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Demand Defrost

 All commercial refrigeration evaporators that run below 32°F will accumulate frost on their tube and fin surfaces.  Most unit coolers used in commercial refrigeration do run at temperatures below freezing, certainly all freezer applications.  The frost that forms on a unit cooler must be periodically removed.  The evaporator must be defrosted.

Everyone in the refrigeration industry knows that frost is an insulator.  Frost crystals trap air, similar to glass fiber insulation.  This insulating effect reduces heat exchange, resulting in reduced refrigerant flow.  As the TXV throttles to try and maintain its superheat setting, velocity drops, oil return is poor, liquid refrigerant may return to the compressor, air flow is restricted, etc.  In short, the evaporator must be defrosted or system failure will occur.  This is an indisputable fact.

Three defrost systems dominate the methods used to clear the frost from evaporators.

One of these systems can be used only where the box temperature is above freezing.  Walk-in coolers that run about 36°F or more box air temperature usually utilize “off” cycle defrost.  Off cycle defrost largely depends on good system design and stable conditions.  Systems are designed to be “off” 1/4 to 1/3 of their run or “on” time.  The evaporator fans are left running during the off periods so as to circulate the warmer box air temperature through the unit cooler, thereby defrosting it.  As long as box conditions are stable, this system works well.  However, as frequently happens, if a greater load is imposed than design conditions anticipated, such as more product, more door openings, the user turns the thermostat down, etc., the refrigeration system will not be off long enough to completely defrost the coil.  Frost will continue to build up.  The system may not shut off for long periods of time, due to high load.  Often, these systems will have a simple on/off timer added to force the system off long enough to accomplish defrost.  This is not the best solution to air defrost systems.  The timer needs to be readjusted to suit varying conditions, something that is seldom done.  The off defrost cycle may be at the “wrong” time, when refrigeration is most needed.  The forced off may be too long, causing the product to warm up and become contaminated.  The system will have to run much longer to get the box back to temperature.  If there were any power outages, the timing of the forced off periods will be changed.  Energy costs to run such a system will be high, and if one includes possible product loss, the cost could get very expensive.

As we will see, “demand defrost” is the best solution. 

The evaporator is defrosted by two different methods used in conjunction with the defrost controls; hot gas and electric defrost.  Hot gas defrost uses more complicated piping arrangements, valves, and controls than electric defrost, and consequently, costs more to install.  Therefore, electric defrost predominates. 

No matter what system is used, a “defrost tax” is imposed on the user in the form of high-energy bills, refrigeration production, product quality, and maintenance.  Gaining good control of the defrost system can reduce this “tax” immensely!

Let’s see what comprises this tax.

Applied heat from both electric and hot gas defrost must be removed, recovered, from the refrigerated space.  Every defrost cycle applies a lot of heat to the mass of the evaporator, as well as to the frost, before the temperature reaches 32°F and actual frost melting occurs.  This heating is repeated with each defrost cycle, even when an unnecessary defrost is initiated!

When frost is warmed to above room and product temperature, sublimation reverses and frost gets transferred from the evaporator to other surfaces.  This is the box and product in the box.  Once the defrost temperature is attained, more moisture is driven off the evaporator in the form of water vapor to the box and product.  Once enough heat has been applied to melt all the frost from the evaporator and more time for sufficient draining and drying, recovery begins.  Recovery includes removal of all the heat in the box that was added to defrost (Evaporator mass or metal, air temperature, latent heat of vaporization, etc.)

A system in defrost and recovery reduces the available refrigeration capacity.

During the defrost cycle, infiltration, heat transmission, and product heat continue to accumulate on top of the defrost-added heat.  A typical defrost and recovery period exceeds an hour.  With two defrost cycles per day, over two hours of refrigeration capacity is lost.  Too often or too long of a defrost cycle causes frost to form on the box walls and on product.  During run, moisture is attracted to the colder evaporator, but during defrost, will migrate to the box walls and product in the box.  The high temperature spikes of defrost can shorten the shelf life of the refrigerated product.  An iced-over evaporator can also cause product loss.  Maintenance costs also increase with frequency of defrost.

Defrost is necessary, but how can the “defrost tax” be reduced?  By defrosting only when necessary!

The most prevalent defrost system is time initiated, based on a 24 hour clock.  Time termination is very inefficient, and therefore, most modern systems use temperature termination.

Time initiated defrost suffers from many problems.  At best, the clock settings will be based on experience for each individual box or system.  Safety margins are added.  Varying conditions cannot be accommodated.  System improvements can be made by interfacing with devices that monitor compressor run time, humidity, temperature, and average this historical input, but this still cannot account for the unusual situation.  These “improved” systems can get very expensive.  The “defrost tax” is still very high.

Demand defrost systems directly measure frost accumulation on the evaporator’s surface and initiate defrost only when needed!  Defrosting at the same frost thickness each time will reduce the frequency and duration of defrost, compared to time initiated methods.  Demand defrost reduces the number of defrost cycles substantially.  Typical savings of 50% to 80% in energy costs are realized.  A big drop in the “defrost tax.”

With fewer and shorter defrost periods; more refrigeration capacity is available.  If existing capacity is marginal, the added capacity provided by demand defrost may save the cost of additional equipment.  Unusual conditions that cause fast, heavy frost buildup and a loss of evaporator efficiency, while waiting for a timed defrost will be avoided.  Defrosting at the same optimum frost thickness maintains evaporator efficiency.

With longer run times, moisture will migrate to the evaporator where it belongs, maintaining “clean” product and box surfaces.

Defrost generally terminates at 50 to 60°F with a temperature termination control, higher if time terminated.  This affects product close to the evaporator.  With fewer shorter defrosts, product is much less affected.

Time initiated defrost is maintenance intensive.  Times should be changed seasonally and for variations in product loading.  Times are “good guesses.”  Most often, defrost timers are set up once and then not changed until someone notices, “something is wrong.”

Equipment life is longer with demand defrost. With fewer temperature cycles, hot gas valves, controls, and piping will be less stressed, electric heaters will last longer, and even the evaporator itself will have an extended life.

Demand defrost savings can be calculated.

A typical freezer kept at -10°F with a capacity of about 1 ton at -20°F suction, two time initiated defrosts per day, one electric defrost evaporator, will consume about 376 KWH in defrost energy per week.  The same freezer using a demand defrost system will use about 125 KWH in defrost energy per week, a difference of 251 KWH per week.  If a KWH costs 10 cents, the savings in energy costs are $25.10 per week, or $1,305.20 per year!  Besides this, there is an increase of available refrigeration capacity of about 68,000 BTU per day!

For example, let’s say the demand defrost system for the freezer costed approximately $750.00.  One man should be able to install the system in four hours (a generous estimate).  If the dealer charges $75.00/hr for labor, and $50.00 for miscellaneous supplies such as wire, screws, etc., the total would be $1,100.00.  The payback for the freezer owner is less than a year!  This is just direct energy savings, and doesn’t include any product loss savings, maintenance savings, additional capacity gains, etc.

Climatic Control Company sells a demand defrost system made by a company called “Demand Defrost Systems.”  The system uses infrared technology for direct sensing of frost accumulation.  The sensor leads are wired to various solid-state controllers to provide a wide selection of functions.

It is impossible to go over every configuration of all the available sensors and controllers, but an illustration of the basic system using an ELC Heatcraft evaporator will demonstrate the most common system.  (Call the Milwaukee office for help in quoting multiple evaporator systems, PLC controlled systems, etc.)

The mechanical part of installing a system is simplicity itself.  For our example, only a CFDH sensor and DCU-4 controller are needed from Demand Defrost Systems Co. To select the correct CFDH sensor, the tube size of the evaporator needs to be known.  Sensors are available to fit 3/8” O.D. to 1” O.D. tube sizes.

The sensor is installed directly to the evaporator tube within the fin area.  Do not put a sensor on the return bends of an evaporator.  The best sensor placement is where the most frost accumulates.  This is usually close to the refrigerant inlet area of the evaporator.  Clip out about 2” width of fins to accommodate the sensor.  Use 4-wire shielded cable to connect the sensor to the DCU-4.  Mount the DCU outside of the cold room.  Sensors can be up to 1,000’ away from the DCU.  Never run sensor wiring with any other wires.  Keep them isolated.  The DCU is available only as a line voltage control.  It can be ordered as either a 120VAC or 240VAC control (the 240VAC will function a 208VAC and 230VAC).  The DCU provides power to the sensor and contains the electronic circuitry to interpret the sensor, open and close the defrost switch that provides line voltage to a relay and responds to the termination device dry contact closure.  Do not ever apply voltage to the yellow terminate leads!

The drawing, along with the sequence of operation will show you how this all works.

When the CFDH sensor’s infrared beam is broken by a frost buildup the following sequence occurs:  

  1. The electronic relay closes the switch between the two red wires, energizing R1.
  2. Contact 1R1 opens, turning off the evaporator fans.
  3. Contact 2R1 closes, energizing the defrost heaters.
  4. Contact 3R1 opens, de-energizing the liquid line solenoid valve (LLSV) even if the thermostat is closed. 
  5. The system pumps down.  The system is now in defrost. 
  6. When the defrost termination thermostat (DTT) senses a temperature high enough to switch blue to red (N to X), relay R2 is energized, contact 1R2 is closed, signaling the DCU to reset, opening the switch   between the red wires de-energizing R1. 
  7. Contact 2R2 is opened, preventing the fans from starting.
  8. Contact 2R1 opens, de-energizing the heaters.
  9. Contact 3R1 closes, energizing the LLSV and starting the refrigeration cycle.
  10. When the DTT senses a low enough temperature, it switches, breaking blue to red (N to X) de-energizing relay R2.
  11. Contact 2R2 closes, and the evaporator fans run.  Everything is now reset until the next time the CFDH senses enough frost to begin the defrost cycle again.

Drawing 1.

Some things to note:

The ELC needs to be rewired as it comes from the factory as shown; it is not a difficult job.  The contact ratings of R1 and R2 need to be able to carry the load of the fans and heaters.  This can vary, depending on the ELC being controlled.  Consult the Heatcraft catalog to get these loads.  (Other manufacturers’ evaporator nameplates or catalogs would need to be checked to find these ratings when retrofitting.)

With energy costs high, and probably going higher, there should be an increased demand for demand defrost.

Demand defrost can be sold, especially to the end user who is paying the energy bills.  Contractors need to be convinced that the additional time spent selling their customers on demand defrost will be profitable for them.

The difference in cost between a time clock and demand defrost is small, but the results are large!

Infrared Sensor Data Sheets:

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