Handbook of air conditioning and refrigeration / Shan K

10.28 CHAPTER TEN
added to the summer outdoor design dry-bulb temperature to account for the temperature increase of
the roof due to solar heat. Let T o � 90°F (32.2°C). For a typical condenser, entering air temperature
is gradually raised to T al of about 102°F when it leaves the condenser coil. Hot HCFC-22 gas may
enter the top of the condenser coil at a temperature T gas � 170°F (76.7°C) and is desuperheated to a
saturated temperature of about 115°F (46.1°C). Gaseous refrigerant condenses to liquid and releases
the latent heat of condensation. As the refrigerant flows through the condensing coil, the condensing
pressure drops, typically by about 8 psi (55 kPa), before entering the subcooling coil. Because no
latent heat is released, the temperature of the subcooled liquid refrigerant drops sharply to 105°F
(40.6°C) when it leaves the air-cooled condenser.
Equations to calculate heat transfer between refrigerant and air in an air-cooled condenser are
similar to those for a water-cooled condenser, except that the temperature of condenser water
should be replaced by the temperature of cooling air, and the air-side heat-transfer coefficient h o is
discussed in Chap. 17. The condensing coefficient for horizontal tubes can be found in ASHRAE
Handbook 1997, Fundamentals.
Total heat rejection Q rej of an air-cooled condenser in a DX packaged system can still be calculated
by Eq. (10.16a). However, for air-cooled condensers, the refigeration load Q rl in Eq. (10.16a)
is equal to the DX coil load Q c, both in Btu/h (W), and F w, h � 1.
Cooling Air Temperature Rise and Volume Flow
For a specific Qrej, temperature rise between air entering and leaving the air-cooled condenser Tal �
To depends mainly on the volume flow rate of cooling air per unit of total heat rejection V˙ ca / Qu, rej .
Here Qrej is often expressed in tons of refrigeration capacity at the evaporator; therefore, V˙ ca / Qu, rej
is expressed in cfm/ton (L/s�kW). A smaller V˙ ca / Qu, rej usually results in a lower air-side heattransfer
coefficient ho, a greater Tal � To, a higher Tcon, a lower condenser fan power, and a greater
log-mean temperature difference �Tm between refrigerant and air. Conversely, a larger V˙ ca / Qu, rej
and a smaller Tal � To mean a lower Tcon, a smaller �Tm, greater fan power consumption, and probably
more noise from the propeller fans. Based on cost analysis, the optimum value of V˙ ca / Qu, rej is
usually 600 to 1200 cfm/ton (80.5 to 161 L/s�kW) refrigeration capacity at the evaporator. In
comfort air conditioning systems, if the heat rejection factor Frej � 1.25, air volume flow may be
between 40 and 80 cfm per 1 MBtu/h total heat rejection. When V˙ ca / Qu, rej � 900 cfm/ton (120.8
L/s�kW) refrigeration capacity, Tal � To is around 13°F (7.2°C). Fan power consumption in aircooled
condensers is usually 0.1 to 0.2 hp/ton (0.075 to 0.15 kW/ton ) refrigeration capacity.
Condenser Temperature Difference
The condenser temperature difference (CTD) for an air-cooled condenser is defined as the difference
in saturated condensing temperature corresponding to the refrigerant pressure at the inlet and
the air intake dry-bulb temperature T con, i � T o. The total heat rejection Q rej of an air-cooled condenser
is directly proportional to its CTD. The Q rej of an air-cooled condenser operated at a CTD of
30°F (16.7°C) is approximately 50 percent greater than if the same condenser is operated at a CTD
of 15°F (8.3°C). On the other hand, for a specific value of Q rej, an air-cooled condenser selected
with a CTD of 15°F (8.3°C) is larger than a condenser elected with a CTD of 30°F (16.7°C).
Air-cooled condensers are rated at a specific CTD related to the evaporating temperature T ev of the
refrigeration system in which the air-cooled condenser is installed. Typical CTD values are as follows:
Tev, °F CTD, °F
45, for air conditioning 20 to 30
20 15 to 20
�20 to �40 10 to 15