Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
3~ 3~ ~ 9D RA 133~9
The present invention relates in general to new and
¦ improved frost control apparatus, in particular to frost control
¦ apparatus for use with heat pumps or similar closed temperature
¦ conditioning systems which bridge indoor and outdoor environments,
5 ¦ wherein a compressor is used to circulate a volatile fluid through
¦ the system.
I BACKGROUND OF THE INVENTION
l . _ .
. It is well known that systems such as heat pumps or the
l like operate on a reverse cycle principle. In general, such a
¦ system includes indoor and outdoor heat exchange coils which are
exposed to their respective ambient conditions. When the tempera-
l ture of the indoor air is to be raised, the compressor pu~ps the
¦ volatile refrigerant fluid through the indoor coil and subsequentl~
l by way of expansion means, through the outdoor coil of the system.
¦ The latter then acts as an evaporator by absorbing heat from the
outdoor air. Simultaneously, the indoor coil acts as a condenser
which gives off the heat previously absorbed from the outdoor air
to the indoor environment.
l Since the outdoor coil under these conditions operates
¦ at a temperat~re less than the ambient outdoor air, moisture
carried by the latter condenses on the outdoor coil and causes
frost to be built up thereon. This layer of frost acts as an
insulator between the outdoor air and the outdoor coil and so
l prevent.s effective heat transfer to the latter. Under these
¦ conditions, system efficiency is greatly reduced. Further, the
s~
forst on the coil may encase whatever sensing element is posi~ioned
thereon so as to produce a false reading of the condition which is
intended to be sensed.
l A similar action occurs with respect to the indoor coil
hen the latter is used as an evaporator in order to cool the
......
3~3L3~6 9D RA 13389
temperature of the indoor air. Moisture carried by the indoor air
¦is deposited on the indoor coil in the form of frost and reduces
¦ the efficiency of the system.
¦ Workers in the air conditioning field are well aware of
¦ the problem of controlling the build-up of frost in temperature
¦ conditioning systems of this type and numerous schemes have been
¦ advanced to deal with it. It is recognized that the defrosting
¦ action itself may be carried out in a number of ways, e.g. by
¦ reversing the operation of the 3ystem, by terminating the system
¦ operation, by blowing heated air across the frost-carrying coil,
¦ etc., or by a combination of some or all these methods. ~he
difficulty arises primarily in automatically choosing the points
at which the defrosting action is to be initiated and terminated
l respectively, so as to maintain system operation at maximum
¦ efficiency. The problem is compounded by the fact tha~ these
l operating points change with changing ambient conditions. Thus,
/ ¦ w.lile at low ambient outdoor temperatures it takes longer to
accumulate a given thickness of frost than at higher temperatures,
l the defrosting action will also take longer t'nan when a higher
¦ outdoor temperature prevails. Thus, the points at which the
defrosting action o:E the outdoor coil are initiated and terminated
respectively, must vary as a function of the outdoor air tempera-
ture in order for the system to operate at maximum efficiency.
l Other ambienl conditions will also affect the amount of
¦ frost which collects on the outdoor coil. For example, the
humidity of the ambient air will determine in some measure the
rate of frost build-up. Further, precipitation such as snow or
freezing rain will affect the amount of frost deposited on the
l outdoor coil.
¦ An effective frost control system must be capable of
l taking into account all of these ambient outdoor conditions in
13:~!L32~.j 9D RA 13389 1 ;
order to keep the outdoor coil frost-free and so as to operate
! the temperature conditioning system at high efficiency.
The deposit of frost on the indoor coil is affected only
l by some of the factors discussed above. These are primarily the
¦ temperature of the ambient indoor air and its humidity. An
effective frost control system must also be capable of dealing
with the latter conditions in order to provide efficient operation
throughout.
l A further requiremen~ of an effective frost control
¦ system is reliability of opera~ion under different and adverse
conditions. Since the system is ordinarily serviced only at
l infrequent intervals, it must be capable of maintaining efficient
¦ operation for long time intervals without human intervention.
l Such a requirement dictates a reduction of the number of part~
¦ ordinarily found in prior art frost control apparatus which are
subject to breakdown or to malfunction, as well as the ability
to stand up under varying, adverse operating conditions
Finally, the cost contribution of the frost control
l apparatus to the overall cost of the temperature conditioning
¦ system of which it is a part must be considered. This includes
the initial cost, as well as the cost contribution to the mainte-
nance of the overall system. The latter may be considerable if
the frost control apparatus is complex and subject to break-down.
l Earlier attempts at solving the problems associated
¦ with effective frost control have not been successful in all the
areas discussed above. Much of the proposed equipment, particu-
larly that which employs relatively co~plex mechanical apparatus
to achieve frost control, is expensive to build, difficult to
l maintain and it is prone to break down or malft~ction if left
¦ unattended for extended periods of time. Moreover, such
Il 9D RA 13389
1~ ~i3~32~
equipment is difficult to adjus~ accurately for optirnum oper~Lion
I and to maintain in adjustment during operation. As a conseq1lerlce,
¦ optimum operating efficiency is rarely, i.f ever, achieved with
¦ such equipment, while the associated costs are often high. Stlll
¦ other prior art frost control equipment operates strictly on the
¦ basis of temperature check points. The operation of this type of
¦ equipment is frequent~y inaccurate and erratic since it fails to
¦ take into account the varying relationships of the relevant operat
¦ ing factors as ~he ambient conditions change.
1 OBJECTS OF THE INVENTION
I ... ~
I ¦ Accordingly, it is a primary object of the present
invention to provide frost contro]. apparatus which is not subject
l to the foregoing disadvantages.
¦ It is another object of the presen~ invention to provide
¦ frost control apparatus which maintains maximum opera~ing effi-
ciency by au~omatically adjusting to varying ambient conditions.
It is a Eurther object of the present invention to pro-
l vide frost control apparatus which is simple and economical in
J~ I ~
construction and which ls cadily'implemented and maintained.
20 ¦ These and other objects of the present invention together
with the feaLures and advantages thereof will become apparent
from the following detailed specification when read in conjunctior
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates a temperature conditioning system of
the type to which the present invention i.s applied;
FIG. 2 illustrates a preferred embodiment of the present
invention which is capable of controlling frost on both the indoo
and outdoor heat exchange coils; and
- -: - - .: . -
..... , :, ,
" ~ ~7~ 3~ ~ 9D-RA-13389
FIG. 3 illustrates in graph of differential
temperature vs. outdoor ambient temperature.
DESCRIPTION OF THE INVENTION
With reference now to the drawings, Fig. 1 illustrates
a temperature conditioning system 110 of the type in which the
present invention may be used. A reversible cycle, closed system
is shown, which bridges indoor and outdoor environments and
wherein a compressor 114 pumps a volatile fluid, e.g. a
refrigerant, through the system in a direction determined by the
setting of a valve 112. System 110 includes an outdoor heat
exchange coil 116 and an indoor heat exchange coil 118 positioned
in their respective indoor and outdoor environments. In a
practical embodiment of the invention, the outdoor coil may
be positioned entirely outdoors. Alternatively, it may be
positioned in the wall which separates the indoor and outdoor
environments, but protruding to the outdoors and exposed to
ambient outdoor air. In Fig. 1, compressor 114 and 112 have
been arbitrarily placed in the outdoor environment, although
it will be understood that the position of these components may
likewise be varied.
Compressor 114 is energized by an AC line voltage applied
to terminals 120. A switch 122, in series with the AC line, is
coupled to a solenoid relay 124. The latter is adapted to be
actuated by a solenoid coil 126 in accordance with the signal
applied to the coil terminals~ A control unit 130 is coupled
to valve 112 and is adapted to set the latter to a desired
position in accordance with the direction in which the refrigerant
fluid is to be pumped in system 110. Control unit 130 is energized
from a pair of terminals 131 in accordance with an externally
derived signal which determines whether the indoor coil will act
as a cooling device or as a heat source for the indoor environment.
9D RA 13389
3~ ~
A temperature sensingelement 132, which may comprise a
thermistor device having a negative temperature coefficient of
¦ resistivity in a preferred embodiment of the invention, is posi-
¦ tioned proximate outdoor coil 116. Element 132 is preferably
¦ placed in direct contact with the bare coil or, if prevented
¦ from doing so by the fins surrounding the coil, it may be posi-
¦ tioned between the coil fins, spaced no more than l/4" from the
I coil itself. A pair of terminals 134 is associated with element
¦ 132 for connecting the latter into an electronic circuit, as will
¦ be explained in greater detail hereinbelow.
A further sensing element 136 has characteristics
substantially indentical to those of element 132 and is positionec
in the vicinity of coil 116. Element 136 is exposed to the
l ambient outdoor air conditions prevailing near coil 116, but is
¦ spaced sufficiently far from it so as not to be affected by frost
build-up thereon. In a pre~erred embodiment, the temperature
characteristics of sensing element 136 are identical to those of
element 132. A pair of terminals 138 is associated with sensing
l element 136 for connecting the Latter into an electronic circuit.
¦ A catch basin 140 is positioned below outdoor coil 116
so as to collect the droplets 142 of melt water from outdoor coil
116 when the latter is in its defrost cycle. When the outdoor
coil is located in the wall separating the two environments, the
l catch basin is similarly embedded in the wall. A temperature
¦ sensing element 144 is positioned at the entrance to catch
basin 140. In a preferred embodiment of the invention, the
sensing element 144 comprises a thermistor which has a pre-
determined negative temperature coefficient of r esistivity.
I A pair of terminals 148 permits sensing element 144 to be
¦ connected into an electronic circuit.
1 9D RA 13389
I A sensing element 1.50 is preferably positioned in contact
¦ with indoor coil 118. If prevented from doing so by the surround-
¦ ing fin structure, element 150 may be located between the fins,
¦ preferably no further than 1/4" from coil 118. In a preferred
¦ embodiment of the invention, sensing element 150 comprises a
¦ thermistor having a predetermined negative temperature coefficient :
¦ of resistivity. A pair of terminals 152 permits sensing element
¦ 150 to be connected into an electronic circuit.
¦ Fig. 2 illustrates frost control apparatus for use with
¦ a temperature conditioning system of the type shown in Fig. 1.
¦ The circuit shown consists of two por~ions applicable to outdoor
and indoor coil frost control respectively. The first of these
portions comprises a pair of differential voltage bridge circuits
l 10 and 12, while the indoor frost control section utilizes only
¦ a single differential voltage bridge circuit 14.
Bridge circuit 10 includes respective resistive circuit
sections 16, 18, 20 and 22 which are connected by intermediately
positioned nodes 24, 26, 28 and 30. A positive DC voltage, which
l need not be regulated, is applied across nodes 24 and 26. Node
¦ 26 is coupled to a reference point such as ground.
Circuit section 16 is connected between nodes 26 and 28
and comprises the aforesaid temperature sensing element 132
which h~s a resistor 34 connected in parallel therewith. As
l explained above, in a preferred embodiment of the invention
¦ sensing element 132 has a predetermined negative coefficient of
resistivity, such that its resistance increases with a decrease
in the temperature of the environment in which lt is located.
Circuit section 18 is connected between nodes 26 and 30 and
l includes the aforesaid ~emperature sensing element 136 which has
characteristics substantially identical to those of element 132.
- 7 -
3~6 9D RA 13 3 8 9
Circuit section 18 further includes a resistance 38 which is
¦ connected in series wi~h thermistor 136. Although a resistance
¦ of fixed value is shown, it will be understood ~hat a selectively
¦ variable resistance may be employed to assist in adjusting bridge
S ¦ circuit 10. Circuit section 20 is connected between nodes 24 and
¦ 28 and includes a resistor 40 which has a substantially constant
. ¦ resistance value. Similarly, circuit section 22 is connected
¦ between nodes 24 and 30 and comprises a resistor 42 of sub-
¦ stantially constant resistance value.
¦ Nodes 28 and 30 are coupled to the input of a voltage
¦ comparator 44 by way of an RC filter. The latter comprises
¦ resistors 41 and 43 in series with respective comparator input
terminals and a capacitor 45 connected across the latter terminal~ i.
l Comparator 44 is adapted to provide a bi-level output signal.
¦ Comparator 44 may be of the type which is commerci.ally available
from National ~emiconductor Company under the designation 3302 P
and provides a change of voltage level from low to hign voltage,
e.g. from 0 V to 8 V, at its output. Output level switching
I occurs in response to a pol.arity reversal of the signal applied
¦ to the comparator input.
The output of comparator 44 is coupled to node 54 of
¦ differential bridge circuit 12 by way of a diode ~. Bridge
circuit 12 includes respective resistive circuit sections 46,
1 48, 50 and 52 which are connected by means of intermediate nodes
¦ 54, 56, 58 and 60. Node 56 is coupled to ground.
Circuit section 46 is connected between nodes 56 and 58
and includes the aforesaid sensing element 144, the characteristi ~s
of which were discussed above in connection with Fig. 1. Circuit
l section 48 is connected between nodes 56 and 54 and includes a
¦ resistor 64, which is selected so as to balance the resistance
9D ~A 13389
¦ of sensing element 1~4 at the temperature at which switching i5
¦ desired. Circuit section 50 is connected ~etween nodes 54 and
¦ 58 and includes a resistor 66 which has a substantially constant
¦ resistance value. Similarly, circuit section 52 is connected
¦ between nodes 54 and 60 and includes a resistor 68 of sub-
¦ stantially constant resistance value.
Nodes 56 and 60 are coupled to the input of a second
¦ comparator 70 by way of an RC filter comprising resistors 65
¦ and 67 connected in series with the comparator input terminals
¦ and a capacitor 75 connected across the latter terminals. This
comparator is preferably of the type discussed above in conjunc-
l tion with comparator 44 and may have substantially identical
¦ characteristics to the latter. The bi-level signals which appear
l at the outputs of comparators 70 and 44 are coupled to a common
15 -¦ junction point 74 by way of diodes 72 and 47 respectively, as
well as to node 54 of bridge circuit 12.
l Junction point 74 and diodes 47 and 72 serve the function
¦ of a logical OR gate by combining ~he bi-level signals derived
I at the outputs of comparators 44 and 70. In addition to being
¦ coupled to bridge circuit 12, the output signal of the OR gate
is applied to one terminal of a silicon contro:Lled rectifier
76. SCR 76 is connected in series with relay coil 126 between
l an AC line terminal 78 and ground. A resistor 80 and a diode
¦ 82 form a series combination which is connected in parallel with
¦ the relay coil. In lieu of SCR 76, a transistor or a sensitive
triac may be used.
As previously explained, sensing element 132 is posi-
tioned proximate coil 116, i.e. either in direct contact there-
I with or positioned sufficiently close so that it will be encased
¦ if a layer of frost accumulates on the outdoor coil. In either
~¦ 9D RA 13389
3~ '
¦ case, it senses coil temperature. Sensing element 136 measures
¦ the ambient outdoor air in the vicinity of coil 116. Although
¦ the effective resistance of bridge circuit section 16 is sub-
¦ stantially equal to that of circuit section 18 when bridge
S ¦ circuit 10 is balanced, the effective temperature coefficient
¦ of resistivity of the two circuit sections is not the same.
¦ This is due primarily to the presence of resistor 341 which is
¦ connected in parallel with sensing element 132. Thus, the
¦ effective resistance of circuit section 18 will decrease more
¦ rapidly for a sensed temperature change of a given magnitude,
than will the effective resistance of circuit section 16 due
to a sensed temperature change of the same magnitude. Further,
the presence of resistor 38 in series with element 136 moves
l the charactexistic operating curves of the two circuit sections
¦ apart. The function of this feature of the invention will be-
eome apparPnt from the discussion below.
As compared to the outdoor coil portion of the circuit
described so far which relies on sensing elements 132, 136 and
l 144, the control of frost on indoor coil 118 depends on a single
¦ sensing element 150. As shown in Fig. 2, sensing element 150
forms part of differential bridge circuit 14. The latter com-
prises four resistive circuit sections 84, 85, 86 and 87 connecte 1
by intermediate nodes 88, 89, 90 and ~l. Nodes 88 and 89 are
l connected between the DC line voltage and ground respectively.
~ Circuit section 84, which is connected between nodes 89 and 90,
includes the aforesaid sensing element 150. Circuit section 85,
which is connected between nodes 89 and 91, includes a resistance¦
92. The latter is selected to balance the resistance of sensing
l element 150 at the temperature where switching is desired.
¦ Circuit sections 86 and 87 comprise resistors 93 and 94
~ - 10 -
- ` ` 9D RA 13389
1 1 3~ 3
¦ respectively, having substantially constant resistance values
¦ which are equal to each other in a preferred embodiment of the
¦ invention.
¦ The output of differential bridge circuit 14 is derived
¦ across nodes 90 and 91 and is applied to a voltage comparator 95
¦ by way of an RC filter network. Comparator 95 may be sub-
¦ stantially identical to units ~4 and 70, i.e. a bi-level signal
¦ is obtained at the comparator output which changes signal levels
¦ upon a change of polarity at the comparator input. The RC
¦ filter network includes resistors 96 and 97, connected between
¦ respective inputs of comparator 95 and nodes 90 and 91 respective Ly
¦ and a capacitor 90 connected across the comparator inputs. A
feedback resistor 99 is connected between the output of comparato
l 95 and one of the comparator inputs. The co~parator output is
¦ further coupled to junction point 74 by way of a diode lO0.
In operation, the signal applied to terminals 131 of
control unit 130 determines whether system llO will operate in
the heating or cooling mode by suitably setting valve 112.
l Let it be assumed that the heating mode is selected wherein heat
¦ is extracted from outdoor air by outdoor heat exchange coil 116,
to be dissipated through the indoor heat exchange coil 118. In
this mode of operation, outdoor coil 116 acts as an evaporator
which absorbs heat from the outdoor air so as to bring the re-
l frigerant fluid from its liquid to its gaseous state. Con-
¦ versely, indoor coil 118 acts as a condenser which gives offheat to the ambient indoor air in the process of converting
the gas to a liquid.
For the heating mode, valve 112 is set to cause the
l refrigerant fluid to circulate in the direction indicated by the
¦ arrows in Figure l. Initially bridge circuit 10 is unbalanced
1, ~1
9D RA 13389
3~ ~6
so that the potential of node 28 relative to node 30 is negative.
As explained above, comparator 44 will switch to provide a high
~oltage level at its output if the applied input signal has the
polarity shown in Figure 2. Thus, for the unbalanced bridge
condition the signal level at the comparator output remains at
zero. The application of the zero signal level to bridge circuit
12 maintains the latter in a non-energized state. As a conse-
quence comparator 70, which responds to the potential across
nodes 58 and 60, also maintains its output at the zero signal
level. Further, since the indoor defrost circuit portion is
inactive when the system is operating in the heating mode, the
output of comparator 95 is similarly zero.
Since all input signals of the OR gate are zero, junction
point 74 is maintained at the zero voltage level and SCR 76 re-
mains cut off. The ~C voltage at terminal 78 therefore fails to
energize relay coil 126 and hence relay 124 remains in its
nactive -state. In a preferred embodiment of the invention
switch 122, which controls the application of power to compressor
114, is normally closed. Accordingly, while relay 124 is in-
active, the application of power to the compressor ls not
interrupted.
As previously discussed, the ambient outdoor air gives
up heat to coil 116 and in the process deposits moisture on the
coil. This moisture turns into a layer of frost on the cold
coil surface. In tiMe, the frost insulates the coil, as well as
sensing element 132 which is positioned on the coil, from the
ambient alr. Thus, the temperature to which element 132 is
exposed will be increasingly that of the frost layer rather
than that of the coil itself, so as to produce a change in the
resistance of this sensing element. On the o~her hand sensing
- 12_
,
; 9 D RA 13 3 8 9
element 136, which is positioned in the vicinity of coil 116
but out of contact with the latter, will continue to measure
the temperature of the ambient air. As a consequence, the
l temperature differential between sensing elements 132 and 136
¦ will increase progressively as the layer of frost builds up.
This action will eventually balance bridge circuit 10 and
subsequently unbalance it in the opposite direction, thus pro-
ducing a change of polarity across nodes 28 and 30. When node
l 28 becomes positive with respect to node 30, comparator 44
¦ responds by switching its bi-level output signal from zero to
¦ the high voltage level.
The new signal level, which is coupled to junction point
l 74 by way of dlode 47, is effective to turn on SCR 76. As a
¦ consequence relay coil 126 is energi2ed, relay 124 is activated
¦ and switch 122 opens to interrupt the application of power to
the compressor from terminals 120. Thus, when the temperature
differential between the ambient outdoor air and the outdoor coil
116 increases beyond a predetermined threshold, it indicates
l an excessive frost condition of the outdoor coil and the outdoor
¦ ` coil defrost cycle is initiated.
The bi-level comparator signal from unit 44 is further
l coupled to node 54 so as to energize bridge circuit 12. The
¦ parameters of this bridge circuit are chosen such that the bridge
is normally unbalanced and node 58 is positive with respect to
¦ node 60. When bridge circuit 12 is energized under these condi-
tions, comparator 70 responds by providing a high signal level
at its output. The latter signal level is coupled to junction
point 74 by way of diode 72, as well as being fed back to noc'e
54 oi bridge circuit 12.
- 13 -
I
- - ~ ^ r ~
1 ~ 2~ 9D RA 13389
! As explained above, compressor 114 is shut down upon
the initiation of the outdoor coil defrost cycle. The action
of relay 124 may be further utilized to initiate positive de-
frosting, e.g. by heating or the like, if the system is so
¦ equipped. In either case, the temperature of the outdoor coil
116 rises, causing the frost deposited thereon to melt. In the
absence of cooling by the refrigerant, the temperature of coil
116 approaches the ambient outdoor air temperature. The tempera-
l ture sensed by element 132 thus approaches that sensed by element
¦ 136 so as to first balance the bridge and then return it to its
previous unbalanced condition. Accordingly, the output signal
level of comparator 44 switches back to zero.
The latter action does not, however, deenergize bridge
l circuit 12 due to the fact that the bi-level output signal of
¦ comparator 70 is fed back to node 54 of that bridge circuit.
Thus, the positive potential across nodes 58 and 60 of bridge
circuit 12 is maintained and the bi-level signal at the output
of comparator 70 remains high. Due to the logical OR gate
l arrangement described above, the signal level at junction point
¦ 74 also remains high and hence SCR 76 continues to conduct.
Relay coil 126 remains energized and relay 124 continues to hold
switch 122 open to continue the defrost cycle.
As explained above, sensing element 144 is exposed to
l the droplets 142 of melt water which remains at approximately
¦ 32 E' until the defrosting action produces no further melting.
At that point the ambient outdoor air, or such heat as is trans-
ferred through the wall which separates the outdoor and indoor
¦ environments, raises the temperature of the melt water and
¦ hence of sensor 144. At a predetermined temperature, e.g. at
¦ 36 F, the resistance of sensing element 144 changes sufficiently .
1 ~ 3~ 9D RA 13389
to cause bridge circuit 12 to become firs~ balanced and
subsequently unbalanced in the opposite direction. The bi-
level signal at the output comparator 70 then switches to zero,
thereby cutting off SCR 76 and deactivating relay 124 by de-
energizing relay coil 126. This action terminates the defrostcycle by permitting switch 122 to revert to its normally closed
position and to restore power tc comparator 114.
In essence, therefore, in the present invention the
buildup of frost on sensing element 132 causes bridge circuit 10
to become balanced. The resultant signal from comparator 44
initiates the defrost cycle and it further energizes bridge
circuit 12. The latter action produces a signal at the output of
comparator 70. This signal is fed back so as to maintain the
snergization of bridge 12 and continue the defrost cycle. The
defrost cycle is ter~inated by the action of bridge circuit 12
only when all the frost has melted off outdoor coil 116.
It will be noted that bridge circuit 10 becomes active
when the differential in the temperature sensed by elements 132
and 13~ respectively, exceeds a predetermined threshold level.
It is a ~eature of the present invention that this threshold
level, which represents the difference between outdoor coil
temperature and outdoor ambient air temperature, decreases with
decreasing ambient outdoor air temperature. This action is
brought about by the difference in the effective negative
temperature coefficîents of resistivity of circuit sections 16
and 18, as noted above and it effectively compensates for the
fact that at lower outdoor ambient temperatures less heat is
absorbed by the outdoor coil from the ambient outdoor air.
Thus, in accordance with the present invention, maximum operat-
ing efficiency of the system is maintained by initiating the
- 15 -
.. . .
9 D RA 13 3 8 9
~¦ defrost cycle at a smaller diferential temperature when the
ambient outdoor air temperature is low, than is the case at ,
higher outdoor air temperatures.
l A typical relationship of these parameters for effective
¦ operation is shown in Fig. 3, wherein the threshold level, i.e.
the differential ~T between the temperatures sensed by elements
132 and 136 which prevails at the start of the defros~ cycle, is
plotted against outdoor ambient temperature. It will be noted
l that the threshold level decreases with declining ambient outdoor
¦ temperature. No frost build-up occurs beyond an ambient tempera-
ture of approximately +53 F.
If it is desired to operate the temperature conditioning
system 110 in its cooling mode, a suitable signal is applied to
l terminals 131 which causes control unit 130 to switch valve 112
¦ to its opposite setting. For the latter set~ing, the direction
of fluid flow will be opposite to tha~. shown in Fig. 1. In
~his case, indoor coil 118 acts as an evaporator and outdoor coil
116 becomes the condensing coil.
l If the ambient indoor air contains moisture, frost may
¦ be caused to be deposited on coil 118. ~s explained above,
temperature sensing element 150 is positioned proximate coil
118, preferably in contact with the coil. If prevented from
contact with the coil by the surroundi.ng fin structure, element
l 150 is positioned at a spacing su~ficiently close so that frost
¦ build-up on ~he coil will also encase the element.
Bridge circui~ 14, which is normally balanced, becomes ,
unbalanced when frost on coil 118 encases element lS0. The
¦ resultant positive potential between nodes 90 and 91 is applied ¦
l to the input of comparator 95 and causes the latter to switch.
1 The resultant high voltage level at the comparator outpu~ is
.~ - 16 -
.
9D R~ 13389
3~ 3~ ~
applied to junc~ion point 74 and thence to SCR 76 which becomes
conductive, Relay coil 126 is thus energized and activates relay
124 so as to open switch 122. Accordingly, compressor 114 is
shut down.
It will be noted that the output signal of comparator 95
is fed back to the comparator input by resistor 99. Resistors 99
and 90 form a voltage divider by means of which a portion of the
voltage at the comparator output is fed back to the comparator
input. The presence of the feedback resistor connection causes
the comparator to be turned on~ i.e. switched to provide a high
voltage level at its output, when a first positive potential
appears across nodes 90 and 91 representative of a predetermined
indoor coil temperature. The comparator remains turned on even
after the balance of bridge circuit 14 is restored. Thus, the
defrosting action continues until a second predetermined potential
appears across nodes 90 and 91, lower than the first potential.
Power to compressor 114 is restored upon the occurrence of the
latter potential, which is representative of an indoor coil
temperature higher than the one at which the defrost cycle was
initiated.
Thus, since the voltage across nodes 90 and 91 is a
function of the temperature sensed by element 150, i.e. the
temperature of the indoor coil, the circuit arrangement described
maintains the indoor coil defrost cycle throughout a predetermined
temperature range to assure complete defrosting. This range
can be tailored to the precise requirements of the system by
the choice of the values of resistors 92, 99 and 90. In a pre-
ferred embodiment of ~he invention, the start of the indoor coi
defrost cycle is set to occur at a temperature of 32 F and it
terminates at a temperature of approximately 48 F, both sensed
by element 150.
.
¦ 9D RA 13389
I ~ ~ 3~
¦from the basic principles thereof. As stated, the defrost
cycle, whether for the indoor or outdoor coil, may be implemented
in ways other than shutting down the compressor. For example,
l the direction of fluid flow in the closed temperature conditioning
¦ system may be temporarily reversed. In this manner the frost-
carrying evaporator coil temporarily become~ the condenser coil.
¦ Its temperature rises as a resuit of the reversed operation, and
hence the defrosting action is speeded up. Similarly, special
l heaters may be employed, preferably in conjunction with fan motors
¦ which direct heated air at the frost-laden coil when a defrost
cycle is initiated.
As pointed out above, the circuit which combines the
various signals that are coupled to junction point 74 effectively
constitutes a logical OR gate. The presence of a high voltage
level in any one of the signals which are being combined will
provide an output signal to activate SCR 76. In this type of an
arrangement switch 122 is normally closed, but is opened when
one or more of the bi-level signals applied to junction point 74
are at a high voltage level. The present invention is not
limited to such an arrangement. For example, an AND gate may be
employed to tur-n off SCR 76 if the latter is normally conductive
when the compressor is operating. In such an arrangement, switch
122 may be normally closed, to be opened only when relay coil
126 is deenergized in response to the absence of a high voltage
level in one or more of the bi-level signals coupled to junction
point 74.
While the sensing elements preferably comprise thermis~ors¦
the invention is not so limited and requires only the presence
of sensing elements which vary in a predictable manner, increasing'
~or decr sing ln value with a change of temperature
~ - 18 -
. . . . . ..
.
3Z 6 9D RA 13389
Although not specifically shown in Fig. 2, it will be
understood that selectively variable resistors may be included
in one or more circuit sections of each bridge circuit. The
purpose of such resistors is to permit the respective bridge
circuits to be manually balanced if such fine adjustment is
desired.
The invention herein disclosed provides a frost control
system whereby a defrost cycle is initîated and terminated
independently of ambient conditions, such as ambient air tempera-
ture, wind gusts, etc. The invention is implemented in a manner
that permits it to be incorporated at relatively low cost into
a temperature conditioning system such as a heat pump or the like
and where it requires only a minimum of maintenance.
Numerous modifications, variations and changes will now
occur to those skilled in the art, all of which fall within the
spirit and scope of the present invention. Accordingly, the
invention is intended to be limited only by the scope of the
~ppended cl~lm~
:: ' : ~ ~ ~
