Medical Centers Keep Cool With Variable Speed Compressorsfiles.danfoss.com/technicalinfo/dila/17/MedicalCenterCaseStudy... · systems employing Danfoss VSH variable-speed compressors.

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<3years payback

When combined with air-to-air heat exchangers, Danfoss VSH variable-speed compressors are helping Annexair’s rooftop makeup air units use less energy and are expected to have less than a 3-year payback.

Medical Centers Keep CoolWith Variable Speed Compressors As the oldest inland city in South Carolina dating back to King George III,

Camden is proud of its stately homes, steeplechase racing and southern

resort hospitality. It’s also proudly celebrating the 100-year anniversary of

its hospital -- today known as KershawHealth Medical Center. This modern,

multi-story, 215-bed facility provides general medical and surgical care for

65,000 residents in Kershaw County and surrounding communities. It’s also

keeping up with the times by using energy-efficient, dedicated outdoor air

systems employing Danfoss VSH variable-speed compressors.

“Today’s hospitals are built to comply with ASHRAE Standard 62-2007,

which sets the minimum ventilation requirements for acceptable indoor air

quality,” notes Anthony Palucci, director of sales for Annexair, the system

manufacturer. “Our goal was to provide a product that could not only meet

the outside air requirements, but also provide energy savings.”

The ASHRAE standard requires operating rooms to have 25 air changes per

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

surgical suite. Other hospital areas require two changes per hour to control

odors and provide ventilation for comfort. But using a traditional

mechanical cooling for that amount of outside air is extremely expensive.

Annexair’s solution was to use a recuperative dehumidification design in

the makeup air-handling systems. Each unit employed a Danfoss VSH

variable-speed compressor to supply a cooling coil used in conjunction

with a fixed plate heat exchanger to recycle thermal energy.

“There are several ways to meet requirements for 100 percent outdoor air

without paying a high price for electricity,” says Palucci. “Typically, a unit

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.

But that makes the air too cold. Then you’ve got to use hot gas reheat to

raise the sensible temperature, which is extremely inefficient.”

Palucci proposed a dedicated outdoor unit package that takes advantage

of a fixed plate heat exchanger to provide free pre-cooling and free-reheat

functions to handle dehumidification at full load. The variable-speed

compressor provides the balance of cooling needed at part loads.

This design takes advantage of Camden’s maritime climate,” says Palucci.

“In the summer, humidity is high and temperatures average 78 degrees

Fahrenheit, peaking in the 90s in July. The heat exchanger does double

Case Story

MAKING MODERN LIVING POSSIBLE

USCO.EA.014.A1.22 ©Danfoss USCO/mk, 02.2013

duty by providing cooling and reheating to

dehumidify, using the compressor and cooling

coil section as needed during part-load

conditions.”

Controlling airborne moisture in the fresh air

supplied to the facility is important to control

fungus and microbe growth inside. Proper

dehumidification control is also important to

minimize moisture in the airhandling unit itself.

If ambient air is not properly pre-cooled and

reheated, condensation can occur.

Palucci explains that in an air-to-air recuperative

unit, the outdoor air passes through the fixed

plate heat exchangers twice (2-pass). The heat

exchange occurs between the incoming fresh

warm air and the cool air leaving the cooling

coil. On the first pass, the fixed plate pre-

conditions the air reaching the cooling coil by

reducing the sensible temperature, thereby

requiring less mechanical cooling.

At part-load conditions, the VSH compressor

supplies the DX coil with exactly the required

cooling capacity to drop the sensible

temperature even further as needed. The

cooling coil maintains a constant leaving air

dewpoint with the help of the VSH compressor,

without the use of energy wasting hot gas

by-pass. Then in the second pass, the cooled air

is reheated by absorbing heat from the

incoming air stream. Again as needed, a hot gas

reheat coil will raise the sensible temperature to

maintain a neutral dry supply air. But because

the heat exchanger is about 65 percent effective

in handling both precooling and reheating

functions simply through thermal transfer,

electric consumption for additional cooling and

reheats is minimized.

Annexair provided two rooftop makeup air units

(MAU), each using one 22-ton Danfoss VSH

compressor and each delivering 3,800 cfm.

Another MAU employs a 12-ton compressor and

provides 1,700 cfm. The compressors supply the

mechanical cooling section to enable the MAU

to deliver dry air when dry bulb energy air

temperatures range from 95 degrees to 55

degrees and wet bulb temperatures range from

78 degrees to 53 degrees.

“We prefer VSH series compressors for our

dedicated MAU systems because they are a

variable-speed scroll designed specifically for air

conditioning and heat pumps,” says Palucci.

“They come with all the features completely

packaged to make it easy to integrate a

variable-speed compressor into our systems.”

With a 3:1 turndown ratio, the VSH compressors

are ideal for dehumidification. “We take

advantage of the variable-speed control to

deliver only the required cooling capacity,”

notes Palucci. “Even in South Carolina, over 90

percent of operating hours occur at part-load

conditions. The turndown ratio allows

compressor speed to be reduced to match lower

capacity requirements. Between the two MAU

designs, we can go from 22 to 7.5 tons and 12 to

4.”

“Anytime the compressor can reduce capacity, it

makes the size of the cooling coils

thermodynamically larger,” explains Palucci.

“And when the airstream is exposed to more coil

surface area, more water vapor is pulled out of

the air.”

The speed control allows each unit to operate

within +0.2 degrees of setpoint. Fixed-speed

compressors run at full speed and have to be

cycled on/off or use someother capacity-

reduction method that wastes energy -- and

creates more noise.

Noise reduction is a major benefit in a hospital

setting. In the evenings, when sound is critical

and loads are typically low, the VSH compressors

operate at their slowest and quietest -- unlike

fixed-speed compressors that frequently cycle

on and off. The VSH drive controller also

provides a soft start that ramps up the inrush

current slowly. Avoiding a huge amperage surge

makes it easier on the motor winding, thus

extending the life of the motor.

Reliability of the Annexair MAU design is also

enhanced because the air-to-air design’s heat

exchangers involve no moving parts, use no

external connections to water or glycol loops,

and do not use a heat-transfer fluid that could

be subject to freezing.

“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

systems.”

Anthony Palucci, director of salesfor Annexair

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“The facility went through some scorching

summer heat with these units,” says Palucci. “But

a 100 percent dedicated air-to-air MAU design is

ideal for this application. The units can hit

temperature and humidity setpoints exactly.

And compared to the old, generic units they

replaced, they’re using a lot less electricity. In

fact, with the continuous energy savings from

the air-to-air heat exchangers and VSH

compressors, the expected payback is less than

three years. Conventional units have no

payback. Recuperative air-to-air

dehumidification technology is obviously a

great fit for KershawHealth Medical Center.