Demand defrost system

A demand defrost system is provided for detecting the existence of frost build-up on the surface of the heat exchanger of a cooling system. A light beam is directed through the heat exchanger such that frost build-up blocks the path of the light from reaching a photocell. When the light received by the photocell falls below a selected value, a control signal is produced which discontinues cooling operation of the cooling system and initiates defrost heaters which heat the cooling coils and remove the frost build-up thereon. A temperature sensing switch is also provided which reinitiates the cooling cycle after a temperature indicates that frost has been removed from the cooling coils.

TECHNICAL FIELD 
This invention relates to demand defrost systems for use with cooling 
systems. 
BACKGROUND ART 
In refrigeration systems, frost tends to build up on the evaporator or 
cooling coils due to condensation of the moisture in the air cooled by the 
coils. This frost build-up reduces the efficiency of the heat transfer, 
and when substantial, creates a significant resistance to air flow across 
the cooling coils. It is necessary, therefore, to defrost the cooling coil 
surfaces periodically so that it can be restored to its original 
frost-free condition and operate in its normally efficient manner. 
Changes in ambient conditions, such as differences in the moisture content 
of the air, varying numbers of times doors to the cooling or refrigeration 
compartments are opened, variance in the amount of humidity in the air on 
given days, and the diverse applications of cooling systems in different 
environments results in significant variations in the amount of frost as a 
function of time, both from system to system and for any one system 
subjected to such varying conditions. 
Prior art devices have incorporated time controls which arbitrarily fix the 
frequency at which defrosting occurs. These time control devices do not 
take into account the actual amount of frost that is present on the 
cooling coils. Since such systems are preset to initiate a defrost cycle 
at fixed time intervals without regard to the actual need for defrost 
operations, defrost cycles may commence before there is a real need for 
defrosting, or well after a time when a defrost cycle should have been 
started. In either case, the result is a significant waste of electrical 
energy. 
When the defrost cycle does not occur as frequently as needed, the 
efficiency of the cooling system is greatly reduced and more energy is 
expended running the compressor more than is needed. When the defrost 
system operates more frequently than is needed, energy is also wasted due 
to the excessive energy needed to power the heaters which heat up the 
cooling coil surfaces, and to recool the system after defrosting is 
complete. Keeping a time control device in adjustment for the specific 
application and variable weather and humidity conditions would require 
continual maintenance and adjustment and is not really a practical 
alternative. 
Further, one device which presently attempts to control defrosting by 
demand requires installation of a separate sensing unit mounted on the 
cooling coils. Such units sense only the frost build-up on themselves, and 
thus are designed to simulate the frost build-up on the cooling coil. Such 
an indirect method has not proved to be an accurate way to detect the 
actual frost that builds up on the cooling coil, not only because it 
attempts to detect frost indirectly, but also because it is restricted to 
sense frost only at a single point. 
Another approach for detecting frost contemplated a fan motor sensing 
circuit to sense variations in the fan speed as a result of air flow 
resistance due to the frost build-up. Attempts to construct this type of 
system have met with little success. 
Air temperature sensing devices to detect the need for defrosting have also 
been tried. Such a device would clearly only detect the temperature in the 
vicinity of the heat exchanger and not the amount of frost actually 
accumulated on the heat exchanger. This indirect method of detecting frost 
build-up has been generally unreliable or at least too variable to serve 
its intended purpose. 
BRIEF SUMMARY OF THE INVENTION 
In accordance with the present invention, a demand defrost system for use 
with cooling systems of the type having a compressor and heat exchanger 
includes a probing light source positioned to direct a light beam through 
the heat exchanger. A light responsive device, such as a photocell, is 
positioned to receive the light after it passes through the heat exchanger 
such that frost built up on the heat exchanger will obstruct the light 
path and will reduce the amount of light impinging on the photocell. When 
the light received by the photocell drops below a determined amount, a 
control signal is produced which discontinues operation of the cooling 
cycle and initiates a defrost cycle. 
The demand defrost system of the present invention detects frost build-up 
directly on the cooling coil itself and activates a defrost cycle only 
when the need for defrost exists. Such a system does not require the 
continuous monitoring or adjustment required of prior defrost systems. The 
system of the instant invention also senses frost build-up at several 
locations on the heat exchanger surfaces. These features overcome the 
deficiencies of the prior art which only indirectly sense frost build-up 
on the coils and which only sense frost build-up at a single location near 
the cooling coils. 
Substantial energy savings will result from the use of the novel invention 
and after installed, the invention will require a minimal monitoring and 
or adjustment. 
Numerous other advantages and features of the present invention will become 
readily apparent from the following detailed description of the invention 
and embodiments thereof, from the claims and from the accompanying 
drawings in which like numerals are employed to designate like parts 
throughout the same.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
While this invention is susceptible of embodiment in many different forms, 
there are shown in the drawings and will herein be described in detail 
preferred embodiments of the invention. It should be understood, however, 
that the present disclosure is to be considered as an exemplification of 
the principles of the invention and is not intended to limit the invention 
to the embodiments illustrated. 
In the following description, two digit numerals are used to refer to the 
embodiment illustrated in FIG. 2, and corresponding three digit numerals 
are used to refer to the embodiments illustrated in FIGS. 3 and 4. The 
same last two digits in each numeral designate similar or functionally 
analogous elements in the various embodiments. 
Referring now to the drawing, FIG. 1 shows a series of cooling coils 10 
with fins 12 attached thereto. Not shown in FIG. 1 is a fan which blows 
the cool air through and over the coils 10 and fins 12 into the 
refrigeration or freezer area. The coils 10 constitute the evaporator 
portion of the refrigeration system and contain cooling fluid and are 
connected to a compressor as is well known. Techniques for producing the 
coils and fins are well known in the art and do not constitute, as such, 
any part of the present invention. 
Frost or other condensation normally builds up on the surfaces of the 
cooling coils 10 and the fins 12 after a period of time. The result is 
inefficient heat transfer and resistance to the air flow through the coils 
and fins. After a period of time the air flow is severely restricted and 
the cooling system no longer operates efficiently and it is therefore 
necessary to defrost the surfaces of coils 10 and fins 12 so that they can 
be restored to their original frost-free condition. 
To accomplish this defrosting with a minimum of energy expenditure, the 
invention initiates a defrosting cycle only when a predetermined amount of 
frost has built up on the coils 10 and fins 12. In this way the defrost 
cycle occurs only when necessary, and energy expenditure is minimized. 
Light source 14 emits a light beam through cooling coils 10 and fins 12 and 
is received by photo electric cell 16 in housing 18. The light beam 
preferably has about a 1-2 to 2 inch diameter and the fins 12 typically 
are constructed with spacing equal to about 4 to 8 per inch. Thus, an 
acceptable amount of frost build-up can occur before the light beam is 
obstructed from reaching photo electric cell 16. The beam width of this 
light beam can be varied depending on the spacing between the fins or 
coils, without altering the configuration of the fins or coils. The 
sensitivity of this defrost system can also be adjusted by placing a metal 
woven mesh-type screen 20 at one or more locations in the cooling 
evaporator coil area transverse to the path of the light beam. This wire 
mesh screen frosts up before the coils have excessively frosted to inhibit 
the optical signal from reaching the photocell 16. Thus, the defrost 
system initiates the defrost cycle before frost on the coils and finds 
have built up to excessive levels. This screen 20 is especially useful as 
the distance between the fins is increased. By placing one or more screen 
at any one of several locations, an accurate and reliable indication of 
defrosting demands can be obtained for different cooling coils and fin 
arrangements. 
Turning now to the circuitry, a potential of 230 volts is applied across 
lines L1 and L2 in the upper half of FIG. 2. Transformer T1 steps this 
potential down to create a voltage potential of 12 volts across lines L3 
and L4. The potential across L1 and L2 could alternatively be 115 volts. 
The voltage potential across L3 and L4 powers light source 14 which 
transmits light to photo electric cell 16. The photo electric cell 16 
conducts as long as at least a predetermined amount of light is received 
from light source 14. When photo electric cell 16 is conducting, 
indicating that an excess amount of frost has not built up on the fins 12, 
switching transistor 17 is not conducting and no current flows through 
sensing relay R1. R1 could be any other switching control means, as can be 
R2 and R3 described below. 
When the amount of light received by photo electric cell 16 decreases below 
a selected amount because frost build-up on the fins 12 or the screen 20 
blocks the light path, the photo electric cell 16 no longer conducts, 
switching transistor 17 turns on and current flows through the 
transistor's collector-emitter junction. The sensitivity of switching 
transistor 70 in response to photo-electric cell 16 can be adjusted by 
varying the resistance of resistor 17a. 
When the transistor 17 conducts, the light sensing relay R1 conducts and 
produces a control signal which causes normally open contacts R1-1 to 
close. This completes a circuit through defrost control relay R2, if the 
external control contact is closed as normal, and through normally closed 
contacts R3-1 of heater shut-off relay R3. When defrost control relay R2 
is energized, normally open holding contacts R2-1 close thereby locking 
defrost control relay R2 into the circuit independently of whether or not 
contacts R1-1 remain closed. 
Therefore once the light sensing relay R1 is activated and contacts R1-1 
close causing defrost relay R2 to conduct, R2 continues to conduct 
independently of whether a control signal is generated by the conduction 
of transistor 17, and will continue to conduct until the normally closed 
contacts R3-1 are opened by activation of heater shut-off relay R3 
indicating that the defrost cycle is complete, the operation of which will 
be discussed below. 
During the normal operation of the compressor and fan motor 18, the defrost 
termination fan delay (DTFD) temperature sensing switch 21 is in the low 
temperature position indicating a relatively cold temperature around the 
cooling condensor coil and fin area. The DTFD sensing switch 21 is a SPDT 
switch sensitive to temperature in the evaporator cooling coil area. In 
its normal low temperature position, DTFD switch completes a circuit 
through fan motor 18 and the normally closed contacts R2-2 so fan motor 18 
continually blows air through the cooling coil and fin area into the 
refrigeration or freezer area as long as normally closed contacts R2-2 
remain closed. 
The compressor controller 22 also continues to operate if the normally 
closed contacts R2-2 remain closed, if thermostat contacts 24 are closed 
indicating a demand to cool, and if high pressure safety switch 25 and low 
pressure safety switch 26 remain closed indicating a safe condition for 
the compressor to continue operating. These switches 24, 25 and 26 are 
common in refrigeration systems and well known to those skilled in the 
art. 
Once the defrost control relay R2 is activated in response to the control 
signal as described above, the normally closed contact R2-2 in series with 
the compressor controller 22 and the fan 18 open to de-energize the 
compressor controller 22 and fan motor 18. At this time normally open 
contacts R2-3 close to complete a circuit through the DTFD switch 21 which 
is still in the low temperature position, to energize the heater 28 and 
initiate the defrost cycle. The fan 18 is de-energized to prevent liquid 
from being blown into the refrigeration area, and to improve coil heating 
during the defrost cycle. 
The heater 28 continues to operate until DTFD switch 21 assumes its high 
temperature position, which occurs when the temperature around the cooling 
coils 10 and fins 12 reaches a temperature indicating that the defrost 
cycle is complete. When the DTFD switch 21 assumes its high temperature 
position indicating that the defrost cycle should be terminated, heater 
shut-off relay R3 is energized. When heater shut-off relay R3 is 
energized, its normally closed contacts R3-1 are opened to de-energize 
defrost control relay R2. As a result, contacts R2-3 return to their 
normally open position to de-energize heater 28, contacts R2-2 return to 
their normally closed position to energize the compressor controller 22 
and to enable fan 18, and contacts R2-1 return to their normally open 
position to preclude defrost control relay R2 from being energized when 
contacts R3-1 close. 
The compressor controller 22 starts the compressor to circulate coolant 
through the coils 10. Since the DTFD switch 21 is still in the high 
temperature position, fan motor 18 is not yet energized. This prevents 
moisture from being blown off the cooling coils 10 and fins 12 into the 
refrigeration area. 
Before the temperature sensed by the DTFD switch 21 drops to a selected 
value and indicates a cold temperature condition, the moisture in the coil 
area either evaporates, falls to a drain pan or freezes. When the DTFD 
switch 21 returns to its low temperature position, a circuit is completed 
through DTFD switch 21, fan motor 18 and normally closed contacts R2-2, 
and the whole refrigeration system returns to operate in the normal 
manner. This normal operation continues until frost builds up, the light 
from light source 14 is obstructed, and the defrost cycle again initiates. 
FIG. 3 illustrates a second embodiment circuit employing a voltage 
comparator to provide a control signal to trigger the light sensing relay 
R1. A light emitting device 114 directs light towards the photoelectric 
cell 116. Light emitting device 114 could be a semiconductor such as a 
photoemissive diode and photoelectric cell 116 could be a semiconductor 
such as a phototransistor. Adjustment of variable resistor 117a changes 
the level at which light received by the photoelectric cell 116 will 
affect the operation of the voltage comparator 117 to produce the control 
signal. 
A full wave rectifying bridge 119 creates a dc voltage potential between L5 
and L6. This dc potential is filtered by filter capacitor 123 and supplies 
operating voltage to voltage comparator 117 which in one embodiment is an 
LM301 manufactured by National Semiconductor. The voltage comparator 
compares the voltage of its input terminals and normally has a high output 
at its output terminal 117-3 when the voltage at input terminal 117-2 is 
higher than at input terminal 117-1. When the voltage at 117-1 is higher 
than at 117-2, then the output at output terminal 117-3 drops to a low 
state. Proper selection of resistors 117a, 130 and 132 will result in 
terminal 117-1 being held at a lower voltage potential than 117-2 when 
photoelectric cell 116 is conducting. When frost builds up and obstructs 
light from light source 114 from reaching photoelectric cell 116, 
photoelectric cell 116 no longer conducts and the voltage potential of 
117-1 is greater than 117-2. The voltage comparator 117 output terminal 
117-3 drops to a low voltage potential relative to L5. This creates a 
voltage potential across relay R1 and causes this relay to conduct 
producing a control signal which causes normally open contacts R1-1 to 
close. The remainder of the circuit of FIG. 3 operates in a similar 
fashion to the circuit of FIG. 2 which is described above. 
The embodiment shown in FIG. 4 is similar to FIG. 2, but incorporates 
additional operational and fail-safe capabilities. The 230 volt potential 
is applied across main line L1 and L2 to step down transformer T1 which 
steps this voltage down to 12 volts across lines L3 and L4. 
The light source 214 is connected in series with balancing register 230 
across lines L3 and L4. The junction between balancing register 230 and 
light source 214 is connected to the control terminal of a triac 232 in 
line L3 to disable the light responsive control circuit in the event the 
light source 214 fails. As a result, defrosting is not initiated when the 
light source fails. 
When this occurs, the circuit through the balancing register 230 and the 
light source 214 opens and the signal to the control electrode of the 
triac 232 terminates. The triac ceases to conduct. This opens the circuit 
to the circuitry connected between that position of line L3 below the 
triac, line L3', and line L4 to preclude energization of relay R1. Since 
relay R1 is not energized, relay R2 connected in series with normally open 
contacts R1-1 is not energized, and the defrost cycle is not initiated. 
A normally energized pilot light 234 is connected between lines L3' and L4. 
If light source 214 fails, pilot light 234 will be extinguished to 
indicate the failure of the light source. An alarm relay RA is also 
de-energized to close its normally open contacts RA-1 connected in series 
with an alarm 236 across lines L3 and L4. The alarm is thereby energized 
to provide an alert that the light source has failed. 
In the circuit of FIG. 4, when frost causes light to the photocell 216 to 
be interrupted, the transistor 217 conducts. The result is a control 
signal applied to the control electrode of an SCR 238 connected in series 
with relay R1 and the normally closed contacts R3-1 of relay R3. When 
relay R3 is energized, as described above, to terminate the defrost cycle, 
contacts R3-1 are opened to de-energize relay R1. 
In the circuit of FIG. 4, relay R2 is a 220 volt relay connected between 
lines L1 and L2. Except for that change, the operation of relay R2 is 
substantially the same as described above with respect to FIG. 2. Thus, 
when relay R2 is energized, contacts R2.1 close to keep relay R2 
energized, contacts R2-2 open and contacts R2-3 close to initiate the 
defrost cycle. When the cycle is complete, the contact of DTFD switch 221 
shifts to the high side to energize relay R3. Contacts R3-1 open to 
de-energize relay R2 and contacts R3-2 open to de-energize relay R1. 
Since automatic initiation of the defrost cycle is disabled when the light 
source 214 fails, a manual switch 240 is provided in parallel with relay 
contacts R1-1 and R2-1. Closure of the switch manually energizes relay R2 
to initiate operation of the defrost cycle which then operates as 
described, although it cannot be initiated automatically until the light 
source 214 becomes operative. 
Additional safety is provided by a high temperature limit switch 242 
connected in series with relay R3. This normally open switch closes to 
energize relay R3 and terminate the defrost cycle if temperatures become 
too high due to failure or switch 221. If desired, a safety timer 244 can 
be connected across relay R2. This timer would be energized simultaneously 
with relay R2 when the defrost cycle is initiated. The timer contacts T-1 
connected in parallel with limit switch 242 close if the timer T times 
out, which only occurs on failure of the normal DTFD switch 221 and the 
limit switch 242. 
Relays R1, R2, and R3 can be any other switching control means. 
It should be appreciated that other forms of signals can be used as a 
detection signal in the instant invention, such as infrared signals or 
ultraviolet signals. 
From the foregoing, it will be observed that numerous variations and 
modifications may be effected without departing from the true spirit and 
scope of the novel concept of the invention. It is to be understood that 
no limitation with respect to the specific apparatus illustrated herein is 
intended or should be inferred. It is, of course, intended to cover by the 
appended claims all such modifications as fall within the scope of the 
claims.