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
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 71/2-ton unit with a reheat device 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 71/2-ton unit.
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.
