Bryant 50YEW Design Manual page 5

The 50YEW series compressors have a wide operating map, which
allows high temperature operation, up to 145°F [63°C] leaving
water temperature, even at 32°F [0°C] ground loop temperatures.
The ground loop heat exchanger [evaporator] is called the "Source"
heat exchanger in Bryant technical literature, and the heating
system heat exchanger is called the "Load" heat exchanger. The
terminology is not as important for heating only water-to-water
units, since the ground loop heat exchanger is always an evaporator,
but for reversible units, the evaporator and condenser change,
depending upon operating mode, heating or cooling.
Figure 1-3 shows a Bryant reversible water-to-water unit. With the
addition of a reversing valve, the Source and Load heat exchangers
can change functions, depending upon the desired mode of
operation. In the heating mode, the "Load" heat exchanger
functions as the condenser, and the "Source" heat exchanger
functions as the evaporator.
In fi gure 1-4, the reversible water-to-water heat pump now
provides chilled water on the load side instead of hot water. The
load heat exchanger becomes the evaporator, and the source heat
exchanger becomes the condenser. Because the evaporator is
susceptible to freezing under adverse operating conditions (e.g.
failed pump, controls problem, etc.), a coaxial heat exchanger is
used on the load side for reversible units.
When selecting equipment for systems that require cooling, all
aspects of the system design should be considered. In many cases,
a separate water-to-air unit for forced air cooling is more cost
effective than using a chilled water / fan coil application due to the
complication in controls and seasonal change-over. For ground
loop applications, the water-to-water and water-to-air units can
share one ground loop system.
Figure 1-5: COP vs TD
Bryant Geothermal Heat Pump Systems
WATER-TO-WATER HEAT PUMP DESIGN
Design Temperatures
Various types of hydronic distribution systems have been used
successfully with geothermal heat pumps. Radiant fl oor systems
use relatively mild water temperatures, whereas baseboard
radiation and other types of heat distribution systems typically
use hotter water temperatures. When designing or retrofi tting
an existing hydronic heating system, it is especially important to
consider maximum heat pump water temperatures as well as the
effect water temperatures have on system effi ciency.
Heat pumps using R-22 refrigerant are not designed to produce
water above 130°F [54°C]. Some heat pumps with R-410A and
R-407C refrigerant are capable of producing water up to 145°F
[63°C]. Regardless of the refrigerant, the effi ciency of the heat pump
decreases as the temperature difference (TD) between the heat
source (generally the earth loop) and the load water (the distribution
system) increases. Figure 1-5 illustrates the effect of source and load
temperatures on the system. The heating capacity of the heat pump
also decreases as the temperature difference increases.
As the temperature difference increases, the Coeffi cient of
Performance (COP) decreases. When the system produces
130°F [54°C] water from a 30° [-1°C] earth loop, the TD is
100°F [55°C], and the COP is approximately 2.5. If the system is
producing water at 90° F [32°C], the TD is 60°F [33°C] and the
COP rises to about 5.0, doubling the effi ciency.
If the water temperature of the earth loop is 90°F [32°C], and
the distribution system requires the same temperature, a heat
pump would not be needed. The system would operate at infi nite
effi ciency, other than the cost of pumping the water through the
Water-to-Water System Design Guide
Part I: System Overview
3