HVACR318 Refrigeration II

1. HVACR318 Refrigeration II Commercial Defrost Timers

2. Commercial Defrost Timers • Commercial refrigeration uses a more complex time clock than domestic refrigeration to control the defrost cycle.

3. Commercial Defrost Timers • Commercial defrost timers have a couple of advantages over the domestic type: • The timer is connected directly across the power line; this separates the timer from the operation of the compressor. • The defrost cycle can be started during periods when the unit is in minimum use.

4. Defrost Timer Settings • Commercial defrost timers generally have two timed settings. • The first setting determines the time of day or night the timer turns defrost on. • The second setting determines how long the defrost remains on.

5. Defrost Timer Settings • Commercial defrost timers, Cont’d. • The center knob sets how long the contacts are energized before they return to their normal position. • The contact can be set to remain in their energized position for a minimum of 2 min to a maximum of 120 min.

6. Defrost Timer Settings • Commercial defrost timers, Cont’d. • This timer has a separate timer release solenoid incorporated into its design. • When the timer release solenoid is energized, it causes the contacts to return to there normal de-energized position immediately. • This permits the action of some type of external limit switch, such as a temperature or pressure switch, to terminate the defrost cycle.

7. Commercial Defrost • The defrost in refrigeration is divided into: • Medium temperature ranges. • Low temperature ranges. • There are different components used in the defrost cycle for these temperature ranges.

8. Defrost Methods • There are two methods of defrost: • Off cycle or random. • Timed or planned.

9. Off Cycle Defrost • Characteristics: • Off cycle is generally used with medium temperature. • No additional electrical device needed to defrost the coil. • Medium temperature coil normally operates below freezing and rises above freezing during the off cycle. • The time between the units shuts off until it turns on is the defrost period.

10. Off Cycle Defrost Schematic

11. Timed Defrost • Characteristics: • Uses a defrost time clock • Sends the system into an enforced defrost period • Timed defrost can be used with: • no supplemental heat. • electric heat. • hot gas.

12. Timed Defrost No Supplemental Heat • Characteristics: • Time clock turns on and off the compressor. • Compressor stays off for a longer period of time (45 to 60 min). • Time clock is a 24 hour clock type device with “N/O” and “N/C” contacts. • Timer motor runs continuously.

13. Timed Defrost No Supplemental Heat

14. Timed Defrost Using Electric Heat • Characteristics: • Low temperature applications • Time clock turns off the compressor and turns on the electric defrost heaters on the evaporator. • Evaporator fan motors shuts off during defrost, with a delay in starting after the defrost cycle.

15. Timed Defrost Using Electric Heat • Characteristics, cont’d: • Straight time can end the defrost cycle or it can be terminated by sensing the coil temperature as it warms up. • A pump down cycle may also be used to remove the refrigerant from the evaporator during defrost.

16. Timed Defrost Using Electric Heat

17. Timed Defrost Using Hot Gas • Characteristics: • Low temperature applications. • Time clock energizes hot gas solenoid valve. • EFM shuts off during defrost with a delay in starting after the defrost cycle. • Compressor stays running during defrost. • Hot gas from the discharge line of the compressor is routed into the evaporator.

18. Timed Defrost Using Hot Gas • Characteristics, cont’d. • Straight time can end the defrost cycle, or it can be terminated by sensing the coil temperature as it warms up. • Less expensive to use than the electric defrost. • More wear and tear on the compressor, as it turns on in both the refrigeration and defrost modes.

19. Timed Defrost Using Hot Gas