Heat Recovery Vapor Trap

Abstract

A heat recovery vapor trap is provided for removal of vapor condensate from a container, while preventing any appreciable escape of vapor, and for achieving beneficial heat recovery from said condensate prior to temperature degradation associated with depressurization, whereby useful heating to much higher temperature is possible. The trap is indirect acting, and is preferably mechanically actuated by a float.

Description

Conventional steam traps decrease the pressure of the saturated condensate, usually down to the condensate drain pipe pressure. A decrease in pressure of a saturated liquid represents a serious degradation of the ability to do useful heating by that liquid. The pressure reduction yields a two phase saturated mixture at the lower pressure with a corresponding lower temperature. For example, consider the use of 100 psig steam (saturation temperature 337 F), used to heat a fluid from 220 F to 300 F. Condensate is supplied to the steam trap at 337 F. The trap reduces the condensate pressure to 5 psig, at a saturation temperature of 227 F. With a conventional steam trap, each pound of steam at 100 psig delivers 881 BTU of useful heating. Conversely, when the condensate is first used to provide useful sensible heat before being depressurized, an additional 114 BTU of useful heat can be obtained from each pound of steam—heat that otherwise ends up as flash vapor at 5 psig. The vapor temperature at 5 psig (227 F) is too low to do the required heating, and often that means it is reject heat. At best it can only be used for low temperature heating, e.g. in a condensate tank vent condenser. What is needed is a steam trap that does not depressurize the condensate until after sensible cooling of the condensate. That is an objective of this invention. The trap must still maintain the desired liquid level between steam and condensate, such that condensate does not flood any of the heat exchange surface.

DISCLOSURE OF INVENTION

A heat recovery steam trap is disclosed, comprised of a mechanical float-activated level control valve that controls level in a container, plus a heat recovery heat exchanger (HRHX). Liquid condensate removed from said container is first supplied to said HRHX for cooling, and then supplied to the indirect acting valve mechanism in said trap for depressurization, and then discharged into the condensate drain pipe. “Indirect acting” means that the liquid being depressurized in the trap is not directly from said container, but is indirectly from the container via said HRHX heat exchanger. The key advantage achieved is that the fluid being heated in the HRHX can be heated to a temperature higher than the saturation temperature corresponding to the condensate drain pipe pressure (typically equal to the deaeration tank pressure).

The fluid being heated can be any fluid. If it is the same fluid as that being heated by the steam, then it is important that the HRHX be in parallel with another heat exchanger heating the same fluid, to achieve maximum gain in efficiency.

The steam trap can either be located externally to said container (in fluid communication) or internally to said container. The heat exchanger for sensible cooling of the condensate can be either an independent dedicated HRHX heat exchanger, or can be a portion of another heat exchanger, such as the one the trap is servicing, and including the option of being contained in said container.

The disclosed level controlling heat recovery vapor trap may be used in conjunction with any saturated vapor, i.e. is not limited to steam.

The level control mechanism can be other than a mechanical float, provided that it be capable of achieving the requisite “indirect acting” function.

The pressure drop through the HRHX should be low enough on the condensate side that no appreciable flashing occurs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a first embodiment of the heat recovery steam trap, comprised of a mechanical float-actuated indirect acting level control valve that is located inside the steam-containing container.

FIG. 2 illustrates two different applications of the heat recovery vapor trap, applied in the same apparatus.

FIG. 3 depicts one preferred configuration of the float actuated indirect acting level control valve. FIG. 3A is a top view; FIG. 3B is a cross-sectional view of the rod; FIG. 3C is a side view with the float in the upmost position; FIG. 3D is a side view with the float in the lower-most position.

FIG. 4 illustrates the application of one embodiment of the heat recovery steam trap to a double effect LiBr absorption chilling cycle.

FIG. 5 illustrates applying two heat recovery steam traps to a double effect LiBr cycle, one on internal condensate and one external.

BEST MODE FOR CARRYING OUT THE INVENTION

FIG. 1 illustrates a container 10 wherein a fluid 1 is being heated by steam via heat exchanger 11. Condensate is discharged from said container via conduit 14, to a heat recovery heat exchanger 13, wherein it countercurrently heats fluid 2. Then the cooled condensate is routed to indirect acting level controlling steam trap 12. The mechanical float-actuated steam trap 12 maintains the desired zero level of condensate in container 10 by being located sufficiently below the container that the level in the trap, controlled by float 15, cannot rise to the container height. The two fluids may be the same or different.

FIG. 2 illustrates two simultaneous uses of the heat recovery vapor trap in the same apparatus. A desorption column 20 is provided for stripping a sorbate from a rich sorbent via thermally activated multistage vapor liquid contact. The desorber is reboiled by supplying preheated rich sorbent to a steam heated reboiler 21. The steam condensate from the reboiler is first routed in counter current heat exchange 22 with part of the rich sorbent, then to the valve section of the level controlling trap 23, and then to the discharge piping 24. Thus both the steam and the hot condensate are causing the rich sorbent to desorb, and the rich sorbent is supplied to them in parallel. Although the steam heat exchanger 25 and the condensate recovery heat exchanger 22 can be physically separate components, that arrangement would require a controlled split of the rich sorbent. Hence a preferred arrangement is to combine both exchange duties in parallel in a single exchanger 21, as shown. In that way the sorbent split occurs automatically and inherently. Numerous heat exchanger geometries are available that permit two hermetically separate fluids on one side of a heat exchanger, including plate and frame; brazed plate; opposed slant tube; and triple helix heat exchangers.

The lean sorbent collects at the bottom of the desorption column, and must be controllably removed to maintain fixed sorbent level. That is accomplished by heat recovery vapor trap 26, an indirect acting level control valve. The bottom liquid is withdrawn thought heat exchange bundles 27 located on the vapor-liquid contact trays 28 (e.g. perforated trays). Then it is further cooled in the rich sorbent preheater 29. Finally it is conveyed to the valve section of the trap 26, for depressurization.

This arrangement is useful for any type of desorption: for example, desorbing ammonia vapor from aqueous ammonia sorbent in an ammonia absorption refrigeration cycle; or desorbing CO₂ from a CO₂ scrubbing sorbent in a CO₂ scrubbing cycle. It also applies to distillation.

FIGS. 3a, b, c, and d, illustrate one preferred arrangement for the float-actuated valve section of the vapor trap. A float 30 is connected by a yoke 31 to a rod 32 that rotates in a fixed cylinder 33. Float movement causes the rod to rotate in the cylinder. An opening 34 in the rod (e.g. a drilled hole) is positioned such that when the float is in the up position (high level, FIG. 3c) the opening aligns with inlet and outlet openings in the cylinder. That allows liquid to flow in pipe 35 and out pipe 36 thus lowering the level. When the float is in the down position (low level, FIG. 3d), the opening does not align with the cylinder openings, and the flow is blocked.

Hence the rod and cylinder comprise the actual valve, and the float causes it to actuate responsive to liquid level. Note that in order to be indirect acting, the valve must have two connections to outside its container: both inlet and outlet. As is known in the prior art, the steam trap can additionally have a vapor purge port for removal of noncondensable gases. The trap may be located inside a housing 37. Alternatively, the trap may be located inside the level-controlled container, as in FIG. 1. When located in its own housing, it requires fluid connections 38 and 39 to reproduce the required level.

FIG. 4 provides more specific details regarding how the disclosed heat recovery steam trap can be applied to an absorption refrigeration cycle in order to achieve about 10% reduction in the amount of steam necessary to drive the cycle. In this example a double-effect LiBr absorption cycle is illustrated, comprised of high temperature generator (HTG), low temperature generator (LTG), condenser (COND), absorber (ABS), evaporator (EVAP), LiBr pump, and evaporator water pump. The novel aspect is the treatment of the steam condensate exiting the HTG steam heat exchanger. The condensate level is controlled by the level-controlling indirect acting steam trap 40. The condensate is conveyed first to the float portion of valve 40 plus to a high temperature heat exchanger (HTHE) wherein it countercurrently exchanges heat with part of the LiBr solution enroute to the HTG; then it is conveyed to a low temperature heat exchanger (LTHE) wherein it countercurrently exchanges heat with part of the LiBr solution enroute to the LTG; and then it is conveyed to the mechanical float-actuated level control valve 40 for depressurization and routing to the condensate return piping. Steam trap (valve) 40 is located sufficiently below the HTG that it maintains the desired zero level in the HTG. The actual level is inside the housing of valve 40. The HTHE and LTHE are also known as feed-effluent heat exchangers.

Once again the two condensate heat recovery heat exchangers can be independent ones, but a preferred arrangement is to combine them with the HTHE and LTHE that are already present for heat exchange with returning LiBr, as shown. Also, the same enhancement can be applied to a single effect LiBr cycle, which does not have the HTHE.

FIG. 5 illustrates an additional opportunity to reduce the driving heat requirement of a double effect LiBr absorption cycle. This opportunity arises inside the cycle, at the LTG that is being heated by desorbed vapor from the HTG. The condensate is supplied to the float side of HRST 41, and to the HRHX (part of the LTHE), then to the valve side of HRST 41, and finally to COND. There is still scope for external savings as well with the steam supply HRST per FIG. 4. An optional alternative configuration is illustrated in FIG. 5, wherein the indirect-acting level control valve is not a mechanical float-actuated type. Instead it is comprised of electronically actuated valve 46, controlled by level sensor 45. Also, the heat recovery heat exchanger is stand-alone. With that configuration, to get the full benefit of the higher condensate temperature, the rich sorbent must be split into two parallel paths, with only one supplied to the HRHX, e.g. by coordinated action of valves 43 and 44.

Claims

1. An apparatus for controlling the liquid level in a container containing saturated vapor and liquid, and for recovering heat from the liquid discharged from said container by said level control apparatus, comprising:

a. A float actuated mechanical control valve wherein said float is supported by a liquid level that is in vapor and liquid communication with said level in said container;

b. A heat recovery heat exchanger having hot and cold ends;

c. A first conduit conveying liquid from said container to said hot end;

d. A second conduit for conveying cooled container liquid from said cold end to said mechanical float-actuated control valve; and

e. A third conduit for removal of cooled reduced pressure liquid from said valve

2. The apparatus according to claim 1 wherein said container is comprised of a steam-heated heat exchanger for a fluid, wherein said saturated vapor is steam, and additionally comprised of a feed-effluent heat exchanger for the fluid supplied to and from said steam-heated heat exchanger, and wherein said heat recovery heat exchanger is arranged for parallel fluid flow with said feed-effluent heat exchanger.

3. The apparatus according to claim 1 wherein said float is contained in said container.

4. The apparatus according to claim 1 additionally comprised of a housing for said float that is external to said container, plus at least one fluid conduit connecting said housing to said container.

5. The apparatus according to claim 1 wherein said valve is comprised of:

a. A rod that rotates inside a cylinder;

b. Inlet and outlet ports in said cylinder that connect to inlet and outlet fluid conduits of said valve;

c. A mechanical connection between said float and said rod, whereby float movement causes the rod to rotate; and

d. An opening in said rod that aligns with said cylinder openings at one float position, and misaligns with them at another.

6. The apparatus according to claim 1 wherein said container is a desorption column and said saturated vapor is sorbate, and said liquid is sorbent.

7. The apparatus according to claim 1 wherein said container is a distillation column.

8. A heat recovery steam trap for a steam heated fluid heat exchanger, comprised of:

a. A condensate level sensing steam trap, that prevents level buildup in said fluid heat exchanger;

b. A condensate heat recovery heat exchanger; and

c. A means for routing condensate from said fluid heat exchanger sequentially first to said condensate heat recovery heat exchanger, then to said steam trap.

9. The apparatus according to claim 8 additionally comprised of a float for said steam trap that senses said level.

10. The apparatus according to claim 8 additionally comprised of a level sensor that sends an electronic signal to said steam trap, which is an electronically-actuated valve.

11. The apparatus according to claim 8 additionally comprised of a means for splitting said heated fluid into two parallel streams and routing one stream to said fluid heat exchanger and the other stream to said condensate heat recovery heat exchanger.

12. The apparatus according to claim 8 additionally comprised of a second indirect acting steam trap connected to said condensate heat recovery heat exchanger.

13. An apparatus for removing condensate from a container, comprised of:

a. An indirect acting level control valve for said container;

b. A heat recovery heat exchanger in fluid communication with said container; and

c. A conduit connecting said heat recovery heat exchanger to said indirect acting level control valve.

14. The apparatus according to claim 13 additionally comprised of an electronic level sensor, and wherein said valve is an electronically actuated valve that receives a signal from said sensor.

15. The apparatus according to claim 13 wherein said valve is float actuated and additionally comprised of a housing for said valve and float that is external to said container.

16. The apparatus according to claim 13 wherein said valve and said float are located internal to said container.

17. The apparatus according to claim 13 wherein said valve is a float valve comprised of:

a. A float;

b. A rod that rotates inside a cylinder;

c. Inlet and outlet ports in said cylinder that connect to inlet and outlet fluid conduits of said valve;

d. A mechanical connection between said float and said rod, whereby float movement causes the rod to rotate; and

e. An opening in said rod that aligns with said cylinder openings at one float position, and misaligns with them at another.

18. The apparatus according to claim 13 wherein said container is a mass transfer column.

19. The apparatus according to claim 13 additionally comprised of a second indirect acting valve and a second conduit that connects said heat recovery heat exchanger to said second valve.

20. The apparatus according to claim 13 wherein said heat recovery heat exchanger is at least partly located inside said container.