GB2268975A

GB2268975A - Fan assembly for refrigerated display case

Info

Publication number: GB2268975A
Application number: GB9214926A
Authority: GB
Inventors: Mark Anthony John Webb
Original assignee: NORTHAMPTON REFRIGERATION COMP
Current assignee: NORTHAMPTON REFRIGERATION COMP
Priority date: 1992-07-14
Filing date: 1992-07-14
Publication date: 1994-01-26
Anticipated expiration: 2012-07-14
Also published as: GB9214926D0; GB2268975B

Abstract

The arrangement includes a fan 102 fitted in a plenum 106 for building up an air pressure therein. The plenum 106 is connected to an air discharge duct 116 via a shielding member 124 which shields the discharge duct 116 from the direct outlet of the fan so that the airflow into the discharge duct is formed mainly by the static air pressure in the plenum, so as to form a uniform airflow throughout the length of the discharge duct. A control grille (146 Fig.5) can be fitted to the entrance of the discharge duct to control and improve the shielding effects of the shield member 124. <IMAGE>

Description

FAN ASSEMBLY FOR REFRIGERATED DISPLAY CASE This invention relates to a fan assembly for circulating air in a refrigerated display case.

Refrigerated display cases, for example of the type which have a base compartment and a number of shelves above the base which are open to the customer, are commonly used in shops and supermarkets.

The applicants have a co-pending UK patent application No 9201326.7 entitled REFRIGERATED CABINET filed on 22 January 1992 which describes such a case. The opening of the case is shielded by an air curtain which is formed by continuously blowing air from one edge of the opening to an opposite edge. Such an air curtain is formed by different bands of air flow at different temperatures so that the outer layer of the air curtain may be at an ambient temperature while the inner layers may be made colder and colder toward the interior of the cabinet.

The different temperatures of these layers are maintained by circulating the air of different layers via different circulating paths. In this way, the temperature inside the cabinet can be kept stable and well below 0 C.

In an example where three separate circulating air bands are used, the primary air band is the coldest possibly around -20 C) and follows a path closest to the goods on display. The secondary-air band is nonrefrigerated and is directed adjacent -the rirtary air band, and the tertiary air band (called the ambient feed) is typically at around +20"C and follows a path adjacent the secondary air band.

The three air bands also travel at different velocities, with the primary being the fastest at typically 300 metres per minute. The secondary air band is typically at 250 metres per minute and the tertiary is the slowest at typically 200 metres per minute. This difference in velocities improves the cooling effect. These velocities are given only as examples - they could alternatively be around 200/180/150 metres per minute - it is the differential which is important.

It is known to circulate the air in each band using a common "propeller"-type fan, of the kind where a rotating set of angled blades draws air in from behind the fan and propels it out of the front of the fan. One fan is used for each band. The discharge of this type of fan can be considered to be an expanding cone with the air in the cone rotating in the direction of rotation of the fan blades. It is convenient to think of this discharge as being shaped like a torch beam.

The design of refrigerated cases is usually such that either one fan or one bank of fans is required to provide each band of air over a length of the face of the case of typically between one and two metres, i.e. the band of air is fairly narrow and fairly long. If there are three bands of air it is desirable that these three bands do not mix as they pass down the face of the case. If such mixing occurs some of the cooling effect is lost and moisture tends to be transferred from the tertiary ambient band through to the primary band and then deposited onto the goods in the case and/or onto the cooling coil of the case - an undesirable result.

The discharge of an propeller fan is such that when it is directed at a long narrow slot such as is used to provide the desired size of air band described above, the velocity of the air tends to be non-uniform.throughout the band. In turn this tends to encourage the mixing of the air bands and the resulting undesirable effects.

Discussion of the reasons why an propeller fan produces an air band with non-uniform velocity throughout can be found later in the discussion of the figures.

Propeller fans also have the inherent disadvantage that, since air must pass through them in what is effectively a straight line, they take up rather a large amount of space in the refrigerated case. Finally propeller fans may be light and cheap, but they are also relatively inefficient - typical energy conversion efficiencies are between 30 and 40%. It is clear that the greater the amount of energy that must be supplied to circulate the air the hotter the air becomes, and thus the more energy that must be used to subsequently cool the air to the required temperature.

The present invention provides a fan assembly for circulating air in a refrigerated display case which is adapted to fit in a pressure plenum of such a case (and which may in fact be a conversion kit for an existing case), including a fan which is capable of building up the air pressure inside the plenum and shielding means mountable in the plenum, such that the discharge duct through which the air may exit the plenum is shielded from the direct outlet of the fan.

In a second aspect, the invention provides a refrigerated display case in which air is circulated, including a fan which is capable of building up inside a plenum pressure to circulate the air, and shielding means mounted in the plenum, such that the discharge duct through which the air may exit the plenum is shielded from the direct outlet of the fan. The refrigerated display case may include a plurality of pressure plenum and shielding means and thus provide a plurality of air circulation bands.

Thus the ar pressure inside the plenum is increased and the air is circulated in the case primarily under the action of the built-up pressure, rather than being impelled directly at the discharge duct opening by the fan as before. Such a fan is preferably a backwards curved centrifugal fan, in which the air enters the fan axially and exits the fan radially i.e. the air undergoes an approximately 90" direction change whilst passing through the fan.

The primary consequence of this is that since the air is forced through the discharge duct under what is mainly the action of the pressure in the plenum rather than being impelled directly at the duct opening, the uniformity of the air velocity along the length of the air band is improved. This then means that the different air bands used to cool the case have less tendency to mix, thus improving the cooling capability of the case and reducing the likelihood of moisture being deposited from the ambient atmosphere onto the goods on display in the case, or onto the cooling coil of the case.

A propeller fan has a limited capacity to generate static pressure in such a pressure plenum ie. if an obstruction is placed in front of a propeller fan it has a tendency to stall and cannot build up any further pressure, and thus it must be used to propel the air directly at the duct opening. On the other hand, a backwards curved centrifugal fan is suited to building up pressure and is also more efficient (typical efficiencies of between 50 and 70t); this means that a smaller fan can be used and that less heat is introduced into the system through inefficiency, which improves the performance of the system as explained above.

Preferably the entrance to the discharge duct is covered by a control grill which contains a large number of holes or slits along its length. The volume of the air exiting the plenum and entering the discharge duct can thus be controlled by varying the size and number of slots/holes in the grill. Thus the characteristics of the air band(s) can be controlled.

Since a backwards curved centrifugal fan introduces an approxinlately 90" change in the direction of air passing through it, it also has the added advantage that it usually requires less space for its operation in the case than a typical propeller fan - this can be seen more clearly from the following figures.

This invention also relates to a method of providing circulating air in a refrigerated display case including the steps of building up the air pressure inside a pressure plenum of a case using a fan capable of building up such pressure, and passing the air through a discharge duct in the plenum, the discharge duct being shielded from the direct outlet of the fan by shielding means.

An embodiment of the present invention will now be described with reference to the following drawings in which: - Fig. 1 shows a side elevation view of an embodiment of the prior art described in GB 9201326.7.

Fig. 2 shows a section taken sideways through a refrigerated display case embodying the present invention; Fig. 3 shows an enlarged segment of Fig. 1; Fig. 4 shows two graphs of discharge velocity/ length of a discharge outlet; Fig. 4(a) shows the graph for a case according to the prior art, and Fig. 4(b) shows the graph for a case according to an embodiment of the present invention; Fig. 5 shows a section through a fan and shielding member according to an embodiment of the present invention; Fig. 6 shows a face view in the direction of arrow A in Fig. 5; Fig. 7 shows a face view in the direction of arrow B in Fig. 5.

Fig. 1 shows an embodiment of a refrigerated cabinet as described in GB 9201326.7. The cabinet 10 has an upright configuration with an opening 20. Anything kept inside the cabinet 10 is readily visible and accessible via this opening 20. The temperature inside the cabinet is kept well below 0 C by an air curtain 30 which is formed by three bands 31, 32 and 33 of continuous air flow. The three air flow bands represented by arrows 32, 32 and 33 follow different routes or paths so that they are kept at different temperatures.

The first air band 31 is of ambient air driven by a fan 51 via a path 41, and it flows to the lower edge 21 of the opening 20 outside the cabinet. The second band 32 is driven by a fan 52 to circulate in a path 42 within the cabinet and, therefore, its temperature is reduced when the air goes through the path 42 by its contact with outer walls of the inner part of the cabinet. The air passing through the fan 52 enters the path 42 at an entrance 45, and it is here that the non-uniform velocity effects through the length of the air band may occur. The innermost air band 33 is circulated via a path 43. The air flow of this band enters the body of the cabinet at the lower edge 21 of the opening 20. Along this path 43 from the entrance position at the edge 21, there are arranged a cooler 60, a fan 53, a first refrigerating evaporator 70 and a second refrigerating evaporator 80.

The air from the band 33 is cooled by the cooler 60. At this point, most of the moisture picked up by the air flow will become frost accumulated on the cooler 60.

The dried and cooled air is driven by the fan 53 to the first evaporator 70 and then the second evaporator 80. The temperature of the air is further reduced by the heat exchange between the air and fins fixed to the evaporators 70 and 80.

After the air has passed through the evaporator 80, its temperature is fully reduced and then it is blown out from the top edge of the opening 20 to form the innermost band 33 of the air curtain 30. During the operating of the cabinet, the cooler 60 is regularly defrosted so as to avoid any blocking of the system and to keep the system working efficiently. However, there will still be some moisture in the air stream after the cooler, and this will tend to deposit as frost on the leading edge of the first evaporator 70.

In Fig. 2 a backwards curved centrifugal fan 102 is mounted on a mounting member 104 in a plenum 106 of a refrigerated display case 108. There are three separate air flow paths i which air may circulate in the case. The primary air band passes through a cooling coil (not shown) where it is cooled to around -20 C, along a double-skin insulated duct 110 and through an outlet 112. It then flows down across the open face of the cabinet and is recirculated through the base of the case. Similarly the second air band flows through a duct-114 through a fan 102, along a second double-skin duct 116, out through a second outlet 118 along the open face of the case adjacent the primary air band and is recirculated through the base of the case.The third air band, which is at an ambient temperature of around 20oC enters a second backwards curved centrifugal fan 120 from above the case and passes through a channel 122, out of the outlet 118, and across the face of the case adjacent the secondary air band.

The air bands flow at different velocities, with the primary air band being the fastest at typically around 300 metres per minute, the secondary air band at around 250 metres per minute and the tertiary air band at around 200 metres per minute. These velocities may be lower, ie.

200/180/150 metres per minute. This difference in velocities improves the cooling effect. The case as a whole is around 3 metres long (i.e. that is the length of the face) and this is divided into three 1 metre long sections, the air curtain in each being supplied by separate fans. Thus the duct 116 is typically around 1 metre long and 2.5 centimetres high and the air bands are of corresponding dimensions.

Fig. 3 shows more clearly the passage of the air of the secondary air band through the fan 102 and along the duct 116. The fan is mounted on a mounting member 104 in the plenum 106 and the entrance 122 to the discharge duct 116 is shielded from the direct outlet plane (shown as Z-Z in Fig. 3) of the fan 102 by a shielding member 124. The shielding member may be attached to, or even be an integral part of, the mounting member.

Thus the air is not directed at the entrance of the discharge duct, but can be considered to be forced through the entrance of the discharge duct by the build-up of pressure inside the plenum 106 due to the fan 102. It is the "shading" effect achieved by the shielding member 124 and general positioning of the fan 102 which achieves this effect. The shaded area is shown with a dashed line marked 126 in Fig. 3.

There will be a "positive" section of the plenum 105 in which the pressure increases as desired, and a "negative" section 107 where the air is drawn into the fan and the pressure is decreased.

As can be seen from Fig. 2 a similar fan 120 and associated "shaded" area 128 are used to circulate the tertiary air band. A similar arrangement could again be used for the primary air band, but the primary air band has already passed through a cooling coil which has an equivalent effect to a backwards curved centrifugal fan in producing a more uniform velocity across the length of the air band.

Since the air is forced along the duct 118 substantially by the action of the build-up of pressure inside the plenum 106, the air travels at a more uniform velocity across the whole length of the duct (see Fig. 4 for a graph describing this). This in turn means that the secondary air band passing down the front of the case is of a more uniform and stable nature. Similarly the primary and tertiary at bands are also more uniform.

Thus the air bands are less likely to mix and eddy currents between the bands are less likely to form.

As explained earlier this means that the cooling effect is improved and the likelihood of moisture being transferred from the ambient through the air bands onto the goods on display in the case or onto the cooling coil is decreased.

The line X-X in Fig. 2, drawn diagonally across the plenum 106 from upper left to lower right, shows the plane on which a standard propeller fan would normally be mounted in a refrigerated case (eg. as in Fig. 1).

Mounting the propeller fan at such an angle clearly increases the space required for the effective operation of the fan, since the air supplied to the propeller fan must come from behind it, ie. at the upper right of the plenum.

Thus the plenum must be enlarged to allow the air to pass up from duct 114, round behind the fan, through the fan and be directed at the entrance to the discharge duct 122. As can be seen, the 90" "characteristic" of the backwards curved centrifugal fan 102 (and the fan 120) enables the design of the plenum to be more compact. The backwards curved centrifugal fans 102,120 are also more efficient than standard propeller fans, and thus less energy is needed to achieve the same effect.

Fig. 4(a) shows a graph of discharge velocity/length of discharge outlet for the previous art with a propeller fan in an inclined plane with the direction of discharge being towards the entrance to the discharge duct. Air discharges from the fan in a "torch beam" pattern causing unequal distribution of air entering into the duct, with a "sun ray" air flow direction within the duct towards the duct outlet.

This arrangement relies on fan discharge velocity (rather than static pressure) to promote air flow through the duct and, thus any obstructions such as structural support members in the airstream cause disruption to the airflow with resultant velocity peaks and troughs evident at the final discharge point. (This is analogous to a shadow in a torch beam).

Fig. 4(b) shows a graph of discharge velocityjlength of discharge outlet for a backwards curved centrifugal fan used thin the plenum 102. A restriction is provided by way of a control grille at the entrance to the discharge duct to provide a pressurization of plenum and distribution of air into the duct by the apertures of the control grille under the effect of static pressure to give a uniform and parallel airflow within the plenum.

However, due to the rotational effect of the fan ;speller, a recirculation of air within the plenum occurs in a "raceway" pattern within the plenum walls. This rotational velocity gives the air velocity pressure in its flow direction over and above the plenum static pressure.

When the air is rotating in such a fashion around the plenum, irregular velocity effects would occur at the ends of the entrance to the discharge duct without the use of the shielding member. At the right most end 134 the air is flowing towards the opening of the discharge duct (shown by arrow 136). The air would therefore enter the discharge duct at a greater velocity than the air entering most of the rest of the length of the duct, where its velocity is determined merely by the overall static pressure ie. the forcing of the air into the duct. Similarly at the opposite end of the plenum the air is flowing away from the entrance to the discharge duct and the air flowing into the duct at that end would do so at a slightly reduced velocity.This can be seen in the accompanying graph of Fig. (b). With the inclusion of the "shielding means" to the arrangement, the major component of velocity pressure within the recirculating "raceway" air is obscured from the outlet path and control grille distributor is more reliant on the desired static pressure of the plenum only. Thus, the air enters the discharge duct at a more uniform velocity along its whole length.

Fig. 5 shows a backwards curved centrifugal fan 102,120 mounted on a mounting member 104, with a foamed rubber sealing strip 140 being between the fan and the mounting member. A velocity reducing baffle 142 is mounted on the face of the fan opposite the mounting member 104, and is retained in place by baffle-retaining screws 144 connecting the baffle to the mounting member.

A shielding member 124 is attached to one end of the mounting member 104 so as to form a "shaded" area, in order to shade the entrance to the discharge duct. The entrance to the discharge duct may be covered by a control grill 146 (seen more clearly in Fig. 6). The control grill could either be separate from the mounting member or attached to it. This control grill contains a number of holes or slots through which the air passes from the plenum into the discharge duct. By varying the number and size of these holes/slots the velocity of the air and the characteristics of the air band(s) may be controlled. The velocity of the air entering the discharge duct could also be varied by altering the speed of the fan, but it is more convenient to alter the characteristics of the control grill. The size and distribution of the holes may vary along the length of the control grill in order to alter, or compensate for, any undesired velocity effects.

Finally Fig. 7 shows a view of the fan assembly from above, i.e. the air comes up into the fan perpendicularly to the plane of the figure and is expelled from the fan through the sides, ie. parallel to the plane of the figure. This shows clearly the 90" "characteristic" of a backwards curved centrifugal fan.

Claims

CLAIMS:

1. A fan assembly for circulating air in a refrigerated display case which is adapted to fit in a pressure plenum of such a case, including a fan which is capable of building up the air pressure inside the plenum and shielding means mountable in the plenum, such that the discharge duct through which the air may exit the plenum is shielded from the direct outlet of the fan.

2. A refrigerated display case in which air is circulated, including a fan which is capable of building up inside a plenum pressure to circulate the air and shielding means mounted in the plenum, such that the discharge duct through which the air may exit the plenum is shielded from the direct outlet of the fan.

3. A fan assembly or refrigerated display case according to either claim 1 or claim 2 in which the fan is a backwards curved centrifugal fan.

4. A fan assembly or refrigerated display case according to any one of the above claims including a control grill with which the entrance to the discharge duct may be covered, the control grill containing a number of holes or slits along its length.

5. A fan assembly or refrigerated display case according to any of the above claims in which the shielding means is attached to the fan.

6. A method of providing a circulating air band in a refrigerated display case including the steps of building up the air pressure inside a pressure plenum of a case using a fan capable of building up such pressure, and passing the air through a discharge duct in the plenum, the discharge duct being shielded from the direct outlet of the fan by shielding means.

7. A fan assembly or refrigerated display case substantially as herewith described with reference to the accompanying drawings.

8. A method according to claim 7 in which the circulating air provides part of a curtain of air across part of an open face of the display case.

8. A method according to claim 6 in which the circulating air provides part of a curtain of air across part of an open face of the display case.

CLAIMS: 1. A fan assembly for circulating air in a refrigerated display case having an air discharge duct along its length, which fan assembly is adapted to fit in a pressure plenum of such a case, including a fan which is capable of building up the air pressure inside the plenum by a closed air circulation therein and shielding means for fitting between the plenum and the discharge duct, wherein said shielding means are arranged to shield the air exiting the plenum from the direction of the air circulation caused by the fan, so as to form a uniform airflow from the duct throughout its length.

2. A refrigerated display case in which air is circulated, including a fan which is capable of building up inside a plenum an air pressure by circulating the air therein and shielding means fit between in the plenum and an air discharge duct along its length, such that the discharge duct through which the air may exit the plenum is shielded from the direction of the air circulation caused by the fan, so as to form a uniform airflow from the duct throughout its length.

4. A fan assembly or refrigerated display case according to any one of the above claims including a control grille with which the entrance to the discharge duct may be covered, the control grille containing a number of holes or slits along its length.

6. A fan assembly or refrigerated display case substantially as herewith described with reference to the accompanying drawings.

7. A method of providing a circulating air band in a refrigerated display case including the steps of building up the air pressure inside a pressure plenum of a case by using a fan to form an air circulation within said plenum, forming an airflow through a discharge duct connected to the plenum, and shielding said discharge duct from the direction of said air circulation caused by the fan in the plenum so that the airflow is formed by the static pressure in the plenum.