Dehumidifiers

Abstract: Air drawn in by a fan 4 is cooled by evaporator coils 2 and re-warmed by a condenser 3. A microcontroller MC and sensor S reads the temperature of the incoming air at regular intervals, e.g. once every minute, and controls a compressor 1 to operates in successive run periods, during which the evaporator 2 removes moisture from the air, separated by defrost periods in which the compressor is switched off so that warm incoming air melts any ice on the evaporator. The temperature at the start of a run period determines the duration of the respective run period, and the temperature at the end of a run period determines the length of the following defrost period. The length of the run period is constant at low temperatures but increases to a longer constant period at higher temperatures. The length of the defrost period is a maximum close to freezing point, but is reduced by decreasing increments as air temperature increases. At higher temperatures the dehumidifier operates continuously with no defrost.

Description: TECHNICAL FIELD OF THE INVENTION This invention relates to dehumidifiers for extracting moisture from the air in a building. BACKGROUND In known dehumidifiers a refrigerant is circulated by a compressor through an evaporator, which becomes cold, and a condenser, which becomes warm, and air is passed over the evaporator so that any moisture in the air condenses on the evaporator, following which the air passes over the condenser to be warmed before leaving the dehumidifier. Such dehumidifiers (see for example DE-A-28 02 550) are commonly used to reduce dampness or condensation in a building. The water that collects on the evaporator may freeze, but the dehumidifier periodically enters a defrost mode which allows the ice to melt. In DE-A-28 02 550 the defrosting period ends when the sensed temperature of the evaporator reaches a predetermined level. The water is collected in a water container, which usually includes a float switch that switches off the dehumidifier when the container is full. The defrost mode can be achieved in several ways: 1. A passive defrost system is sometimes used, in which the compressor is switched off for a fixed period every hour, i.e. there is a set running period and a set defrost period. The fan which draws air through the dehumidifier continues to run during the defrost period so that the incoming, relatively warm air eventually melts any buildup of ice on the evaporator. 2. In other cases a defrost heater may be included to melt ice on the evaporator. Again, the length of the defrost period is fixed, as is the length of the running period. 3. In hot gas bypass defrost systems, hot refrigerant from the compressor outlet is diverted by a solenoid valve directly into the frosted evaporator to melt the ice. In this case too, the defrost period is initiated for a preset period every hour (e.g. 5 minutes). An aim of the present invention may be viewed as being to improve the efficiency of existing dehumidifiers. SUMMARY OF THE INVENTION The present invention proposes a dehumidifier as defined in the appended Claims. BRIEF DESCRIPTION OF THE DRAWINGS The drawing is a diagrammatic representation of a dehumidifier of the invention, by way of non-limiting example. DETAILED DESCRIPTION OF THE DRAWINGS The illustrated dehumidifier has a passive defrost phase, although the invention could be applied to dehumidifiers which employ other defrost methods. A compressor 1 pumps refrigerant around a hermetically sealed circuit which includes evaporator coils 2 and a condenser 3. A refrigeration effect causes the evaporator to become cold and the condenser to become warm. A fan 4 draws incoming air over the evaporator coils so that any moisture in the incoming air condenses on the evaporator 2. The condenser 3 is positioned between the evaporator 2 and the fan 4, so that the air passes over the condenser and is warmed before leaving the dehumidifier. A drip tray 5 is mounted beneath the evaporator coils 2 to collect any water which runs off the evaporator and channel the water into a collecting vessel 6. A float-operated microswitch (not shown) is mounted in the collection vessel to switch off electrical power to the dehumidifier (e.g. fan and compressor) and prevent it from operating when the vessel 6 is full. In accordance with the invention, a temperature sensor S is positioned in the incoming air flow to sense the temperature of the incoming air. The output signals from the sensor are fed to a microcontroller MC, which reads the sensed temperature at regular periods, e.g. once every minute. The microcontroller uses this information to control the compressor 1 such that the compressor operates in successive run periods, during which the evaporator 2 removes moisture from the incoming air as described above, separated by defrost periods in which the compressor is switched off but the fan 4 continues to run to draw relatively warm air over the evaporator 2 causing any ice thereon to melt. The sensed temperature at the start of a run period determines the duration of the respective run period, as explained below. The temperature at the end of a run period determines the length of the following defrost period, as illustrated, by way of example, in Table 1. Air Temp. (°C) Run period (min.s) Defrost Period (min.s) 2.5 30 25 3.5 30 18 4.5 30 14 5.5 30 12 6.5 30 9 7.5 30 8 8.5 30 7 9.5 30 6 10.5 - 14.5 30 5 15.5 - 21.5 45 4 Above 21.5 Continuous 0 It will be seen from Table 1 that the length of the run period is constant below about 15°C but increases to a longer fixed period above this temperature when there will be less ice buildup and higher humidity levels will generally occur. At close to freezing point the length of the defrost period is a maximum since the incoming air will only defrost the evaporator slowly, but as the air temperature increases the length of the defrost period is gradually reduced. Only small reductions inthe defrost period take place above about 10°C, and above 21.5°C the dehumidifier operates continuously with no defrost since the temperature of the incoming air will always be high enough to prevent icing up of the evaporator. It will be appreciated that the operating characteristics of the dehumidifier can be varied within the scope of the claims. For example, the dehumidifier may also operate according to the conditions set out in Table 2. Again, the length of the run period is constant below about 13°C but increases to a higher constant figure above this temperature. When the sensed temperature falls below about 4°C the temperature of the incoming air will not be high enough to achieve passive defrosting of the evaporator. In this case, the microcontroller will put the dehumidifier into a 30 minute defrost period and then shut down the dehumidifier altogether. The unit will only come back on when the sensed air temperature rises to about 5°C. The length of the defrost period is a maximum around 4 to 5°C but as the air temperature increases the length of the defrost period is gradually reduced by decreasing Air Temp. (°C) Run period (min.s) Defrost Period (min.s) < 4.0 - refer to text - 4 - 5 45 30 5 - 7 45 25 7 - 8 45 18 8 - 9 45 15 9 - 10 45 13 10 - 11 45 11 11 - 12 45 9 12 - 13 45 7 13 - 15 75 6 15 - 18 75 5 18 - 27 75 4 Above 27 Continuous 0 increments. Above 27°C the dehumidifier operates continuously with no defrost since the temperature of the incoming air will be high enough to prevent icing. In practice there may be a small discrepancy between the temperature of the sensor and the ambient air temperature. The dehumidifier of the invention thus operates with a high level of efficiency for the following reasons: a) When defrosting takes place, the dehumidifier is only inoperative for as long as is necessary for complete defrosting, irrespective of the incoming air temperature. b) Defrost only takes place when the incoming air temperature is low enough to permit ice formation. c) At low temperatures defrosting takes place more frequently (i.e. there is a shorter run period) so that the ice never becomes thick. The operating temperature may be sensed in a number of positions. For example, it is conceivable that the temperature of the condenser or evaporator could be used, e.g. by terminating the defrost period when the evaporator temperature rises above 0°C. In order to provide accurate and repeatable results however, it is preferred to sense the temperature of air passing through the dehumidifier.

Claims: A dehumidifier in which a refrigerant is circulated by a compressor (1) through an evaporator (2), which becomes cold, and a condenser (3), which becomes warm, and air is passed over the evaporator so that moisture in the air condenses on the evaporator, following which the air passes over the condenser to be warmed before leaving the dehumidifier, the dehumidifier being arranged to operate with alternating run and defrost periods, characterised in that the length of the defrost period is varied in a predetermined relationship with sensed operating temperature. A dehumidifier according to Claim 1, in which there is a non-linear relationship between the sensed operating temperature and the length of the defrost periods. A dehumidifier according to Claim 2, in which, for a given change in operating temperature, the length of the defrost periods reduces with increasing operating temperature. A dehumidifier according to any preceding claim, in which the length of the defrost period becomes zero above a predetermined sensed operating temperature. A dehumidifier according to any preceding claim, in which the operating temperature is sensed by a temperature sensor (S) which is arranged to sense the temperature of air passing through the dehumidifier. A dehumidifier according to Claim 5, in which the temperature sensor is arranged to sense the temperature of incoming air before it is cooled by the evaporator or heated by the condenser. A dehumidifier according to any preceding claim, in which the length of a defrost period is determined by the temperature at the end of a preceding run period. A dehumidifier according to any preceding claim, in which the length of the run periods is reduced at low operating temperatures. A dehumidifier according to any preceding claim, in which the length of a particular run period is determined by the sensed operating temperature at the start of said run period. A dehumidifier according to any preceding claim, in which the dehumidifier uses a passive defrost arrangement.

Inventors: TAYLOR SIMON ANDREW, TAYLOR, SIMON ANDREW
Applicants: EBAC LTD, EBAC LIMITED
Application year: 1995
Main classification: F24F3
IPC codes: F24F3, F25D21