Carrier NDK33BN0KB Operation & Service Manual Download Page 88

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4-6

Defrost based on coil condition will be initiated when:



blockage is sufficient to cause an air pressure dif-

ferential across the coil great enough to close the
contacts of the defrost air switch(DAS) .



when the RAT - DTT temperature is greater than

20°C (36°F) for 15 continuous minutes. This meth-
od  of  defrost  initiation  will  only  be  used  for  3  con-
secutive  defrosts.  After  three  consecutive  defrost
initiations  by  this  method  the  counter  will  be  reset
to  allow  this  defrost  initiation  again  after  a  defrost
cycle  is  initiated  by  another  method  (timer,  air
switch or manual).

TIP

Ice is not the only thing that will cause the air
differential  to  increase  across  the  evaporator
coil. Shrink wrap, paper, plastic bags, and oth-
er  such  items  when  caught  in  the  return  air
stream  and  pulled  up  against  the  evaporator
coil  or  the  return  air  grille  can  also  cause  the
DAS contacts to close.

Time interval between defrosts is selected in the mi-
croprocessor Functional Parameters. The parameter
value may be set to 1.5, 3, 6, or 12 hours. The factory
default setting is 6 hours.

NOTE

The defrost timer will not count when the unit
is in defrost, the engine is in an off cycle or
DTT is greater than 4.4°C (40.0°F).

2. Defrost Modes

There are two Defrost Modes available:

NOTE

When the unit is in either defrost mode and in
Engine operation:



the condenser fan will operate to supply air for

the radiator.



the engine will operate in high speed when

HTCON1 and/or HTCON2 are energized (to
supply power for the heaters).



the engine will operate in low speed when both

HTCON1 and HTCON2 are de-energized.



Normal Defrost Mode will be entered if RAT is less

than 0.0°C (32.0°F)

and

SAT is less than 7.2°C

(45.0°F). Defrost will be accomplished by shutting
down the refrigeration system while the heaters
(HTCON1 & HTCON2) are energized.



Natural Defrost Mode uses a combination of the

heat from the product and the electric heaters to
melt any ice.
Natural defrost is available when:



the Natural Defrost microprocessor Configura-

tion is set to YES

and



defrost is initiation by closing of the DAS con-

tacts with RAT greater than or equal to 4.0°C
(39.2°F)

and



at least one normal defrost has been performed

since power up

and



this initiation will not result in more than 3 natu-

ral defrosts between normal defrosts

when  a  natural  defrost  is  initiated,  the  unit  will  op-
erate  for  the  first  five  minutes  with  the  evaporator
fan and HTCON2 de-energized and only HTCON1
energized.  After  5  minutes  the  evaporator  fan  is
energized while both HTCON1 & HTCON2 are de-
energized.

3. Defrost Termination

When  in  either  defrost  mode,  defrost  will  terminate
when the DTT rises to 12.8°C (55°F). When operating
in Return Air mode, defrost will continue for a minimum
of  5  minutes,  a  maximum  of  10  minutes,  or  until  the
SAT is equal to or higher than the RAT.
During  Defrost  Termination,  the  heaters  will  de-
energize, and the engine will go to low speed. "dF" will
continue to be displayed in place of refrigerated com-
partment Temperature, and the Cool LED will come on
when the compressor starts.
The EVXV will open, and the evaporator coil will begin
to  cool  down.  The  evaporator  fans  will  not  come  on
right  away,  so  that  warm  air  is  not  blown  into  the  re-
frigerated compartment, but will wait for the evaporator
to cool down, up to a maximum of 8 minutes. Once the
evaporator fan has started, "dF" will no longer be dis-
played,  and  refrigerated  compartment  Temperature
will  again  be  shown  in  the  main  display.  If  there  is  a
DTT sensor alarm present, defrost will terminate after
10 minutes.

NOTE

Defrost  Mode  uses  a  Defrost  Duration  Timer
that allows for a maximum of 45 minutes in de-
frost. If defrost is not terminated during the 45
minutes,  the  microprocessor  will  end  the  de-
frost  cycle,  and  show  the  alarm  "DEFROST
NOT COMPLETE". When this occurs, the De-
frost  Interval  Timer  is  set  for  1.5  hours,  at
which time the unit will go into defrost again.

4.6

USER SELECTED OVERRIDE OPERATION

Three optional software override programs are availa-
ble  to  the  user.  These  programs  include  Supply  Air
Limit  Control,  Temperature  Range  Lock  and  Product
Shield.  Information  on  how  the  unit  operates  under
these  programs  is  provided  in  the  following  sub-
paragraphs.

4.6.1 Supply Air Limit Control

Under supply air temperature control the compressor
suction modulating valve (CSMV) is modulated to
maintain supply air to a minimum in accordance with: