Shel lab SRI3, Operation Manual

Page: 31 / 42

31 | P a g e
USER MAINTENANCE
R
EFRIGERATION AND
D
EFROSTING
The refrigeration compressors of most home refrigerators run periodically in order to maintain an
average chamber temperature centered on the thermostat setting. This results in temperature
oscillations of up to 2° or 3°C and reflects the fact that affordable refrigeration compressors cannot
provide the high degree of temperature stability needed for laboratory applications — at least not
without assistance.
To counteract this instability, refrigeration compressors in the SRI family of incubators and plant
growth chambers run continually to provide constant chilling. At the same time, precise applications
of heat maintain thermal stability in the incubation or cultivation chamber to approximately
±
0.5
°
C.
Freezing and Defrosting
Air is chilled in the chamber by blowing over a refrigeration coil. The surface temperature of the coil
may dip below freezing. This happens frequently when the unit is run toward the low end of its
performance range. At these temperatures, humidity in the air will condense and freeze on the coil.
Figure 3: Refrigeration Coil and Heating Element
To counteract this, a defrost thermostat checks the temperature of the coils for 20 minutes, once
every 12 hours. If the coil surface temperature is cool enough for ice to form during the 20-minute
interval, the defrost thermostat deactivates the compressor. This allows heat from the nearby heating
element to melt the ice on the coil without having to compete with the chilling flow of coolant fluid
through the coil. The defrost thermostat reactivates the compressor when the surface temperature
rises above the defrost trigger set point or when the 20-minute interval comes to an end.
If the incubator is running at the low end of its temperature range, defrosting can result in short-lived
but significant heat spikes in the unit incubator. This may preclude its use in biological oxygen
demand studies below 20°C.
Defrost Thermostat Heating Element
Refrigeration Coil