GB2163245A - Improvements in or relating to fluid flow controls for refrigerators and heat pumps - Google Patents

Abstract

A self-regulating capillary flow control for the fluid circuit of a refrigerator or heat pump, incorporating a reservoir 7 on the high pressure side of the circuit between the condenser 2 and the capillary 8. Fluid can reach the capillary 8 from the reservoir 7 by two routes 15, 18, a cooled route 15 leaving the reservoir 7 by an overflow outlet 10 at relatively high level and a warmer route 11 leaving it at lower level. Slow flow through the capillary 8, indicating warm fluid, causes the reservoir 7 level to rise. When it overflows, cooler fluid reaches the capillary 8 so that the flow rate within it rises. This draws further fluid from the reservoir 7 by the warmer route 11, so that the reservoir 7 level falls, the overflow stops, and flow within the capillary 8 falls. The liquid level then stabilises. Several alternative designs of reservoir are described, and also both unvalved designs of the warmer route and a design fitted with a pressure-dependent valve. <IMAGE>

Description

SPECIFICATION Improvements in or relating to fluid flow controls for refrigerators and heat pumps This invention relates to fluid flow controls for refrigerators and heat pumps, the apparatus for which essentially comprises in sequence a compressor, a condenser, a fluid flow control and an evaporator. While the invention applies also to apparatus using the absorption cycle, and to heat pumps in which useful energy is extracted from the apparatus at the condenser, it will be described principally with reference to refrigerators using the vapour compression cycle and in which energy is usefully extracted by the evaporator.

At present the most widely used flow controls in refrigerators and heat pumps are the thermostatic expansion valve and the capillary. The purpose of the flow control is to ensure that flow takes place at such a rate that the fluid medium is always gaseous when it enters the compressor, and substantially liquid as it enters the evaporator, and to maintain a pressure difference between the condenser and the evaporator. The action of a thermostatic expansion valve is to ensure that no liquid enters the compressor by maintaining constant superheat in th vapour as it leaves the evaporator. This however can only be achieved at the expense of reduced evaporator efficiency. The simple capillary flow control is favoured for its simplicity, low cost and reliability, and for its effect of promoting pressure equalisation throughout the cycle when the compressor is shut down.However, any variation from design conditions in capillarycontrolled apparatus quickly results in a loss of efficiency. If the supply of liquid medium is inadequate, vapour enters the capillary: on the other hand if the supply of liquid is too great, liquid accumulates upstream of the control, filling not only the conduit between the control and the condenser outlet but also the later stages of the working space of the condenser itself. The efficiency of the condenser is thus impaired, and the evaporator is starved.

The present invention seeks to provide a practical and self-regulating modification of the known capillary flow control, whereby a known head of liquid medium is maintained between the capillary and the condenser, that is to say on the high pressure side of the capillary. According to the invention a flow control, for location between the condenser and the evaporator in the fluid circuit of a refrigerator or a heat pump, comprises a resistance of capillary type, having a manifold inlet and having an outlet for connection to the evaporator; a reservoir to receive liquid from the outlet of the condenser, and having first and second outlets; a first connection, subject to substantial cooling action and operable, when the liquid level in the reservoir attains a predetermined level, to conduct liquid from within the reservoir to the manifold by way of the first outlet and a second and parallel connection subject to less cooling action and operable, at any liquid level within the reservoir, to conduct liquid from within the reservoir to the manifold by way of the second outlet. The first and second connections and the reservoir thus together resemble a manometer.

The invention is further defined by the claims and will now be described, by way of example, with reference to the accompanying diagrammatic/schematic elevations in which: Figure 1 shows a refrigerator circuit incorporating a flow control; Figure 2 shows a refrigerator circuit incorporating another flow control; Figure 3 shows an alternative form of reservoir for the refrigerator circuits of Figs. 1 and 2; Figure 4 shows part of a refrigerator circuit including yet another flow control, and Figure 5 shows an alternative layout of pipework for the part of the flow control shown in Fig. 4.

The vapour-compression refrigerator of Fig.

1 comprises the customary basic units of compressor 1, condenser 2, evaporator 3 and liquid accumulator 4 linked by conduit 5. The flow control to which this invention relates lies within the schematic broken-line outline 6 and comprises, in flow succession, a reservoir 7 to receive the refrigerant in liquid form which drains into it from condenser 2, and a capillary 8 the outlet 9 of which is connected to the inlet of the evaporator 3. The reservoir 7 has a first outlet 10 and a second outlet 11 at a lower level, and capillary 8 has an inlet 1 3 which will be referred to as an inlet manifold because refrigerant can enter it by more than one route. Liquid passes from reservoir 7 to manifold 1 3 by either of two parallel streams, one of which is cooled and the other is not.

Provided the liquid within reservoir 7 reaches the level of outlet 10, it tends to overflow by way of that outlet into one limb 14 of a Utube 15, the other limb 1 6 of which is cooled by a jacket 1 7 through which the output of capillary 8 passes on its way to evaporator 3.

Limb 1 6 terminates at manifold 1 3. The other stream of liquid passes from reservoir 7 to manifold 1 3 by way of outlet 11 and an uncooled conduit 1 8 including a ball-type pressure-operated valve 1 9.

Having recently left condenser 2, the liquid 20 within reservoir 7 will be relatively hot.

The cooling (by jacket 17) of the stream of that liquid which enters and passes through the U-tube 1 5 induces a fast flow through capillary 8. The liquid level in the left-hand limb of U-tube 1 5 therefore tends to fall. The pressure difference AP across the valve 1 9 approximately equals hpg, where h as illus trated is the difference in height between the liquid levels in limb 14 and in the reservoir 7, and p is the density of the hot liquid. When AP is sufficient to lift the ball of valve 1 9 off its seat, hot liquid refrigerant reaches manifold 1 3 by way of the uncooled stream through outlet 11 and conduit 18 and so the flow rate through capillary 8 tends to fall again.Equilibrium is reached when the flow rate through capillary 8 equals that from condenser 2.

When refrigerant is cooled, its density increases and this gives rise to a small additional pressure effect. This is small compared with AP and is ignored here.

The liquid head 1 2 within reservoir 7 equals or slightly exceeds the difference in level between outlets 10 and 11 and determines the proportion of the output of condenser 2 which overflows into the U-tube 1 5 and is cooled, and hence the degree of sub-cooling of liquid which enters capillary 8 and so the overall flow rate.

Liquid which passes through the U-tube 1 5 may be cooled by means other than that shown: for instance the cooling jacket 1 7 instead of receiving the system fluid as it passes from the capillary 8 to the evaporator 3, could be located instead between the evaporator 3 and the inlet to compressor 1.

Alternatively, liquid within the U-tube 1 5 could lose heat to ambient air, to the supporting framework or to another medium such as water, such loss constituting either the sole means of cooling or being in addition to heat exchange with the system fluid as already described.

Within the uncooled conduit 18 a simple orifice, or length of small bore, could serve as a pressure-dependent flow regulator in place of a pressure-operated valve such as the ballvalve 1 9. The resistance of such an orifice or length should be chosen so that 100% flow through outlet 11 corresponds to a convenient value of h.

In the alternative design of Fig. 2, references common to Fig. 1 indicate parts of comparable function. In Fig. 2 the coil of capillary tube 8 is cut away to reveal the Utube 15, both legs 23 and 24 of which pass through the coil.

The difference in height h1 between liquid levels serves the same purpose as quantity h in Fig. 1, but here it is established by reason of the unequal liquid densities in the two limbs of the U-tube 1 5. For instance when R 1 2 refrigerant is cooled from 40"C to - 20"C its density increases by one sixth. For this reason, and also because the design of Fig. 2 includes no component like the ball valve 1 9 of Fig. 1 requiring physical movement, h, is typically far smaller than h. The difference in height h1 is approximately equal to the product of the fractional increase in density and the difference in the level between the mid-point of the cooling jacket 1 7 and the manifold 1 3.

In the refrigerator of Fig. 2, the density effect assists the flow of refrigerant through a cooled first connection and no pressure-dependent flow regulator is required in an uncooled second connection. The action of the fluid flow control is now as follows: when the compressor 1 starts, hot liquid from the condenser 2 drains into the reservoir 7 and thence via outlet 11 into leg 24 of U-tube 1 5.

From leg 24 it enters the capillary 8 and a slow flow through the capillary is established.

The level of liquid 20 within the reservoir 7 rises until liquid overflows through outlet 10.

Liquid then passes down leg 23 of U-tube 1 5 and in so doing is cooled by jacket 17 which surrounds that leg. Cold liquid therefore reaches the manifold 1 3 and enters capillary 8, an increased flow through the capillary is established, and the liquid level in reservoir 7 is stablised. When the compressor 1 is shut down, liquid levels fall and finally vapour passes through the capillary and pressures in the condenser and evaporator equalise. A cylindrical insert as shown at 21, supported with clearance within leg 23 by perforated spacers 22, may increase the effectiveness of the heat exchanger and reduces the quantity of liquid in the flow control, thereby hastening pressure equalisation when the compressor is shut down.

Fig. 3 shows an alternative design of reservoir 7 for an embodiment of the invention otherwise as shown in Fig. 1 or Fig. 2.

Outlets 10 and 11 are now both located in the base of the reservoir, the interior of which is spanned by a weir 25 which divides the interior into two parts ?6 and 27 of substantially equal size. Outlet 10 is in part 26, outlet 11 is in part 27 and h2 is the head of liquid.

Fig. 4 shows features corresponding to those that appear towards the upper left-hand side of the upper end of outline 6 in Fig. 1, and at the upper end of the same outline in Fig. 2 also. In the alternative and potentially more economical design shown in Fig. 4, references common to Fig. 2 indicate parts of comparable function. The reservoir 7 in this case is defined as the pipework situated within the inner schematic broken-line outline 28. The end of leg 23 of the U-tube 1 5 is partially flattened where it joins leg 24, and protrudes into the leg 24 as shown. An alter native form of pipework is shown in Fig. 5, and in that case the partial flattening is optional and there is no protrusion of the free end. In both cases, the slope of the upper part of leg 23 is arranged so that liquid only passes through the first connection 23, via the first outlet 10, when the liquid level in the reservoir rises high enough to flow over the "hump" 29. The flow controls of Figs. 4 and 5 may be formed from a single piece of pipe, jacketed over part of its length, and this reduces both the productin cost and the chance of gas leakage.

In any version of the fluid flow control, an optional element such as a strainer or filter/drier may be placed between the manifold 1 3 and capillary 8 to clean the refrigerant.

Claims (20)

1. A flow control, for location between the condenser and the evaporator in the fluid circuit of a refrigerator or heat pump, and comprising: a resistance of capillary type, having a manifold inlet and having an outlet for connection to the evaporator: a reservoir to receive liquid from the outlet of the condenser, and having first and second outlets: a first connection, subject to substantial cooling action and operable, when the liquid level in the reservoir attains a predetermined level, to conduct liquid from within the reservoir to the manifold by way of the first outlet, and A second and parallel connection subject to less cooling action and operable, at any liquid level within the reservoir, to conduct liquid from within the reservoir to the manifold by way of the second outlet.

2. A flow control according to Claim 1, in which the second connection includes a pressure-dependent flow regulator.

3. A flow control according to Claim 2, in which the pressure-dependent flow regulator is a pressure-operated valve.

4. A flow control according to Claim 3, in which the pressure-operated valve is a ball valve.

5. A flow control according to Claim 2, in which the pressure-dependent flow regulator is an orifice.

6. A flow control according to Claim 2, in which the pressure-dependent flow regulator is a length of small bore.

7. A flow control according to Claim 1, in which the first connection is cooled by heat exchange with system fluid passing between the capillary and the evaporator.

8. A flow control according to Claim 1, in which the first connection is cooled by heat exchange with system fluid passing between the evaporator and the inlet to the compressor.

9. A flow control according to Claim 1, in which the cooling action upon the first connection is due to loss of heat to its surroundings.

10. A flow control according to Claim 1, in which cooling action upon the first connection is exerted by a medium external to the fluid circuit.

11. A flow control according to Claim 10, in which the external medium is water.

1 2. A flow control according to Claim 1, in which the second and parallel connection is subject to no substantial cooling action.

1 3. A flow control according to Claim 1, in which the first outlet is located higher than the second outlet.

14. A flow control according to Claim 1 in which the first and second outlets are separated by a weir which defines the predetermined level.

1 5. A flow control according to Claim 1 in which the first connection comprises a length of tubing of substantially uniform section and the reservoir a connected vessel of larger section.

1 6. A flow control according to Claim 1 comprising a length of tubing one free end of which is turned back through 360 to rejoin the body of the tubing and form a continuous loop. in which part of this loop constitutes the first connection and the second connection is included within the remainder of the loop, the reservoir coinciding with where the free end of the tubing rejoins the body of it.

1 7. A flow control according to Claim 1 6 in which the free end of the tubing protrudes through the wall of the body of the tubing at the point of rejoining.

1 8. Apparatus using the vapour compression heat cycle and including a flow control according to any of the preceding claims.

1 9. A flow control according to Claim 1, substantially as described with reference to the accompanying drawings.

20. Apparatus using the vapour compression heat cycle, according to Claim 1 8 and substantially as described with reference to the accompanying drawings.