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A significant advance in defrost control
Economic factors will become more and more prominent in the future. Thus the potential of compact heat exchangers ~ is largely unexploited. More progress is also desirable in the development of enhanced heat transfer surfaces using roughness elements, fins and porous structures and by the application of internal turbulators in tubes 2~3°. A lot of empirical w o r k has been done but w e need more understanding of fundamentals 39. Furthermore w e should not forget the fouling problem 4°, because sophisticated heat exchanger designs are useless unless w e have reliable methods to eliminate or control fouling, Finally, m u l t i c o m p o n e n t boiling 41, also including condensation heat transfer presents a continuously g r o w i n g field of interest. N e w w o r k i n g substances for absorption heat pumps are coming up and nonazeotropic refrigerant mixtures are used to improve the performance of heat pumps and refrigerators. We need to provide reliable heat transfer calculation methods for such systems 31-35
13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
References
37 38 39 40 41 42
1 2 3 4 5 6 7 8 9 10 11 12
Marchal et al. Proc XVI IIR Congress of Refrigeration, Paris (1983) 81. 122 Kuni8 et el. ibid (1983) BI. 138 Briuon Lope= ibid (1983) BI. 143 Giat et el. ibid (1983) BI. 202 Came=-Pintaux lit el. ibid (1983) B1. 203 Kalinowski et el. ibid (1983) BI. 401 James tit el. ibid (1983) B1. 452 Gori ibid (1983) BI. 26 Latyehev et el. ibid (1983) BI. 31 Srendeng ibid (1983) Bl. 42 A n u r i lit al. ibid (1983) B1.56 Soetee et el. ibid (1983) B1, 139
43 44 45
Roriz ibid (1983) B1. 200-282 Gonzalez et al. ibid (1983) B1. 371 Powell ibid (1983) B1. 379 Fournier et el. ibid (1983) BI. 590 De Ponta et el. ibid (1983) BI. 590-591 Deqenne et el. ibid (1983) BI. 655 Dhar lit el. ibid (1983) B1.317 Hihara lit al. ibid (1983) BI. 400 8andru ibid (1983) B1. 518 8teiner ibid (1983) BI. 610 Ella ibid (1983) B1. 621 Hahne et el. ibid (1983) BI. 330 Arora et el. ibid (1983) B1.54 Poredoe et el. ibid (1983) BI. 179 Srendeng ibid (1983) BI. 459 Malek et al. ibid (1983) BI. 490 Giovannoni et el. ibid (1983) BI. 493 Bonca et al. ibid (1983) BI. 600 Tendon i~ el. ibid ,(1983) B1, 95 Singal et el. ibid (1983) BI. 96 Jain et el. ibid (1983) BI. 315 Gareia et el. ibid (1983) B1. 587-588 Alpay et al. ibid (1983) BI. 596 Keltin Proc 7th Int Heat Transfer Conference, Munich, Hemisphere Pub Corp I (1982) RK 15 Butterworth et al. ibid (1982) RK 15 Shah ibid (1982) RK 12 Nakayama et el. ibid (1982) RK 13 K n u d N n e t e l . ibid (1982) RK11 Stephan ibid (1982) RK 14 Comini, G., Del Giudiee, S., Strada, M., Rebellato, L. The finite element method in refrigeration engineering Int J Refrig 1 2 (1978) 113-118 Taborek, J. Evolution of heat exchange design techniques Heat Transfer Engineering I (1979) 15-29 Hewitt, F. G. Measurement of two phase flow parameters. Academic Press, London (1978) Gorenflo, D. Stand der Berechnungsmethoden zum W~rmeL~bergang bei der Verdampfung yon K<emitteln in freier Konvektion DKV-Tagungsbericht 9 Jahrgang, Essen (1982)
A significant advance in defrost control D. S. LLewelyn Keywords: refrigeration, defrost, control
Un progres important dans la r6gulation du d6givrage On d~crit un progr~s important dans la r#gulation du d#givrage pour ~liminer la glace des r#frig#rateurs, des
cong~lateurs, des entrepSts frigorifiques et des serpentins frigorifiques des pompes ~ chaleur. Cette r~gulation s'appuie sur la rnesure de I'effet d'isolation thermique de la couche de glace, avec fonctionnement automatique du d~givrage sur demande.
A s i g n i f i c a n t a d v a n c e in d e f r o s t c o n t r o l f o r ice r e m o v a l in r e f r i g e r a t o r s , f r e e z e r s , cold stores and h e a t p u m p c o o l i n g coils is described.
This Control is based on m e a s u r e m e n t of t h e t h e r m a l i n s u l a t i o n e f f e c t o f t h e ice layer, w i t h a u t o m a t i c o p e r a t i o n f o r d e f r o s t on d e m a n d .
Regular removal of the ice accretion on refrigerator, freezer, cold store and heat pump cooling coils is of increasing importance as energy costs soar and the
need for conservation of energy and profit margins increases. Ice is a g o o d thermal insulator. The thicker the ice layer the greater the insulating effect and the greater the demand on the compressor to maintain the desired heat removal rate. W i t h o u t defrosting a point may be reached where, even w i t h the compressor running continuously, it will prove impossible to hold the
The author is from Newtech Controls Ltd., Pucklechurch Industrial Estate. Bristol, Avon, BS17 3QH, UK. Paper received 9 December 1983.
334
0140-7007/84/050334-02S3.00 © 1984 Butterworth Et Co (Publishers) Ltd and IIR
Revue Internationale du Froid
temperature down. In fan-assisted evaporators, once the evaporator matrix is choked by ice, the calorific efficacy falls dramatically. All current defrost controls are based on time cycles. A range of electromechanical controls, using a synchronous motor driving a tappet-carrying dial through a gear train, has been available for many years. By setting the tappets at appropriate points around the dial defrost initiation times can be set, The more sophisticated of these controls incorporate a subsidiary tappetted dial driven by the gear train which may be used to set defrost durations. Provision for fan delay is also included in some controls. Electronic defrost controls, more precise and accurate than the older controls, are beginning to take a share of the market, These controls are generally microprocessor based, and can provide additional features such as 'fail safe', malfunction warning, local and remote alarm, etc. All such controls use a clock in one form or another. Since the rate of ice deposition is not a function of time, but is dependent on difficult to predict parameters such as humidity, relative temperatures, frequency of air changes, type and mix of goods stored, etc, any control which initiates a defrost based on time is at best an energy wasteful compromise, A control is needed which initiates a defrost only when it is justified, which terminates the defrost when the ice has cleared and which delays re-energizing the fan(s) until any residual water droplets in the evaporator matrix have been reconverted to ice. This paper reports a successfully completed research and development programme aimed at producing a rugged, reliable, simple to instal and cost-competitive defrost control to satisfy these requirements. Several parallel approaches were pursued. These included: Monitoring the capacity of a plate capacitor installed in the evaporator unit. Since the dielectric constant of ice differs from the air it displaces, a signal proportional to the ice thickness was derived. Although practical trials established its feasibility, this approach was discarded because very stable high gain amplifiers and high frequencies were involved. Monitoring the resonant frequency of an acoustic oscillator installed in the evaporator unit. Since the resonant frequency is a function of mass and compliance, a marked shift in frequency occurs with deposition of frost. Although this method gave reliable predictable results it was discarded on the grounds of unacceptable high unit cost. Various optical methods were explored using continuous and pulsed beams, in and beyond the visible range. Again several methods gave good results but all were found to be susceptible to false signals. Various configurations of pressure/flowrate change across or through the evaporator matrix were tried. All were discarded because of inconsistency, insensitivity and proneness to false signals.
Volume 7 Num~ro 5 Septembre 1 984
All the foregoing involved the measurement of ice deposit thickness, whereas the important parameter is the thermal insulation effect of the ice layer. The author therefore focussed on this thermal conductivity approach. The result is a new automatic (defrost on demand) defrost control, called the DD5, which initiates a defrost only when the thermal insulation of the ice layer on the evaporator reaches a pre-determined value, and terminates the defrost when the ice has cleared. This control is now being produced commercially and also incorporates a fan delay. It is 'fail safe', simple to install, immune from power interruption and, most important, it eliminates premature, belated and unwanted defrosts. Two identical sensors are used, one in intimate contact with the evaporator coil, and one positioned in the outlet air stream from the evaporator unit. As ice builds up on the evaporator, three separate effects occur. (1) The coldest point on the evaporator coil moves from its 'ice free' position toward the low pressure end of the coil: (2) the heat transfer efficiency of the system reduces; and (3) the mean evaporator temperature reduces and a temperature gradient develops through the ice layer. DD5 harnesses all three effects to provide a precise and repeatable value for defrost initiation. The sensors form part of a bridge network, the output of which is electronically processed to trigger a latching circuit, which initiates the defrost cycle. The sensor in intimate contact with the evaporator coil is then automatically incorporated into a second bridge circuit which provides the signal to reset the latching circuit, terminate the defrost and re-energize the freezing system when the ice has cleared. This same sensor is then automatically incorporated into a third bridge network which provides the signal to re-activate the fan(s) when any residual water droplets adhering to the evaporator have been reconverted to ice. Only one user adjustment is provided. This allows the user to tune the DD5 for optimum performance on any installation. DD5 defrost controls are giving good service on vehicle freezer units, refrigerated holds in ships, in cold stores and on domestic and industrial heat pumps. DD5 is available with power input options of: 240V, 50/60 Hz; 120V, 50/60 Hz; 24V, ac/dc; 12 V, dc. A four-channel defrost diversity sister matching unit is also available for use with DD5 in multievaporator installations, where a need exists to prevent simultaneous defrosting of the evaporators. A matching defrost status monitor which displays 'power on', 'freeze', 'defrost' and 'fan on' is also available. This unit incorporates a pair of volt free contacts which provide the facility to connect a remote warning system, e,g. a telephone auto-dialler. With the exception of relays the foregoing units are all solid state and have no moving parts. They are designed to be inexpensive and to give many years of service with no attention or adjustment.
335
13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36
References
37 38 39 40 41 42
1 2 3 4 5 6 7 8 9 10 11 12
Marchal et al. Proc XVI IIR Congress of Refrigeration, Paris (1983) 81. 122 Kuni8 et el. ibid (1983) BI. 138 Briuon Lope= ibid (1983) BI. 143 Giat et el. ibid (1983) BI. 202 Came=-Pintaux lit el. ibid (1983) B1. 203 Kalinowski et el. ibid (1983) BI. 401 James tit el. ibid (1983) B1. 452 Gori ibid (1983) BI. 26 Latyehev et el. ibid (1983) BI. 31 Srendeng ibid (1983) Bl. 42 A n u r i lit al. ibid (1983) B1.56 Soetee et el. ibid (1983) B1, 139
43 44 45
Roriz ibid (1983) B1. 200-282 Gonzalez et al. ibid (1983) B1. 371 Powell ibid (1983) B1. 379 Fournier et el. ibid (1983) BI. 590 De Ponta et el. ibid (1983) BI. 590-591 Deqenne et el. ibid (1983) BI. 655 Dhar lit el. ibid (1983) B1.317 Hihara lit al. ibid (1983) BI. 400 8andru ibid (1983) B1. 518 8teiner ibid (1983) BI. 610 Ella ibid (1983) B1. 621 Hahne et el. ibid (1983) BI. 330 Arora et el. ibid (1983) B1.54 Poredoe et el. ibid (1983) BI. 179 Srendeng ibid (1983) BI. 459 Malek et al. ibid (1983) BI. 490 Giovannoni et el. ibid (1983) BI. 493 Bonca et al. ibid (1983) BI. 600 Tendon i~ el. ibid ,(1983) B1, 95 Singal et el. ibid (1983) BI. 96 Jain et el. ibid (1983) BI. 315 Gareia et el. ibid (1983) B1. 587-588 Alpay et al. ibid (1983) BI. 596 Keltin Proc 7th Int Heat Transfer Conference, Munich, Hemisphere Pub Corp I (1982) RK 15 Butterworth et al. ibid (1982) RK 15 Shah ibid (1982) RK 12 Nakayama et el. ibid (1982) RK 13 K n u d N n e t e l . ibid (1982) RK11 Stephan ibid (1982) RK 14 Comini, G., Del Giudiee, S., Strada, M., Rebellato, L. The finite element method in refrigeration engineering Int J Refrig 1 2 (1978) 113-118 Taborek, J. Evolution of heat exchange design techniques Heat Transfer Engineering I (1979) 15-29 Hewitt, F. G. Measurement of two phase flow parameters. Academic Press, London (1978) Gorenflo, D. Stand der Berechnungsmethoden zum W~rmeL~bergang bei der Verdampfung yon K<emitteln in freier Konvektion DKV-Tagungsbericht 9 Jahrgang, Essen (1982)
A significant advance in defrost control D. S. LLewelyn Keywords: refrigeration, defrost, control
Un progres important dans la r6gulation du d6givrage On d~crit un progr~s important dans la r#gulation du d#givrage pour ~liminer la glace des r#frig#rateurs, des
cong~lateurs, des entrepSts frigorifiques et des serpentins frigorifiques des pompes ~ chaleur. Cette r~gulation s'appuie sur la rnesure de I'effet d'isolation thermique de la couche de glace, avec fonctionnement automatique du d~givrage sur demande.
A s i g n i f i c a n t a d v a n c e in d e f r o s t c o n t r o l f o r ice r e m o v a l in r e f r i g e r a t o r s , f r e e z e r s , cold stores and h e a t p u m p c o o l i n g coils is described.
This Control is based on m e a s u r e m e n t of t h e t h e r m a l i n s u l a t i o n e f f e c t o f t h e ice layer, w i t h a u t o m a t i c o p e r a t i o n f o r d e f r o s t on d e m a n d .
Regular removal of the ice accretion on refrigerator, freezer, cold store and heat pump cooling coils is of increasing importance as energy costs soar and the
need for conservation of energy and profit margins increases. Ice is a g o o d thermal insulator. The thicker the ice layer the greater the insulating effect and the greater the demand on the compressor to maintain the desired heat removal rate. W i t h o u t defrosting a point may be reached where, even w i t h the compressor running continuously, it will prove impossible to hold the
The author is from Newtech Controls Ltd., Pucklechurch Industrial Estate. Bristol, Avon, BS17 3QH, UK. Paper received 9 December 1983.
334
0140-7007/84/050334-02S3.00 © 1984 Butterworth Et Co (Publishers) Ltd and IIR
Revue Internationale du Froid
temperature down. In fan-assisted evaporators, once the evaporator matrix is choked by ice, the calorific efficacy falls dramatically. All current defrost controls are based on time cycles. A range of electromechanical controls, using a synchronous motor driving a tappet-carrying dial through a gear train, has been available for many years. By setting the tappets at appropriate points around the dial defrost initiation times can be set, The more sophisticated of these controls incorporate a subsidiary tappetted dial driven by the gear train which may be used to set defrost durations. Provision for fan delay is also included in some controls. Electronic defrost controls, more precise and accurate than the older controls, are beginning to take a share of the market, These controls are generally microprocessor based, and can provide additional features such as 'fail safe', malfunction warning, local and remote alarm, etc. All such controls use a clock in one form or another. Since the rate of ice deposition is not a function of time, but is dependent on difficult to predict parameters such as humidity, relative temperatures, frequency of air changes, type and mix of goods stored, etc, any control which initiates a defrost based on time is at best an energy wasteful compromise, A control is needed which initiates a defrost only when it is justified, which terminates the defrost when the ice has cleared and which delays re-energizing the fan(s) until any residual water droplets in the evaporator matrix have been reconverted to ice. This paper reports a successfully completed research and development programme aimed at producing a rugged, reliable, simple to instal and cost-competitive defrost control to satisfy these requirements. Several parallel approaches were pursued. These included: Monitoring the capacity of a plate capacitor installed in the evaporator unit. Since the dielectric constant of ice differs from the air it displaces, a signal proportional to the ice thickness was derived. Although practical trials established its feasibility, this approach was discarded because very stable high gain amplifiers and high frequencies were involved. Monitoring the resonant frequency of an acoustic oscillator installed in the evaporator unit. Since the resonant frequency is a function of mass and compliance, a marked shift in frequency occurs with deposition of frost. Although this method gave reliable predictable results it was discarded on the grounds of unacceptable high unit cost. Various optical methods were explored using continuous and pulsed beams, in and beyond the visible range. Again several methods gave good results but all were found to be susceptible to false signals. Various configurations of pressure/flowrate change across or through the evaporator matrix were tried. All were discarded because of inconsistency, insensitivity and proneness to false signals.
Volume 7 Num~ro 5 Septembre 1 984
All the foregoing involved the measurement of ice deposit thickness, whereas the important parameter is the thermal insulation effect of the ice layer. The author therefore focussed on this thermal conductivity approach. The result is a new automatic (defrost on demand) defrost control, called the DD5, which initiates a defrost only when the thermal insulation of the ice layer on the evaporator reaches a pre-determined value, and terminates the defrost when the ice has cleared. This control is now being produced commercially and also incorporates a fan delay. It is 'fail safe', simple to install, immune from power interruption and, most important, it eliminates premature, belated and unwanted defrosts. Two identical sensors are used, one in intimate contact with the evaporator coil, and one positioned in the outlet air stream from the evaporator unit. As ice builds up on the evaporator, three separate effects occur. (1) The coldest point on the evaporator coil moves from its 'ice free' position toward the low pressure end of the coil: (2) the heat transfer efficiency of the system reduces; and (3) the mean evaporator temperature reduces and a temperature gradient develops through the ice layer. DD5 harnesses all three effects to provide a precise and repeatable value for defrost initiation. The sensors form part of a bridge network, the output of which is electronically processed to trigger a latching circuit, which initiates the defrost cycle. The sensor in intimate contact with the evaporator coil is then automatically incorporated into a second bridge circuit which provides the signal to reset the latching circuit, terminate the defrost and re-energize the freezing system when the ice has cleared. This same sensor is then automatically incorporated into a third bridge network which provides the signal to re-activate the fan(s) when any residual water droplets adhering to the evaporator have been reconverted to ice. Only one user adjustment is provided. This allows the user to tune the DD5 for optimum performance on any installation. DD5 defrost controls are giving good service on vehicle freezer units, refrigerated holds in ships, in cold stores and on domestic and industrial heat pumps. DD5 is available with power input options of: 240V, 50/60 Hz; 120V, 50/60 Hz; 24V, ac/dc; 12 V, dc. A four-channel defrost diversity sister matching unit is also available for use with DD5 in multievaporator installations, where a need exists to prevent simultaneous defrosting of the evaporators. A matching defrost status monitor which displays 'power on', 'freeze', 'defrost' and 'fan on' is also available. This unit incorporates a pair of volt free contacts which provide the facility to connect a remote warning system, e,g. a telephone auto-dialler. With the exception of relays the foregoing units are all solid state and have no moving parts. They are designed to be inexpensive and to give many years of service with no attention or adjustment.
335