An Ice Making System

This invention relates to an evaporator refrigerator system for the production of solid or slurry ice.

Ice making systems have been known for some time and a typical example is described in US-A-3,537,274. Their basic function can be seen from Figure 1 , in which a tank 100 contains a water, brine or other suitable liquid, which can be pumped, by pump 200, to distributors 300a to c which create a falling film on to the sides of respective evaporator plate which together form an evaporator head a, b and c. The evaporator plates are connected to a refrigerant circuit or condensing pack R, so that films of ice are formed on them. The intention is that the resulting ice should then descend into the tank 100 so that the tank 100 has an ice slurry in it. Initially the ice or ice slurry may simply slip off the surface of the evaporator plates 400, but normally a de-frost cycle is required to harvest the ice and restore thermal efficiency. The ice that builds up on the plate can slide into the tank during defrost. A common method of effecting a defrost is to temporarily divert hot compressor exhaust gas to the evaporators, this defrost method is subject to important energy losses. A less common but more energy efficient method is the warm liquid defrost.

In US-A-3537274, as one plate is being refrigerated the other plate is connected to receive refrigerant in its hot liquid condition to defrost whilst the other plate is subjected to flow of low pressure expanded refrigerant, which captures heat from the plate and freezes the descending liquid. As the heat is being supplied to the plate in the defrost mode, defrost can be more efficient because only a thin film on the surface of the plate has to be melted for the ice

to fall off the plate. HoweverO, this arrangement can only operate with a pair of plates and is still subject to significant energy losses.

Flooded evaporator systems are also known in which excess unboiled refrigerant leaving the evaporators is separated from the vapour in a low pressure receiver and is recirculated to the evaporator head.

At least some embodiments of the invention provide more energy efficient approaches.

From one aspect the invention consists in an ice making system including a plurality of evaporators, a cooling circuit for circulating the refrigerant through the evaporators and a defrost circuit for defrosting at least one evaporator at a time and for circulating fluid through said evaporator to defrost that plate, characterised in that the system includes a reservoir for the defrost fluid and a heat exchanger in a cooling circuit for allowing the defrost fluid to cool the refrigerant prior to it passing through the evaporators.

Conveniently the evaporators may be referred to as an evaporator head. The evaporator head may consist of any practical arrangement of heat exchangers where the liquid to be chilled and frozen is brought into contact with the cold surfaces of the evaporator and where the refrigerant flows in such a way as to chill the evaporator. In the preferred embodiment used here to illustrate the principle of operation the evaporator head consists of three vertical plates with internal channels through which refrigerant flows and evaporates. The liquid to be frozen is distributed along the top of each plate and flows down each external side of each plate, falling into a collection tank, from where it can be recirculated or extracted for use. Other evaporator geometries may be used,

such as tubes, coils, spiral plates etc and the evaporator head may be submerged in the tank containing the fluid to be frozen such that ice released during defrost floats upwards.

If the system is to be used to produce pure solid ice then pure water alone is brought into contact with the evaporator surfaces. If the system is to be used to produce slurry ice then a binary fluid such as brine, water/glycol, water/ethanol etc is brought into contact with the evaporator surfaces.

In a particular embodiment the defrost circuit may include a valve system for connecting an evaporator to the reservoir to allow relatively hot fluid to pass from the reservoir to the evaporator and for the liquid refrigerant in the evaporator to flow to the reservoir.

The system may include a compressor and condenser connected in series upstream of the reservoir, in which case there may be a liquid receiver, which may be connected downstream of the condenser.

Preferably the refrigerant can pass through the reservoir when all evaporators are in a refrigerant mode and through the heat exchanger when at least one evaporator is in a defrost mode.

The system may be a flooded evaporator system, in which case a low- pressure receiver may be located between the reservoir and the evaporators. Additionally or alternatively the system may include a buffer downstream of the condenser.

In any of the above cases, the defrost circuit may create a thermo-syphon for defrost fluid between the reservoir and the evaporator to be defrosted. It is preferred in this and other embodiments that the refrigerant and the defrost fluid

are the same.

From a further aspect the invention consists in the method of operating an ice making system including evaporators and incorporating a reservoir for defrost fluids; the method including cooling the refrigerant by passing it through the defrost fluid.

This is conveniently achieved by means of a heat exchanger within the reservoir, but alternatively the refrigerant and defrost fluid may be in direct contact, in at least some stages of operation.

The liquid refrigerant within an evaporator may be returned to the reservoir during defrost, to increase energy efficiency.

From a still further aspect the invention includes a method of defrosting an evaporator including:

(a) storing heat from a warm liquid refrigerant in a fluid reservoir; and

(b) creating a thermosyphon through the evaporator to defrost the plate.

Liquid refrigerant released from the evaporator during defrost may be stored in the reservoir to cool the circulating refrigerant.

From another aspect the invention consists in a method of detecting the ice loading of an evaporator including: applying a mechanical excitation to the evaporator; and detecting the resultant vibration of the evaporator to obtain an indication of ice loading.

The invention may be performed in various ways and specific embodiments will now be described with reference to the accompanying

drawings: in which:

Figure 2a is a diagram of a first embodiment of an ice making system and Figure 2b is a modification thereof;

Figure 3a is a corresponding diagram for a flooded evaporator system and Figure 3b is a modification thereof; and

Figure 4a shows a suction line heat where one heat exchanger is added to the circuit of Figure 2a or Figure 3a and Figure 4b is an modification of Figure 4a.

Figure 2a shows how the three evaporator plates 7a are connected to a supply line 13, an exhaust line 14 and a defrost receiver 5, in such a way that any individual plate may be isolated from the condensing pack, which includes compressor 1 , condenser 2 and liquid receiver 3, during defrost by means of an arrangement of pipes and valves.

The high pressure liquid refrigerant is supplied to a heat exchanger 4 in receiver 5 by line 13 and then may pass through one or more of the expansion valves 6a, b, c, enters one or more plates 7a,b,c chilling said plates and then returns to the inlet of compressor 1 via line 14. When all three plates are in freezing mode then valves 10a,b,c are open and valves 8a,b,c and 9a,b,c are closed.

Let it be assumed that plate 7a requires defrost whilst plates 7b, c continue in freezing mode. In this case valves 6a and 10a are closed (conveniently sequentially), and valves 8a and 9a are opened, exposing the higher pressure warm liquid in defrost reservoir 5 to the lower pressure environment in plate 7a. The liquid in 5 boils and the vapour passes

thermosyphonically to the plate 7a, along line 11, where it condenses and releases heat, thereby defrosting plate 7a. When conditions in 7a reach an equilibrium determined by heat transfer to the internal evaporator surfaces, the chilled liquid refrigerant in 7a returns by gravity to reservoir 5 via line 12 where it recuperates and stores sufficient energy from the hot liquid refrigerant passing through heat exchanger or coil 4 to perform a subsequent defrost whilst simultaneously subcooling the liquid passing to the expansion valves via line 13.

Following defrost of 7a valves 8a and 9a are closed and valves 6a and 10a are opened and plate 7a returns to freezing mode.

The timing and also sequencing of valve operation can be optimised for maximum system efficiency and the plates may be defrosted from the top, the bottom, or from the top and bottom simultaneously. The types of valve which may be employed in the refrigerant flow circuit include check valves, solenoid valves, expansion valves, three-way valves and four-way valves. The duration and frequency of defrost are selected to minimise energy consumption or maximise capacity. The duration of the defrost should be minimised and the frequency optimised. The duration of the defrost is in part determined by storing the appropriate quantity of energy in the defrost receiver 5 prior to transfer of this energy to an individual plate. Thus the thermal capacity of the defrost receiver needs to be matched to the energy requirements for rapid defrost and the release of ice from the evaporator surfaces needs to be detected so that the evaporator can be returned to freezing mode without further delay.

A variety of means for the detection of ice build up on the evaporator surfaces and for ice release from the evaporator surfaces may be employed.

Mechanical excitation of the evaporator and monitoring of the harmonic response may be used to obtain an indication of ice loading and ice release using changes in frequency, amplitude or phase of the vibrations. Attenuation or interaction of electromagnetic waves, such as radio waves, by the ice film may also be used to detect ice accumulation and release. Other means based on changes in optical properties of the evaporator surfaces or ice conductivity effects may also be employed.

The frequency of defrost depends mainly on the rate at which ice builds up on the evaporator surfaces and the consequent fall in thermal efficiency of the overall heat exchange process in the evaporator head. Loss of efficiency caused by ice film growth will vary with evaporator head design so that defrost frequency is specific to each variant of the present invention, and optimal frequency must be determined by experimentation.

Figure 2b shows a modification of the refrigeration circuit which may be used in cases where lubricating oil return is problematic. As has been described in connection with Figure 2a, when a plate enters defrost mode, valves 15 and 17 are closed and valve 16 is open so that the warm liquid refrigerant from receiver 3 passes directly through coil 4 where it is cooled whilst warming the contents of reservoir 5. At other times, when defrost is not active, the warm liquid refrigerant passing through line 13 is diverted by valves in order to flush any accumulated oil from reservoir 5. This is achieved by closing valve 16, and opening valves 15 and 17. This arrangement also allows faster energy recuperation in the defrost reservoir.

In a second embodiment the operation of the system employing a flooded

evaporator may be explained using Figures 3a, b.

In Figure 3a a low pressure receiver 18 links the condensing pack to the evaporator head.

On the condensing side, vapour leaving receiver 18 passes via line 20 to the compressor 1 and warm liquid refrigerant leaving the condenser 2 enters liquid receiver 3 from where it flows through line 13 to coil 4 located in defrost receiver 5. The cooled liquid then passes along line 21 via expansion valve 19 to receiver 18. Expansion valve 19 is controlled to maintain the liquid level in receiver 18.

On the evaporating side, when the plates are in freezing mode, valves 6a, b, c and 10a,b,c are open valves 8a,b,c and 9a,b,c are closed so that liquid refrigerant from receiver 18 flows freely by gravity, or is pumped, via line 22, to the base of plates 7a,b,c and floods the plates and boils. The two-phase (liquid/vapour) refrigerant mixture leaving the plates via line 14 enters the low pressure receiver 18.

When a plate requires defrosting, for example plate 7a, valves 6a and 10a are closed and valves 8a and 9a are opened, allowing a thermosyphon flow of refrigerant from defrost reservoir 5 into plate 7a, thus defrosting 7a as previously described.

Figure 3b shows an alternative arrangement for connecting the defrost reservoir into the refrigeration circuit when oil flushing is required and the operation is the same as that already described in embodiment 1.

Figure 4 shows a third embodiment of the present invention, in which a suction line heater exchanger 23 is included in the refrigeration circuit of either

Figure 2 or Figure 3. This is a standard unit which allows subcooling of the hot liquid in line 13 by indirect contact in coil 24 with the cold suction gas leaving the evaporator head along line 14 (Figure 2) or subcooling of the hot liquid in line 21 by cold suction gas in line 20 (Figure 3). This unit, sometimes referred to as an accumulator, also serves to trap any liquid in the suction line before the suction gas enters the compressor 1.

It will be appreciated the reduced disturbance of a defrost on the refrigeration cycle (a defrosting plate is isolated from the rest of the circuit, and reintroduced with a load of refrigerant and a pressure close to the working conditions), avoids fast variations of the working conditions and facilitates the superheat regulation of the evaporators.

The evaporator plates are designed to prevent oil accumulation during normal functioning.

The liquid receiver 3 is principally provided to buffer the refrigerant from a plate during defrost. It is conveniently placed in the condensing pack but may be elsewhere in the circuit or its function may be combined with another receiver in the circuit.

Thus the preferred characteristics of the recuperative icemaker are:

The energy used for the defrost is taken from the sub cooling of the warm liquid refrigerant leaving the condenser. The energy used for the defrost is continuously collected and stored in the form of the temperature of a volume of liquid refrigerant (defrost receiver) placed at a lower level than the plates. Before a defrost the frosted plate and the defrost receiver are

isolated from the rest of the circuit. The defrost is performed by opening the high pressure warm defrost receiver to the lower pressure cold frosted plate. Pressure differences quickly drive the warm refrigerant into the cold plate, gravity brings the cold liquid refrigerant back into the defrost receiver.

A heat exchange coil placed inside the defrost receiver, exchanges heat between the warm liquid leaving the condenser and the refrigerant stored in the defrost receiver. During a defrost, a thermosyphon establishes a heat flux, through this coil, between the cold defrosting plate and the warm liquid refrigerant leaving the condenser.