrenheit, peaking in the 90s in July. The heat exchanger does double
mechanical cooling for that amount of outside air is extremely expensive.
<3
years payback
When combined with
air-to-air heat exchangers,
Danfoss VSH variablespeed compressors are
helping Annexair’s
rooftop makeup air units
use less energy and are
expected to have less than
a 3-year payback.
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duty by providing cooling and reheating to
“We prefer VSH series compressors for our
dehumidify, using the compressor and cooling
dedicated MAU systems because they are a
coil section as needed during part-load
variable-speed scroll designed specifically for air
conditions.”
conditioning and heat pumps,” says Palucci.
“They come with all the features completely
Controlling airborne moisture in the fresh air
packaged to make it easy to integrate a
supplied to the facility is important to control
variable-speed compressor into our systems.”
fungus and microbe growth inside. Proper
dehumidification control is also important to
With a 3:1 turndown ratio, the VSH compressors
minimize moisture in the airhandling unit itself.
are ideal for dehumidification. “We take
If ambient air is not properly pre-cooled and
advantage of the variable-speed control to
reheated, condensation can occur.
deliver only the required cooling capacity,”
notes Palucci. “Even in South Carolina, over 90
Palucci explains that in an air-to-air recuperative
percent of operating hours occur at part-load
unit, the outdoor air passes through the fixed
conditions. The turndown ratio allows
plate heat exchangers twice (2-pass). The heat
compressor speed to be reduced to match lower
exchange occurs between the incoming fresh
capacity requirements. Between the two MAU
warm air and the cool air leaving the cooling
designs, we can go from 22 to 7.5 tons and 12 to
coil. On the first pass, the fixed plate pre-
4.”
conditions the air reaching the cooling coil by
reducing the sensible temperature, thereby
“Anytime the compressor can reduce capacity, it
requiring less mechanical cooling.
makes the size of the cooling coils
thermodynamically larger,” explains Palucci.
cooling coil maintains a constant leaving air
The speed control allows each unit to operate
dewpoint with the help of the VSH compressor,
within +0.2 degrees of setpoint. Fixed-speed
without the use of energy wasting hot gas
compressors run at full speed and have to be
by-pass. Then in the second pass, the cooled air
cycled on/off or use someother capacity-
“We prefer VSH series compressors
for our dedicated MAU systems
because they are avariable-speed
scroll designed specifically for air
conditioning and heat pumps. They
come with all the features completely
packaged to make it easy to integrate
a variablespeed compressor into our
is reheated by absorbing heat from the
reduction method that wastes energy -- and
systems.”
incoming air stream. Again as needed, a hot gas
creates more noise.
At part-load conditions, the VSH compressor
“And when the airstream is exposed to more coil
supplies the DX coil with exactly the required
surface area, more water vapor is pulled out of
cooling capacity to drop the sensible
the air.”
temperature even further as needed. The
reheat coil will raise the sensible temperature to
Anthony Palucci, director of sales
for Annexair
maintain a neutral dry supply air. But because
Noise reduction is a major benefit in a hospital
the heat exchanger is about 65 percent effective
setting. In the evenings, when sound is critical
in handling both precooling and reheating
and loads are typically low, the VSH compressors
“The facility went through some scorching
functions simply through thermal transfer,
operate at their slowest and quietest -- unlike
summer heat with these units,” says Palucci. “But
electric consumption for additional cooling and
fixed-speed compressors that frequently cycle
a 100 percent dedicated air-to-air MAU design is
reheats is minimized.
on and off. The VSH drive controller also
ideal for this application. The units can hit
provides a soft start that ramps up the inrush
temperature and humidity setpoints exactly.
Annexair provided two rooftop makeup air units
current slowly. Avoiding a huge amperage surge
And compared to the old, generic units they
(MAU), each using one 22-ton Danfoss VSH
makes it easier on the motor winding, thus
replaced, they’re using a lot less electricity. In
compressor and each delivering 3,800 cfm.
extending the life of the motor.
fact, with the continuous energy savings from
Another MAU employs a 12-ton compressor and
the air-to-air heat exchangers and VSH
provides 1,700 cfm. The compressors supply the
Reliability of the Annexair MAU design is also
compressors, the expected payback is less than
mechanical cooling section to enable the MAU
enhanced because the air-to-air design’s heat
three years. Conventional units have no
to deliver dry air when dry bulb energy air
exchangers involve no moving parts, use no
payback. Recuperative air-to-air
temperatures range from 95 degrees to 55
external connections to water or glycol loops,
dehumidification technology is obviously a
degrees and wet bulb temperatures range from
and do not use a heat-transfer fluid that could
great fit for KershawHealth Medical Center.
78 degrees to 53 degrees.
be subject to freezing.
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USCO.EA.014.A1.22
©Danfoss USCO/mk, 02.2013