Bryant COMMERCIAL SINGLE PACKAGE ROOFTOP GAS HEAT/ELECTRIC COOLING 581B User Manual page 50

Standard Unit Refrigerant Cycle
At point no. 1 in the pressure enthalpy curve, vapor leaving the
compressor at a high pressure and a high temperature enters
the condenser. The condenser removes heat from the refriger-
ant, lowers its temperature, and changes it to a liquid. At point
no. 2, the liquid leaves the condenser and enters a fixed expan-
sion device that lowers the pressure of the refrigerant. At point
no. 3, the liquid enters the evaporator coil, where the refrigerant
increases in temperature and changes back to a vapor. At point
no. 4, the vapor leaves the evaporator and reenters the com-
pressor.
Refrigerant Cycle Using Perfect Humidity™ Dehumidification
Package
When a subcooler coil is added to the rooftop unit, the refriger-
ant is affected in such a way that the unit latent capacity is
increased. The refrigerant cycle follows the same path from
point no. 1 to point no. 2 as the standard refrigerant cycle with-
out a subcooler (see the pressure enthalpy curve). However, at
point no. 2, the liquid refrigerant enters the subcooler coil where
the temperature is lowered further. At point no. 2A, this sub-
cooled liquid enters the TXV, which drops the pressure of the
liquid. At point no. 2B, the liquid enters the fixed orifice metering
device. The refrigerant leaves this device as a saturated vapor
and enters the evaporator at point no. 2C. The improved refrig-
eration effect can now be seen between point no. 2C and point
no. 3. The increase in the total refrigeration effect is the addi-
tional enthalpy gained from point no. 2C to point no. 3. However,
the subcooler coil rejects this added refrigeration effect to the
air downstream of the evaporator coil, thus maximizing the over-
all latent effect. This improved latent effect is a direct result of
the addition of the Perfect Humidity subcooler coil to the refrig-
erant cycle.
Latent Capacity Effects
Refer to the psychrometric chart to see how the sensible heat
factor decreases when the optional Perfect Humidity dehumidifi-
cation package is installed. This chart contains data for the
5-ton 581B unit operation, both with and without the Perfect
Humidity package, at 1750 cfm. Point no. 1 on the chart repre-
sents the return-air dry bulb (80 F) and wet bulb (67 F) condi-
tions. Point no. 2 represents the supply-air conditions for a
standard 581B rooftop unit without the Perfect Humidity dehu-
midification package. Point no. 3 represents the supply-air con-
ditions for a 581B rooftop unit with the Perfect Humidity
package. By connecting point no. 1 and point no. 2 on the chart
and finding the intersection on the sensible heat factor scale,
the sensible heat factor is 0.73. Connect point no. 1 and point
no. 3, and see that the sensible heat factor is 0.58. This is a
17.5% increase in latent capacity for the given conditions. This
increase in latent capacity allows the 581B rooftop units to
remove more moisture from the conditioned space; thus lower-
ing the humidity levels.
Dehumidification Effects
Further evidence of dehumidification can be seen by analyzing
the pounds of water per pound of dry air found in the supply air.
At point no. 2 in the psychrometric chart, there are 65 grains
(0.0092 lb) of moisture per pound of dry air. At point no. 3, there
are 58 grains (0.0083 lb) of moisture per pound of dry air. This
is a 12.1% decrease in the amount of water in the supply air.
17. PERFECT HUMIDITY DEHUMIDIFICATION PACKAGE
OPERATING PERFORMANCE — Perfect Humidity dehu-
midification package operation does not affect the electrical
data. The electrical data remains the same either with or
without the Perfect Humidity package.
The operating and shipping weights will be slightly
increased with the addition of the Perfect Humidity sub-
cooler. See the Physical Data table for added base unit
weight with this option.
Refer to cooling performance data, both with and without
the Perfect Humidity dehumidification package. Note the
APPLICATION DATA (cont)
greatly improved latent capacity with the Perfect Humidity
dehumidification package.
Static pressure is also slightly affected by the addition of
the Perfect Humidity dehumidification package. See Static
Pressure Drop table on page 40 when using this option.
18. PERFECT HUMIDITY DEHUMIDIFICATION PACKAGE
FEATURES AND BENEFITS
• The Perfect Humidity dehumidification package can improve
the humidity control of your rooftop equipment by up to 40%.
This greatly reduces the risk of sick building syndrome by
reducing biological growth in both ductwork and the rest of the
building.
• The Perfect Humidity dehumidification package improves
comfort levels in the building by better controlling the humidity.
This improved comfort level allows building tenants to raise
the cooling set point on the thermostats for accumulated
energy savings.
• Better humidity control lowers the risk of humidity-induced
property damage.
• The Perfect Humidity dehumidification package permits build-
ing refrigerators and freezers to operate more efficiently due
to lower relative humidity levels. This is the perfect solution for
supermarket applications.
• The Perfect Humidity subcooling circuit can be operated by a
humidity sensor. If the sensor is used, the Perfect Humidity
circuit will then only operate when needed. If the humidity lev-
els in the occupied space are acceptable (such as in the
spring and fall seasons), the Perfect Humidity circuit will not
operate. The rooftop unit is then able to operate to its full sen-
sible potential, which provides more efficient performance and
energy savings.
• At lower outdoor temperatures, rooftop units with dehumidifi-
cation devices are subject to low suction pressure conditions.
The Perfect Humidity dehumidification package contains a
low-pressure switch that deactivates the Perfect Humidity
dehumidification package under low suction pressure condi-
tions without deactivating the compressors.
• Improved humidity control allows increased outdoor-air venti-
lation in hot and humid geographical areas. Humidity control
also helps to improve the indoor-air quality of the building.
• The Perfect Humidity dehumidification package is factory
installed. There are no additional field installation costs. There
is also no need to purchase a roof curb from another manu-
facturer, as standard Bryant roof curbs will accommodate the
581B rooftop units which have the Perfect Humidity dehumidi-
fication package option installed.
• The Perfect Humidity dehumidification package is engineered
and manufactured by Bryant for Bryant rooftop units. All appli-
cation support, service, and warranty issues can therefore be
handled through one company.
• The slightly lower sensible capacities obtained when using the
Perfect Humidity dehumidification package allow the unit to
operate for an extended period of time. The more the unit
operates, the more air is exposed to the subcooling coil. This
rooftop unit increased latent capacity results in lower relative
humidity levels in the occupied space.
• It is no longer necessary to oversize equipment and add
reheat devices to properly dehumidify your building. In a typi-
cal scenario, a building owner may need 39,000 Btuh of sensi-
ble capacity and 23,000 Btuh of latent capacity (62,000 total
Btuh). To accomplish this without the Perfect Humidity dehu-
midification package, a 7
would be necessary to attain the higher latent capacity
required. This results in a large up-front expense to oversize
the equipment from a standard 5-ton to a 7
The building owner can now purchase a 5-ton unit with the
Perfect Humidity dehumidification package for a small addi-
tional up-front charge, and no reheat device will be necessary
to satisfy the cooling requirements. This reduces both installa-
tion costs and operating costs throughout the life of the
product.
50
1 / -ton unit with a reheat device
2
1 / -ton unit.
2