Handbook of air conditioning and refrigeration / Shan K

31.20 CHAPTER THIRTY-ONE
Temperature Gradient and Thermocline
Diffusers
FIGURE 31.7 Double-octagon diffuser for a cylindrical stratified tank: (a) plan view of bottom diffusers; (b) 12-in.
diameter PVC pipe diffuser.
Vertical temperature profiles are formed during charging or discharging in stratified tanks at various
time intervals. Temperature profiles may be illustrated on a height-temperature (H-T) diagram at the
beginning, the middle, and near the end of the charging process, as shown in Fig. 31.8. In the middle
of the charging process along the vertical height of the storage tank, chilled water is divided into
three regions: bottom colder-and-heavier stored chilled water, thermocline, and top warmer-andlighter
return chilled water.
A thermocline is a stratified region in which there is a steep temperature gradient. The water temperature
often varies from 42 to 60°F (5.6 to 15.6°C). The thermocline separates the colder stored
chilled water from the warmer return chilled water. The thinner the thermocline, the smaller the mixing
loss.
The layout and configuration of diffusers in a stratified tank have a significant effect upon the mixing
of the colder and warmer chilled water, as well as on the formation of the thermocline. The purpose
of the diffusers and their connecting piping is to distribute the incoming chilled water evenly,
so that it flows through the inlet openings with sufficiently low velocity (usually lower than 0.9 ft/s,
or 0.27 m/s) to minimize mixing of colder and warmer chilled water. Wildin (1990) recommended
that diffuser design and installation take into account the following:
● Warmer return chilled water should be introduced as closely as possible to the top water level in
the stratified tank. Colder stored chilled water should be introduced just above the bottom floor of
the tank.
● The inlet temperature of chilled water should be controlled within a narrow band (� 2°F or �
1.1°C) during charging to avoid additional mixing.