Application Data - Carrier Weathermaster 48HJD Series Product Data

Application data (cont)
5. Equipment Inefficiency:
Humidity can cause inefficient operation of refrigera-
tors and freezers.
6. Increased Energy Costs:
Because of high humidity levels and less comfortable
conditions, thermostat set points are lowered to force
the HVAC (heating, ventilation, and air conditioning)
equipment to run longer and work harder to lower the
humidity levels. Also, in an attempt to control humid-
ity, system designers typically oversize HVAC equip-
ment and add reheat capability to get the desired
latent capacity. This results in higher initial equipment
costs, as well as increased energy expenses through-
out the life of the unit.
Applications — There are many different rooftop unit
applications that are susceptible to problems caused by
high humidity levels. Some common applications include:
1. Restaurants:
The kitchen areas of restaurants have many humidity-
producing activities, such as dish washing and
cooking.
2. Supermarkets:
High humidity levels cause inefficiency in operation of
refrigeration and freezer systems.
3. Museums and Libraries:
Humidity can damage books and artwork.
4. Gymnasiums, Locker Rooms, and Health Clubs:
Shower areas and human perspiration cause uncom-
fortable occupied space conditions.
5. Hot and Humid Climates:
The southeastern United States is a good example of
this application. The MoistureMiser dehumidification
package becomes particularly useful when increased
amounts of the hot and humid outdoor air need to be
brought into the building for proper ventilation.
MoistureMiser dehumidification package design
effects — To fully understand the operation of the Mois-
tureMiser dehumidification package, refer to the pressure
enthalpy curve, and analyze the MoistureMiser package ef-
fects on the refrigerant in the rooftop unit. The pressure
enthalpy curve shows the refrigerant cycle for an 48HJ
rooftop unit.
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 condens-
er. The condenser removes heat from the refrigerant, low-
ers 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 compressor.
66
Refrigerant Cycle Using MoistureMiser Dehumidification
Package — When a subcooler coil is added to the rooftop
unit, the refrigerant 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 stan-
dard refrigerant cycle without a subcooler (see the pressure
enthalpy curve). However, at point no. 2, the liquid refrig-
erant enters the subcooler coil where the temperature is
lowered further. At point no. 2A, this subcooled liquid en-
ters the TXV, which drops the pressure of the liquid. At
point no. 2B, the liquid enters the Acutrol™ device. The re-
frigerant leaves this device as a saturated vapor and enters
the evaporator at point no. 2C. The improved refrigera-
tion effect can now be seen between point no. 2C and
point no. 3. The increase in the total refrigeration effect is
the additional enthalpy gained from point no. 2C to point
no. 3. However, the subcooler coil rejects this added refrig-
eration effect to the air downstream of the evaporator coil,
thus maximizing the overall latent effect. This improved la-
tent effect is a direct result of the addition of the Mois-
tureMiser subcooler coil to the refrigerant cycle.
Latent Capacity Effects — Refer to the psychrometric
chart in Fig. 7 to see how the sensible heat factor decreas-
es when the optional MoistureMiser dehumidification pack-
age is installed. This chart contains data for the 5-ton
48HJ unit operation, both with and without the Mois-
tureMiser package, at 1750 cfm. Point no. 1 on the chart
represents the return-air dry bulb (80 F) and wet bulb (67 F)
conditions. Point no. 2 represents the supply-air conditions
for a standard 48HJ rooftop unit without the Mois-
tureMiser dehumidification package. Point no. 3 repre-
sents the supply-air conditions for a 48HJ rooftop unit
with the MoistureMiser package. By connecting point
no. 1 and point no. 2 on the chart and finding the intersec-
tion on the sensible heat factor scale, the sensible heat fac-
tor 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% in-
crease in latent capacity for the given conditions. This in-
crease in latent capacity allows the 48HJ rooftop units to
remove more moisture from the conditioned space; thus
lowering the humidity levels.
Dehumidification Effects — Further evidence of dehumidi-
fication 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.
MoistureMiser dehumidification package operating
performance — MoistureMiser dehumidification pack-
age operation does not affect the electrical data. The elec-
trical data remains the same either with or without the
MoistureMiser package.