Engineered Comfort Fan Coil Controller EZstat Especificaciones descargar pdf (Pagina 4)

8-20-07

FAN COIL UNIT CONTROLS

THE PROBLEM

High humidity at part load conditions has always been a

problem with traditional fan coil unit operation and will

become a greater factor in the selection of equipment by

design engineers in the future. Too much humidity and

comfort zone temperatures decrease to the point that

occupants feel chilled or clammy. This may also create

favorable conditions for mold and mildew growth.

As air moves across a cooling coil, the temperature of the

coil is normally below the dew point of the return air. This

causes the water in the air to condense on the coil surface

where it is gathered in a drain pan and disposed of through

drain lines. The air leaving a coil is typically about 55 to

60ºF. Since the temperature of the coil is usually below the

dew point of the entering air, water has been condensed

from the air and the air is very nearly saturated. This nearly

saturated air warms slightly as it moves through the duct to

the diffusers. By the time it exits into the room, it has risen

a degree or two in a typical system. It then mixes with the

room air and is again warmed, typically to about 74 – 78ºF

(23 – 26

ºC). Both air temperature and water content are

increased in the room; however, relative humidity levels

decrease because the warmer air is capable of holding

more water. The percentage compared to saturated air at

the higher temperature has decreased. See line A-B on the

attached psychometric chart.

THE SOLUTION

The attached sequence of operation (See Figure 2) and

psychometric chart (Figure 3), illustrate how the EPIC

control sequence utilizes variable air volume control, chilled

water valve modulation and constant discharge air

temperature to control a typical space using our unique

controls. Engineered Comfort has chosen 52ºF (11

ºC) as

the optimum discharge air temperature for fan coil

operation.

By lowering the discharge air temperature slightly, the

humidity levels in the room can be lowered. See line C-D

on the attached psychometric chart. This causes the

occupants to feel more comfortable at a slightly higher

temperature. The room temperature required to maintain

acceptable comfort can be raised by as much as 4ºF

(2.5

ºC). Most occupants will be more comfortable at the

increased temperature. This accomplishes five very

important results in addition to the energy and reheat

savings already provided by the ECM motor.

1. Lower relative humidity:

If the air volume and water to the coil are modulated to

maintain the discharge air temperature at all room

conditions as described in the EPIC control sequence,

room relative humidity levels decrease by 10 to 20%, and

there is less chance for wall sweating, which in turn lowers

the chances of mold growth. See line A-B vs. C-D on the

attached psychometric chart.

2. Higher comfort level temperature Setpoint:

The lowered relative humidity allows the occupant to reset

the room temperature higher by 1 – 4ºF (0.5 – 2.5

ºC), while

maintaining acceptable comfort levels. This in turn saves

energy due to higher room set points. (See Figure 1 printed

from ASHRAE Handbook, Fundamentals 2005, Chapter 8,

page 8.12)

Figure 1: ASHRAE Summer and Winter Comfort Zones

(Acceptable ranges of operative temperature and humidity for people

in typical summer and winter clothing during primarily sedentary activity.)

3. Increased chiller efficiency:

If the air volume and water to the coil are modulated to

lower flow during part load conditions, fan energy and

pumping energy is saved by taking advantage of room to

room building diversity on both the water and air sides of

the unit. Additionally, this holds the water and air in contact

longer at the coil allowing greater heat transfer from the air

to the water. This increases the return water temperature to

the chiller and decreases the required pumping energy

while increasing the efficiency of the chiller operation.

Consequently, the pipe sizes needed for the risers and any

duct run outs may be reduced. These reductions may offset

any additional first cost of the equipment.

4. Lower airflow and reduced fan energy cost:

If the supply air temperature is lowered using the EPIC

control sequence, less air from the fan coil is needed to satisfy

the room demand. The reduction in airflow can be

calculated as follows:

Variable Air Volume EPIC Fan Technology

Fan Coil Operation with

Constant Discharge Air Temperature

HUMIDITY PATIO, lb water vapor per 1000 lb dry air

OPERATIVE TEMPERATURE,

DEW-POINT TEMPERATURE,

60 65 70 75 80 85 90

100% rh

50% rh

60% rh

30% rh

Winter

Summer

F Wet Bulb

CFM

x ΔT

= CFM

x ΔT

Where: CFM

= Airflow and ΔT

= EAT – LAT for Std. FCU

CFM

= Airflow and ΔT

= EAT – LAT for EPIC FCU

Therefore:CFM

= CFM

(

ΔT

)

ΔT

1 2 3 4 5 6 7 8 9 ... 19 20

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