US20080190591A1

US20080190591A1 - Low charge refrigerant flooded evaporator

Info

Publication number: US20080190591A1
Application number: US11/695,413
Authority: US
Inventors: Zahid H. Ayub
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-02-08
Filing date: 2007-04-02
Publication date: 2008-08-14

Abstract

A flooded evaporator has a plurality of tubes extending through a shell. Process fluid flows through the tubes and refrigerant flows inside of the shell through gaps between the tubes. Filler beads are located in the gaps between the tubes, thus displacing some of the refrigerant and requiring a lower refrigerant charge. The refrigerant flows through the spaces between the filler beads. The filler beads move thereby dispersing the refrigerant and dislodging bubbles from the outside of the tubes, resulting in an increase in efficiency of heat exchange.

Description

This application claims the benefit of U.S. provisional application Ser. No. 60/900,139, filed Feb. 8, 2007.

FIELD OF THE INVENTION

The present invention relates to shell and tube flooded evaporators for refrigeration applications.

BACKGROUND OF THE INVENTION

A shell and tube flooded evaporator is an integral part of a refrigeration system. In a typical refrigeration system there is an evaporator that cools the process fluid at the expense of boiling the refrigerant that is at a lower saturation temperature and pressure, a compressor that compresses the boiled off refrigerant to an elevated pressure and temperature, a condenser uses a cooling medium to condense the high pressure refrigerant to liquid phase at the expense of heating the cooling medium, and an expansion device that drops down the pressure of the condensed refrigerant back to the low side which then enters the evaporator to repeat the above cycle again. This cycle is called the reverse Rankine cycle.
Such refrigeration systems are found in a variety of installations, such as food processing plants.
A shell and tube flooded evaporator has a shell with tubes extending through the shell. The tubes carry the process fluid. The shell of the evaporator is flooded with the refrigerant. The liquid refrigerant typically enters the bottom of the shell, contacts the tubes, which tubes carry the hot process fluid. The refrigerant vaporizes and exits the shell at the top.
Refrigerants are typically natural, such as ammonia or propane. Synthetic refrigerants are falling out of favor due to environmental concerns. However, even natural refrigerants have drawbacks; ammonia is toxic and propane is flammable.
It is desirable to design an evaporator that has a higher efficiency than found in the prior art. A more efficient evaporator would use less refrigerant, thus minimizing any danger from an accidental refrigerant release. In addition, a more efficient evaporator would be physically smaller, taking up a smaller footprint on a factory or plant floor, thus saving money.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a frontal view of an even-pass (four pass) flooded shell and tube evaporator, partially cut away.

FIG. 2 is a side or end view of the flooded shell and tube evaporator of FIG. 1.

FIG. 3 is a frontal view of an odd-pass (three pass) flooded shell and tube evaporator, partially cut away.

FIG. 4 is a side or end view of the flooded shell and tube evaporator of FIG. 3.

FIG. 5 is a cross-sectional view of a tube bundle of the evaporator of FIG. 1 taken at section A-A.

FIG. 6 is a frontal view of a partially cut away flooded shell and tube evaporator of the present invention.

FIG. 7 is a detailed cross-sectional view of tubes and filler beads.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIGS. 1 and 2, a shell and tube evaporator is shown with a plurality of parallel tubes 6 in horizontal orientation. The tubes 6 are received at each end by two end plates 3 (round or rectangular in shape) called tube sheets. Each tube sheet 3 has a plurality of parallel holes that are machined at a specific distance with respect to each other according to industry standards, viz., Tubular Exchanger Manufacturers Association, TEMA. The tubes are further supported by baffles or tube supports 7 within the span between the tube sheets 3. The distance between the adjacent baffles or tube supports 7 is determined according to industry standards, e.g., Tubular Exchanger Manufacturers Association, TEMA. The baffles or tube supports 7 have a hole pattern identical to the tube sheets 3 as shown in FIG. 5 (larger scale). The combination of tube sheets 3, the tubes 6, the baffles or tube supports 7 and the tie-rods 9 is also known as tube bundle and is welded to the shell 4 at each end, 19 and 20, hence isolating the shell side 16 from the tube side 17. At the ends, the tube side 19 is confined by front and rear heads 1, 2. The tubes 6 are individually joined to the tube sheets 3 at the corresponding holes in the tube sheets 3 via mechanical means or welding.
The process fluid such as water or brine or any other fluid to be cooled enters the tube side 17 at the front head 1 (attached to tubes sheets 3 through bolting 5 or welding) via inlet port 10. Depending upon the nature of the application, the heads 1 and 2 could be arranged for multiple pass or single pass configuration. In the case of multiple pass, the front head 1 and the rear head 2 carry pass partition plates 14 at the corresponding lane 21 on the tube sheets 3 that directs the process fluid in the tubes 6 back and forth through a respective quantity of tubes in each pass until the fluid exits at head 1 via port 11 for even-pass configuration as shown in FIG. 1 and FIG. 2 or at head 2 for odd-pass configuration as shown in FIG. 3 and FIG. 4 via exit port 11. The process fluid entering at inlet port 10 is hot, while the process fluid exiting at outlet port 12 is cooled.
Low temperature and low pressure liquid or liquid-gas mixture of refrigerant enters the shell side 16 via port 12. As the refrigerant travels upwards it extracts heat from the hot fluid in the tubes 6 and progressively evaporates. The vapor/liquid ratio increases along the height of the tube bundle. The wet vapor exits the shell side 16 via risers 15 and enters the separator 8 and leaves the separator 8 as liquid-free vapor via port 13.
From the separator 8, the refrigerant vapor is routed to the compressor (not shown), where the refrigerant is compressed. From the compressor, the refrigerant, which is hot, is cooled in the condenser. After leaving the condenser, the pressure of the refrigerant is dropped by an expansion device, wherein the refrigerant reenters the shell 4 at port 12.
As shown in FIG. 5, the tubes 6 are spaced apart from each other, thus creating gaps 31 between the tubes. The refrigerant flows through these gaps 31. The tubes are grouped into the sections, with each section representing a pass through the shell. Thus, section I is the first pass of the process fluid through the shell, from the inlet port 10 and the front head 1 to the rear head 2. Section II is the second pass, from the rear head 2 back to the front head 1. Section III is the third pass, from the front head 1 to the rear head 2. Section IV is the fourth pass, from the rear head 2 to the front head 1 and the outlet port 11. The tubes within a section are separated from each other by a relatively small gap 31. The tubes in adjacent sections are separated from each other by a larger gap, or lane 21, in order to accommodate the pass partition plates 14. In a prior art evaporator, these gaps 31, 21, which represent the interior volume of the shell, are filled with refrigerant.
In the present invention, much of the interior volume of the shell is filled with filler beads 35 (see FIGS. 6 and 7). The filler beads have a neutral buoyancy when immersed in the refrigerant 33. This minimizes the possibility of the beads accumulating at the bottom of the shell (if negative buoyancy) or at the top (if positive buoyancy) as assures the even distribution of the filler beads throughout the shell. For example, the density of the filler beads 35 could be the same as the density of the refrigerant. Because the refrigerant changes from a liquid state to a vapor state, the density of the refrigerant changes. The filler beads can have a neutral buoyancy relative to the liquid refrigerant.
The filler beads in the preferred embodiment are made of solid plastic. The filler beads remain solid and do not turn to liquid inside of the shell. In the preferred embodiment, the filler beads 35 are spherical, although the beads could be of any shape. In the preferred embodiment, the filler beads are solid and not hollow. Solid beads are easier to manufacture and easier to match neutral buoyancy with the refrigerant. The filler beads 35 are of different sizes. In the preferred embodiment, there are at least three sizes 35A, 35B, 35C (see FIG. 7). The largest size bead 35A is small enough to pass through the gaps 31 between adjacent tubes 6 in a section. Thus, the diameter of the largest size bead is less than P-D, where P is the tube pitch and D is the tube outside diameter. As an example, one type of flooded evaporator has gaps between tubes of 3/16 inches. Thus, the largest size filler bead 35A is less than 3/16 inches. When several of the largest size beads 35A are located so as to contact one another, there are spaces between the beads. The intermediate size beads 35B and the smallest size beads 35C are sized relative to the largest size beads so as to fit within the spaces of the adjacent largest size beads 35A.
The filler beads 35 displace refrigerant inside of the shell 4. The filler beads are located in the gaps 31, 21 between the tubes. The filler beads have the same isothermic state as the refrigerant and consequently are thermally inert. The amount of filler beads inside the shell depends on how efficient the evaporator is to be. For example, filler beads can displace 10% of the volume inside of the shell, thus reducing the volume of refrigerant. Higher evaporator efficiencies can be achieved by using more filler beads. It is believed that up to one half to two thirds of the shell volume can be taken up by filler beads 35. As described below, it is desirable not to overfill the shell with beads to the extent that the beads are immobile. It is desired if the beads can move inside of the shell.
The filler beads 35 can be put into an evaporator before the evaporator's initial operation. Alternatively, an evaporator can be retrofitted with the filler beads. If retrofitted, the beads will quickly reach the same temperature as the refrigerant.
In operation, the refrigerant 33 (see FIG. 6) flows through the spaces 37 between the filler beads 35 and consequently through the gaps 31 between the tubes. The filler beads form a structure similar to sponges, with voids formed by the filler beads. Thus, the filler beads channel the refrigerant through the spaces or gaps between the beads. The filler beads 35 move and disperse the refrigerant resulting in enhanced heat exchange and refrigerant distribution. In addition, the filler beads contact and scrub the outside diameter of the tubes 6. This is useful in dislodging bubbles 39 that are formed on the outside of tubes 6 as the refrigerant boils. A bubble 39 on a tube decreases the heat exchange at that particular location on the tube. Dislodging the bubble increases the heat exchange.
An evaporator equipped with the filler beads is more efficient and utilizes less refrigerant than prior art evaporators. As a more efficient heat exchanger, the size of the evaporator can be reduced, saving material costs and also floor space. The evaporator requires a lower charge of refrigerant for the same heat exchange capacity when compared to the prior art. The requirement of less refrigerant results in a savings in startup and maintenance cost. In addition, any accidental release of refrigerant is less dangerous as there is less refrigerant to release.
The foregoing disclosure and showings made in the drawings are merely illustrative of the principles of this invention and are not to be interpreted in a limiting sense.

Claims

1. A flooded shell and tube evaporator, comprising:

a) a shell having an inlet and an outlet and having a first and second end;

b) a plurality of tubes located in the shell and extending between the first and second ends, the tubes forming a path through the shell, the path comprising at least one pass through the shell;

c) gaps between the tubes;

d) refrigerant located in the shell;

e) filler beads located inside of the shell, the filler beads located in the gaps, the filler beads being smaller than the gaps so as to be able to move through the gaps, the filler beads having a neutral buoyancy relative to the refrigerant.

2. The evaporator of claim 1 wherein the filler beads comprise three sizes.

3. The evaporator of claim 1 wherein the filler beads are made of plastic.

4. The evaporator of claim 1 wherein the filler beads are spherical.

5. A method of heat exchange in a flooded shell and tube evaporator, comprising the steps of:

a) providing filler beads inside of the shell and amongst the tubes;

b) flowing a process fluid through the tubes in the evaporator;

c) flowing a refrigerant through the shell and through the spaces between the filler beads and through gaps between the tubes.

6. The method of claim 5 further comprising the steps of allowing the beads to move as the refrigerant is flowed, wherein the beads disperse the exchange of heat between the tubes and the refrigerant and the beads contact the tubes and dislodge bubbles on the tubes.

7. The method of claim 5 wherein the step of providing filler beads inside of the shell and amongst the tubes further comprises the step of providing filler beads with a neutral buoyancy with respect to the refrigerant.