8 VERIFYING DEFROST

ADEQUACY AND DEFROST CONTROL OPERATION

Introduction Refrigeration systems require a method of defrost to periodically melt ice that accumulates on the coil. Most refrigeration systems have a timer that ini-tiates a defrost cycle at given intervals. A demand defrost control scheme only initiates a defrost cycle when the system determines it is necessary. Defrost also affects the operation of the refrigeration system as a whole. When a sys-tem is undergoing defrost, it is generally not contributing to the load on the refrigeration system. However, defrost takes a significant amount of energy and contributes a high heat load, causing the refrigeration system to use more energy to cool down the unit after defrost. Methods of defrosting that require additional piping and valves and also affect system temperatures and pres-sures include the following:

• Circulating hot gas from the compressor through evaporators

• Circulating gas from the receiver through evaporators Planning and

Design Phase

Ensure that the OPR specifies the following:

• What type of active defrost strategy is used (e.g., hot-gas defrost or electric defrost)

• Manufacturer’s defrost specifications for cases or boxes Ensure that the BoD specifies the following:

• Defrost schedule for each freezer and staging specifications by refrigeration circuit (see Figures D-3 and D-4).

Figure D-3 Example of Freezer Defrost Schedule Shown in Electronic Controller

Figure D-4 Example of Defrost Map Indicating Staging of all Circuits on a Rack Shown in Electronic Controller Refrigeration Commissioning Guide for Commercial and Industrial Systems

• For circuits that use electric defrost:

• Sizing of any electrical equipment (contactors, breakers, etc.) associated with the circuit and determination that maximum electrical load does not exceed available power supply.

• For circuits that use gas defrost:

• Type of defrost (e.g., discharge gas or latent gas).

• Type of piping (e.g., reverse cycle or three pipe).

• Description of any fail-safe measures.

• For reverse-cycle gas defrost, include the maximum percentage of the refrigeration load that may be used for gas defrost at any time.

Common industry guidelines are that no more than 1/4 to 1/3 of the load may be used for gas defrost (see ASHRAE Handbook—Refrig-eration, page 15.19 [2010], and Industrial Refrigeration Handbook [Stoecker 1998]).

• Whether demand defrost is used. If so, describe the control strategy.

Ensure that the design documents specify the following:

• Location of any defrost sensors

• Sequence of operations (SoO) for a complete defrost cycle, includ-ing the followinclud-ing:

• Control setpoints for all steps in the cycle

• For equipment without a manufacturer-provided schedule (e.g., walk-in freezers), any time delays programmed into the system, including pumpdown time, fail-safe defrost termination time, drip time, and fan delay

• For gas defrost, settings for all valves

• If using demand defrost, any conditions that trigger a defrost

• For gas defrost, piping schematics and valve specifications Construction and

Installation Phase

Verify that all sensors are installed according to the design documents.

If the system uses hot-gas defrost, verify that valves and piping are installed according to the design documents.

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System Start-Up At the controller, verify that control setpoints are as specified in the BoD.

For a period of at least 24 h, do the following:

• Monitor or record system load profiles to verify that defrost staging is occurring according to the schedule specified in the BoD, includ-ing proper termination of defrost.

• Log inputs and outputs of the defrost controller and verify that all defrost cycles are occurring according to the sequence in the design documents or the manufacturer’s specifications, including any set-points and time delays.

• If gas defrost is used, verify that the highest observed percentage of the refrigeration load is not greater than the maximum specified in the BoD.

• If the system uses demand defrost, examine the inputs and outputs of the controller to verify that the system is initiating defrosts according to the control strategy in the BoD.

First-Year Operation

At least once per season, do the following:

• At the controller, verify that control setpoints are as specified in the BoD. If control setpoints have been changed, report findings to the building owner and/or system designer to determine the reason. If necessary, update the documentation.

• For a period of 24 h, do the following:

• Monitor or record system load profiles to verify that defrost staging is occurring as specified in the BoD, including proper termination of defrost. Note in exception logs any systems that reach the fail-safe setting before terminating defrost on temper-ature.

• Log inputs and outputs of the defrost controller and verify that all defrost cycles are occurring according to the sequence in the design documents or the manufacturer’s specifications, includ-ing any setpoints and time delays.

• If gas defrost is used, verify that the highest observed percent-age of the refrigeration load is not greater than the maximum specified in the BoD.

• If the system uses demand defrost, examine the inputs and out-puts of the controller to verify that the system is initiating defrosts according to the control strategy in the BoD.

• Visually check a sample of freezer cases or boxes (~10%) for the fol-lowing defrost-related problems:

• Ice on the coil that is not removed by defrost

• Ice in the drain pan or a loose drain pan

• Defrost meltwater or ice on the floor or product below the evap-orator

• Ice on distributor capillary tubes

• Report problems to the building owner, system designer, and/or equipment manufacturer, as appropriate, so that corrective action may be taken. Include in the report any systems that reach the fail-safe setting before terminating defrost on temperature.

Refrigeration Commissioning Guide for Commercial and Industrial Systems

9 MINIMIZING AIR