THIN FILM EVAPORATOR

Abstract

A thin film evaporator has a shell with tubes extending through the shell in at least one pass. The shell has a top and a bottom. Process fluid flows through the tubes. A suction from a compressor is applied to the top of the shell at a refrigerant outlet. Refrigerant is introduced into the shell at the bottom and is distributed across the bottom region of the shell. The refrigerant flows up and contacts the tubes, exchanging heat therewith before flowing out of the shell top. Oil in the refrigerant contacts the shell wall and drains into a sump.

Description

This application claims the benefit of provisional patent application Ser. No. 61/414,059 filed Nov. 16, 2010.

FIELD OF THE INVENTION

The present invention relates to heat exchangers and refrigeration systems and in particular to evaporators.

BACKGROUND OF THE INVENTION

In a typical refrigeration cycle there is an evaporator or chiller that cools the process fluid at the expense of boiling the refrigerant that is at lower saturation temperature and pressure, a compressor that compresses the vaporized refrigerant to an elevated pressure and temperature, a condenser that condenses the high pressure refrigerant to liquid phase at the expense of heating the cooling medium, and an expansion device that reduces the pressure of the condensed refrigerant back to the low side, thus entering the evaporator or chiller 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.

Refrigerants are typically synthetic and/or natural, such as ammonia, carbon dioxide, or hydrocarbons such as propane. Synthetic refrigerants are falling out of favor due to environmental concerns. However, even natural refrigerants have drawbacks; for example, ammonia is toxic and propane is flammable.

It is desirable to design an evaporator that would use a reduced amount of refrigerant, thus minimizing any danger from an accidental refrigerant release. In addition, a more efficient evaporator would be physically smaller, thus saving money.

SUMMARY OF THE INVENTION

A thin film evaporator comprises a shell having two ends, a top and a bottom. A plurality of tubes is located in the shell and extends between the two ends. The tubes form a path through the shell. The path comprises at least one pass through the shell. There is at least one refrigerant inlet which is located in the bottom of the shell. A refrigerant distributor is connected to the refrigerant inlet and is located between the shell bottom and the tubes. The distributor has openings facing the shell bottom. A perforated plate is between the distributor and the tubes. There is at least one refrigerant outlet located in the shell top. A suction is applied to the refrigerant outlet.

In accordance with one aspect, the distributor openings are sized so as to produce a spray of refrigerant.

In accordance with still another aspect, the evaporator further comprises a thin film of liquid refrigerant on the tubes, with vapor refrigerant between the tubes.

In accordance with one aspect, the thin film evaporator further comprises a demister located in the shell between the tubes and the refrigerant outlet.

In accordance with another aspect, the tubes comprise a main body of tubes. They further comprise a super heat body of tubes located between the demister and the refrigerant outlet.

In accordance with another aspect, a sump is located in the bottom of the shell.

There is also provided a method of heat exchange using a thin film evaporator having a shell with two ends, a top and a bottom, and a plurality of tubes in the shell and extending between the ends. A process fluid flows through the tubes. Refrigerant is flowed into the bottom of the shell. The refrigerant is distributed across a bottom region of the shell. A film of refrigerant is provided around the tubes and affects heat transfer between the process fluid and the refrigerant. The refrigerant is allowed to exit through a refrigerant outlet in the top of the shell. A suction is applied at the refrigerant outlet.

In accordance with one aspect, the step of distributing the refrigerant across a bottom region of the shell further comprises spraying the refrigerant against the shell.

In accordance with another aspect, the step of distributing the refrigerant across a bottom region of the shell further comprises passing the refrigerant spray through a perforated member before flowing the refrigerant around the tubes.

In accordance with one aspect, a resistance to flow in the shell is provided between the tubes and the refrigerant outlet.

In accordance with another aspect, the method further comprises the step of coalescing liquid at the top of the shell before exiting through the refrigerant outlet.

In accordance with another aspect, the tubes comprise a main body of tubes. A super heat body of tubes is provided between the resistance and the refrigerant outlet.

In accordance with another aspect, the refrigerant comprises oil. The step of flowing refrigerant into the shell further comprises spraying the refrigerant against the shell before flowing the refrigerant to the tubes. The oil is drained into a sump in the shell.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side cross-sectional view of the proposed thin film evaporator, in accordance with a preferred embodiment.

FIG. 2 is a top cross-sectional view showing the distribution pipes and baffles.

FIG. 3 is a cross-sectional view, taken along lines III-III of FIG. 2.

FIG. 4 is a bottom view of one of the distribution pipes.

FIG. 5 is a side view of the distribution pipes of FIG. 4.

FIG. 6 is a block diagram of a refrigeration system with the evaporator.

FIG. 7 is a cross-sectional view of some of the tubes, showing a thin film of refrigerant.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the Figs., the evaporator 11 has a cylindrical shell 13. Tubes 15, which carry the process fluid, are located in the shell. The evaporator shown in the drawings has two passes of tubes 15, with a lower pass 15L of tubes and an upper pass 15U of tubes. The tubes are not touching one another and are spaced apart to allow the refrigerant to flow around each tube. Baffle plates 17 support the tubes inside of the shell. The ends of the tubes are coupled to tube sheets 19, located at the ends of the shell. Thus, the tubes 15 extend between the tube sheets 19 inside of the shell 13. (The tubes 15 are not shown in FIGS. 1 and 2 so that other details can be shown; however the location of the tube passes 15U, 15L are shown in FIG. 1.)

At one end of the shell, an end bonnet 20 (see FIG. 1) has an inlet chamber 21 communicating with the upper pass 15U of tubes and an outlet chamber 23 that communicates with the lower pass 15L of tubes. A respective inlet 25 and outlet 27 are connected to the inlet and outlet chambers. A divider plate 29 separates the inlet and outlet chambers.

At the opposite end of the shell is another end bonnet 31 with a single chamber so that fluid exiting the upper pass 15U of tubes enters the lower pass 15L of tubes.

The chiller can have a single pass of tubes or more than two passes of tubes. FIG. 3 shows an imaginary horizontal center line which visually separates the upper pass 15U from the lower pass 15L.

The process fluid 30, such as water, brine, gas, etc., flows through the inlet 25 (see FIG. 1) into the inlet chamber 21 and then flows through the upper pass 15U of tubes into the opposite end bonnet 31 and then enters the lower pass 15L of tubes where it then flows into the outlet chamber 23 and through the outlet 27.

The refrigerant enters the shell at the bottom and moves up, where it exits at the top of the shell. The refrigerant flows into the shell by way of distribution pipes 37. The distribution pipes 37 are arranged in assemblies 33. In the preferred embodiment, there are two distribution pipe assemblies 33, arranged end-to-end along the bottom portion of the shell. Each distribution pipe assembly 33 is shaped like an elongated “H” (see FIG. 4). Each distribution pipe assembly has a center feed section 35 that is transverse to parallel outlet distribution pipes 37. Each distribution pipe 37 has openings 39 along the bottom of the pipes. The openings 39 are located along the length of the outlet pipes 37. The openings 39 are oriented straight down. However, the openings could be oriented at some angle relative to straight down. The openings 39 are sized so that the refrigerant exits the distribution pipes 37 as a spray. As shown in FIGS. 3 and 5, a vertical riser pipe 41 depends from each center feed section 35. The riser pipes 41 are the refrigerant inlets. The distribution pipes 33 are located in the bottom portion in the shell 13 and are spaced above the bottom by the vertical riser pipe 41 so that a gap 43 is formed between the distribution pipes and the shell bottom. A perforated plate 45 is located above the distribution pipes. If need be, the distribution pipes 33 can be secured to the perforated plate 45 for support. The perforated plate is located below the lower pass 15L of tubes.

The distribution pipe assemblies 33 can be in various configurations. If the shell is short enough, only a single distribution pipe assembly 33 need be used. Conversely, a longer shell may require more than two distribution pipe assemblies. Likewise, each distribution pipe assembly can have one or more pipes 37. For example, a single pipe can be used, which pipe can be of a larger inside diameter than the pipes 37 shown in FIG. 3. With such a single pipe, some of the openings can be oriented to spray vertically down, while other of the openings can be oriented to spray at an angle to vertical. Alternatively, more than one or two pipes 37 can be used. The number and size of pipes 37 will depend somewhat on the size of the shell. The distribution pipe assemblies 33 are designed so as to provide a distribution of the refrigerant across the bottom of the shell, so that the refrigerant contacts all of the tubes 15. The perforated plate 45 assists in evenly distributing the refrigerant among the tubes 15.

A demister pad 47 is located above the upper pass 15U of tubes. The demister pad is, in one embodiment, a 1″ thick pad of stainless steel wool wire. One or more refrigerant outlets 49 are at the top of the shell, located above the demister pad 47. Between the demister pad 47 and the outlets 49 a single or multiple rows of tubes 15D are located. These tubes 15D are part of the upper pass 15U. The tubes in this section could be the same diameter or type as the tubes in the other sections or passes, or the tubes could be different. For example, the tubes 15D could be of a smaller diameter so as to provide more tubes above the demister 47. The tubes 15D impart super heat to the refrigerant. These tubes 15D act as the final barrier to stop any liquid refrigerant carry-over into the compressor 63 (FIG. 6).

The shell 13 is provided with a sump 51 in its bottom. The bottom wall of the shell at the sump periphery is curved into the sump so as to facilitate drainage into the sump.

The evaporator is installed in a refrigerant system 61 as shown in FIG. 6. The refrigerant outlets 49 are connected to the inlet of the compressor 63. The compressor is connected to a condenser 65. The condenser outlet is connected to an expansion device or valve 67, which in turn is connected to the refrigerant inlets 41 of the evaporator 11. No refrigerant pump is needed to provide refrigerant for the evaporator 11.

The expansion device 67 is provided at the refrigerant inlets to control the flow of refrigerant into the evaporator. Sensors 69 are located at the refrigerant outlets 49. The sensors can be pressure transducers or temperature sensors. As the demand for refrigerant increases, as sensed at the outlets 49, the expansion device 67 can allow more refrigerant into the evaporator, and vice versa.

In operation, the process fluid 30 (FIG. 1) is circulated through the tubes 15 while the refrigerant 70 is circulated through the shell, although exterior to the tubes. The liquid-vapor refrigerant mixture enters by way of the inlets 41, flows into the distribution pipes 37 and passes through the openings 39 as a spray 70 (in FIG. 3, only one side of the distribution pipe assembly 33 is shown as spraying for illustrative purposes). The refrigerant is distributed evenly by the distribution pipes 33 into the bottom shell at the shell wall. The refrigerant impacts the shell wall below the distribution pipe assemblies 33. This action serves to create a homogeneous two-phase (liquid and vapor) refrigerant mixture, which mixture is then evenly distributed across the bottom region of the shell. The perforated plate 45 further helps to evenly distribute the refrigerant mixture across the bottom region of the shell. The compressor 63 suction as applied to the outlets 49 draws the refrigerant up inside of the shell into the tube regions (FIG. 3). The refrigerant forms a thin liquid film 71 on the outside of the tubes 15 (FIG. 7). The refrigerant film has excellent heat transfer characteristics, particularly when compared to a flooded evaporator. As the refrigerant is boiled off of the tubes, the process fluid 75 cools and the refrigerant flows up as a vapor 73. The spaces between the tubes 15 contain the refrigerant in both liquid and vapor phases, with the liquid refrigerant being the size of droplets. This is in contrast with a flooded evaporator where the spacing between the tubes is filled with a pool of refrigerant. The refrigerant vapor first passes through the demister pad 47, then the last batch of tubes 15D and finally out through the refrigerant outlets 49.

At the upper end of the shell, the refrigerant in the spaces between the tubes 15 is mostly vapor and may contain some liquid. The demister pad 47 coalesces any liquid refrigerant and thereby prevents liquid from entering the compressor 63. The coalesced liquid drops back down onto the tubes 15 below the demister 47. The demister pad also applies a back pressure across the refrigerant outlets 49, which serve to evenly distribute the refrigerant across the tube bundle.

As the refrigerant vapor exits the evaporator 11 (FIG. 6), it is super heated. Thus, it will not return to a liquid state prior to being compressed by the compressor. The interaction between the sensor 69 and the expansion device 67 maintains a fixed superheat level.

The sump 51 (FIG. 1) captures oil in the refrigerant and keeps the tubes 15 clean. The refrigerant picks up oil from the compressor. As the refrigerant is sprayed out of the distribution pipes, the oil adheres to the shell wall more readily than does the refrigerant. The oil drains into the sump 51, where it collects and can be removed. Removal of the oil is discussed in U.S. Pat. No. 7,082,774, the entire disclosure of which is incorporated herein by reference.

The thin film evaporator has advantages over other types of heat exchangers. Where a flooded evaporator requires the shell to be flooded with refrigerant, the thin film evaporator requires a much smaller charge of refrigerant. For example, for a 130 Ton-Refrigeration capacity system, a flooded evaporator would require approximately 1200 pounds of ammonia, while the thin film evaporator would require only about 35 pounds. Thus, there is less toxic refrigerant to potentially leak into the atmosphere.

On the other hand, conventional spray evaporators require a pump to spray the refrigerant down onto the tubes. Refrigerant pumps are expensive as they must have special seals and maintenance costs are high due to moving parts in a system. Furthermore, in order to ensure reliability of the refrigeration system, typically a backup pump is called for. The use of two special pumps significantly increases the cost of the refrigeration system. Furthermore, the refrigerant charge is still higher in the spray evaporator as compared to thin film evaporator. However, with the thin film evaporator described herein, no pump is needed as the compressor suction is used to draw the refrigerant up through the evaporator.

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 thin film evaporator, comprising:

a) a shell having two ends, a top and a bottom;

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

c) at least one refrigerant inlet located in the bottom of the shell;

d) a refrigerant distributor connected to the refrigerant inlet and located between the shell bottom and the tubes, the distributor having openings facing the shell bottom;

e) a perforated plate between the distributor and the tubes;

f) at least one refrigerant outlet located in the shell top;

g) a suction applied to the refrigerant outlet.

2. The thin film evaporator of claim 1, wherein the distributor openings are sized so as to produce a spray of refrigerant.

3. The thin film evaporator of claim 1, further comprising a thin film of liquid refrigerant on the tubes, with vapor refrigerant in between the tubes.

4. The thin film evaporator of claim 1, further comprising a demister located in the shell between the tubes and the refrigerant outlet.

5. The thin film evaporator of claim 4, wherein the tubes comprise a main body of tubes, further comprising a super heat body of tubes located between the demister and the refrigerant outlet.

6. The thin film evaporator of claim 1, further comprising a sump located in the bottom of the shell.

7. A method of heat exchange using a thin film evaporator having a shell with two ends, a top and a bottom, a plurality of tubes in the shell and extending between the ends, comprising the steps of:

a) flowing a process fluid through the tubes;

b) flowing refrigerant into the bottom of the shell;

c) distributing the refrigerant across a bottom region of the shell;

d) providing a film of refrigerant around the tubes and affecting heat transfer between the process fluid and the refrigerant;

e) allowing the refrigerant to exit the top of the shell through a refrigerant outlet;

f) applying a suction at the refrigerant outlet.

8. The method of claim 7, wherein the step of distributing the refrigerant across a bottom region of the shell further comprising spraying the refrigerant against the shell.

9. The method of claim 8, wherein the step of distributing the refrigerant across a bottom region of the shell further comprising passing the refrigerant spray through a perforated member before flowing the refrigerant around the tube.

10. The method of claim 7, further comprising the step of providing a resistance to flow in the shell between the tubes and the refrigerant outlet.

11. The method of claim 10, further comprising the step of coalescing liquid at the top of the shell before exiting through the refrigerant outlet.

12. The method of claim 10, wherein the tubes comprise a main body of tubes, further comprising the step of providing a super heat body of tubes between the resistance and the refrigerant outlet.

13. The method of claim 7, wherein the refrigerant comprises oil, wherein:

a) the step of flowing refrigerant into the shell further comprises spraying the refrigerant against the shell before flowing the refrigerant to the tubes;

b) draining the oil into a sump in the shell.