Refrigeration – Processes – Evaporation induced by sorption
Reexamination Certificate
2000-03-14
2001-07-24
Doerrler, William (Department: 3744)
Refrigeration
Processes
Evaporation induced by sorption
C062S476000
Reexamination Certificate
active
06263682
ABSTRACT:
This invention relates to heat pumps of the absorption cycle type, particularly to such heat pumps of a rotary or centrifugal design, and to methods of operating said heat pumps.
Absorption cycle heat pumps comprise the following components: evaporator, absorber, generator, condenser and optionally a solution heat-exchanger; and are charged with a suitable working mixture in the fluid phase. The working mixture comprises a volatile component and an absorbent therefor.
In absorption cycle heat pumps, a high temperature source of heat, so-called high-grade heat, and a low temperature source of heat, so-called low-grade heat, deliver heat to the heat pump, which then delivers (or ejects) the sum of the heat input from both sources at an intermediate temperature.
In operation of conventional heat absorption cycle heat pumps, a working mixture which is rich in a volatile component (which mixture is hereinafter referred to for convenience as “Mixture R”) is heated in the generator, under pressure, by high-grade heat such that vapour of the volatile component is generated and a working mixture which is less rich or lean in the volatile component is produced (which mixture is hereinafter referred to for convenience as “Mixture L”).
In known single stage heat pumps the aforesaid vapour of the volatile component from the generator is condensed in the condenser, at the same high pressure, with the evolution of heat and the formation of liquid volatile component. The liquid volatile component is passed through an expansion valve, to reduce the pressure thereof, and thence to an evaporator. In the evaporator, the aforesaid liquid accepts heat from a low temperature source of heat, typically air or water at ambient temperature, and evaporates. The resulting vapour of the volatile component passes to an absorber where it is absorbed in Mixture L with the re-formation of Mixture R and evolution of heat. The Mixture R is then transferred to the vapour generator and hence completes the cycle. Many variations on this process are possible; for example the heat pump may have two or more stages, where vapour from the volatile component evaporated by the first mentioned (primary) vapour generator condenses in an intermediate condenser which is thermally coupled to supply heat to an intermediate vapour generator which creates further volatile component vapour to condense in the first mentioned (primary) condenser.
Where we wish to emphasize the physical state of the volatile component we shall, for convenience, refer to it as ‘VVC’ (when it is in the gas or vapour state) or ‘LVC’ (when it is in the liquid state). The volatile component may otherwise be referred to as the refrigerant, and the mixtures L and R as absorbent fluid. In the particular example given, the refrigerant is water and the absorbent fluid is a hydroxide solution comprising alkali metal hydroxides as described in EP-A-208427, the contents of which are incorporated herein by reference.
U.S. Pat. No. 5,009,085 discloses an earlier rotary heat pump, and the teachings of that document are incorporated herein by reference. There are various problems associated with a heat pump of the kind described in U.S. Pat. No. 5,009,085, and various aspects of the present invention seek to overcome or at least mitigate these problems.
In heat pumps such as those described in U.S. Pat. No. 5,009,085, there is a risk of catastrophic failure if the working fluid should crystallize or otherwise experience restricted flow. For this reason the heat pump is usually operated with the maximum solution concentration set well away from the crystallization condition, and determined by the desire to avoid crystallization rather than to provide maximum efficiency. We have developed a modification which initiates corrective action when the onset of crystallization is detected, thus allowing safe operation close to the crystallization condition.
Accordingly in one aspect, this invention provides an absorption cycle heat pump including means responsive to the onset of crystallisation of absorbent in the working fluid, or the onset of unacceptably high viscosity, to initiate means for preventing further crystallisation and/or for re-dissolving crystallised material, or reducing said viscosity.
The area most prone to crystallization or flow restriction is normally sited in the absorbent fluid flow path into the absorber from the solution heat exchanger, which is at its lowest temperature and highest concentration.
The means for preventing may comprise clearance means for increasing the temperature and/or reducing the concentration of absorbent in the working fluid at or adjacent said crystallisation site. For example a stream of fluid may be diverted at least temporarily to increase the temperature of the flow past said crystallisation site either directly or indirectly by thermal exchange. This may be activated by detecting the local pressure upstream of the crystallization site.
In one method, where absorbent fluid passing from the vapour generator to the absorber gives up heat to absorbent fluid passing in the opposite direction via a solution heat exchanger, a portion of the absorbent fluid from the path from the generator to the absorber, which will have a relatively high temperature, is diverted to be introduced into the return flow from the absorber back to the generator. In this way, the temperature of the return flow is increased which raises the temperature of the flow upstream of the crystallization site, thereby dissolving or reducing the viscosity of the liquid at said site.
This diversion may be achieved by providing a pressure dependent control such as a valve or a weir between the two flows, whereby said introduction is initiated when the back pressure caused by the onset of crystallization or unacceptably high viscosity exceeds a preset threshold.
Alternatively, coolant fluid may be diverted from the condenser to the evaporator, thereby to raise the evaporation temperature and cause an increased amount of refrigerant to evaporate and be taken up by the absorbent, resulting in a temporary decrease in concentration of absorbent in the working fluid and an increase in temperature of the working fluid in the crystallisation region.
A further problem is that of maintaining a reasonably high efficiency whilst running the heat pump at less than full capacity, when the temperature lift and/or thermal load is reduced. The temperature lift is defined as the temperature difference between the evaporator and the absorber. We have found that it is possible to improve cycle efficiencies in these part-load conditions by controlling the flow rate of absorbent fluid around the cycle in accordance with the thermal load and/or temperature lift. Furthermore, we have found that it is possible to design the heat pump so that the dynamic or static pressures in the pump tend to adjust the flow rate of absorbent fluid to suit the prevailing temperature lift or thermal load, thus obviating the need for adjustable control valves or the like, although we do not exclude the possibility of such control arrangements.
Accordingly, in another aspect, this invention provides an absorption cycle heat pump comprising a vapour generator, a condenser, an evaporator and an absorber so interconnected as to provide cyclic fluid flow paths for a volatile fluid component and an absorbent fluid therefor, and flow rate control means for controlling the flow rate of the said absorbent fluid in accordance with at least one of:
(i) the temperature difference between the absorber and the evaporator, and
(ii) the thermal load on the heat pump, and
(iii) one or more other operating parameters.
The flow rate may be adjusted in various ways but it is preferred not to do so by varying the pump capacity. Thus the flow rate control means may conveniently comprise flow restriction means in the absorbent fluid flow path from said generator. The restriction may be controlled to give the required performance by the use of an active control system, but we have found that suitable control may be achieved by a
Green Richard John
Lorton Robert
Uselton Robert Brownlee
Winnington Terence Leslie
Doerrler William
Interotex Limited
Young & Thompson
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