JPH03211391A - Falling film evaporator - Google Patents

Abstract

PURPOSE:To increase the quantity of a liquid refrigerant flowing into a heat exchanger tube and contrive an increased cooling efficiency of a cooler, by a construction wherein a pressure loss of the liquid refrigerant across a gap between a refrigerant distributing plate and the heat exchanger tube is equal to a pressure loss of a refrigerant gas during passage through a refrigerant vapor extraction pipe from a refrigerant distributing chamber to an evaporation chamber. CONSTITUTION:A liquid refrigerant introduced through a refrigerant inlet part 2 of a shell 1 flows through a refrigerant distributing chamber 30, and flows down on the outside of heat exchanger tubes 4, whereas cold water flows into a water chamber 14 via a cold water inlet part 6. Between a liquid refrigerant distributing plate 15 and each of the tubes 4 provided is a gap 38, through which the liquid refrigerant 37 flows down. A refrigerant vapor generated through evaporation of the liquid refrigerant and a vapor in the distributing chamber 30 are passed through a vapor extraction pipe 32 into an evaporation chamber 31, where the vapors are mixed, and the resulting vapor mixture flows through a refrigerant gas discharge pipe 35 to a compressor. Where a pressure loss during passage through the vapor extraction pipe 31 is designed to be equal to a pressure loss across the gaps 38 provided at the peripheries of the heat exchanger tubes 4 for distribution of the refrigerant, only the liquid refrigerant 37 flows into the heat exchanger tubes 4 via the gap parts 38, and only the refrigerant gas flows through the extraction pipe 32 into the evaporation chamber 31. Thus, cooling efficiency of a cooler can be enhanced.

Description

Detailed Description of the Invention [Field of Industrial Application] The present invention is suitable for devices that supply cold water for cooling various equipment or air conditioning, and plants for ocean thermal power generation. This invention relates to a falling film evaporator that transfers heat.

[Conventional technology]

The conventional falling film evaporator is 1, for example, JP-A-61-16
It is described in No. 8789. In this type of evaporator, the liquid refrigerant flowing from the liquid refrigerant inlet at the top of the evaporator shell flows down in a thin film state on the outer surface of a number of heat transfer tubes in the evaporator shell, and the cold water flowing inside the heat transfer tubes flows down. The heat is removed by evaporation and the cold water is cooled. The gasified refrigerant is
It flows out from the refrigerant gas outlet. Further, the cooled water flows out of the evaporator from the cold water outlet, flows to the system to be cooled, is circulated by the circulation pump, and returns to the cold water inlet of the evaporator again.

[Problem to be solved by the invention]

In the case of the above-mentioned conventional falling film type evaporator, when the liquid refrigerant entering the evaporator flows down into the heat transfer tube, if it flows down in a two-phase flow from the refrigerant distribution hole, the liquid refrigerant scatters and the heat transfer efficiency decreases. Alternatively, excessive liquid refrigerant accumulates in the refrigerant distribution chamber and the liquid level becomes high, and liquid refrigerant from the vapor vent pipe does not touch the heat transfer tubes but accumulates on the lower surface of the evaporation chamber, reducing heat transfer efficiency. These phenomena that reduce heat transfer efficiency are related to the balance between the pressure loss of the liquid refrigerant passing through the refrigerant gap in the refrigerant distribution plate and the pressure loss of the refrigerant gas passing through the refrigerant vapor vent pipe, but this point has not been studied in the past. It wasn't taken into consideration.

[Means to solve the problem]

In order to achieve the above objects, the present invention aims to equalize the pressure loss in the refrigerant distribution gap of the refrigerant distribution plate in the refrigerant distribution chamber of a falling film evaporator and the pressure loss of refrigerant vapor passing through a refrigerant vapor vent pipe. Determine the structure of the refrigerant distribution plate and refrigerant vapor vent pipe so that

[Effect]

By equalizing the pressure loss of the liquid refrigerant passing through the gap between the refrigerant distribution plate and the heat transfer tube in the refrigerant distribution chamber of the falling film evaporator, and the pressure loss when the refrigerant vapor passes through the refrigerant vapor vent pipe, , refrigerant vapor enters the evaporation chamber only from the vapor vent pipe, and liquid refrigerant flows down to the heat transfer tube only from the refrigerant openings, thereby reducing the amount of unevaporated liquid refrigerant and increasing heat transfer efficiency.

〔Example〕

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows an embodiment of the falling film type evaporator of the present invention. In this figure, liquid refrigerant is provided in a shell 1 and passes through a refrigerant distribution chamber 30 from a refrigerant inlet 2 to a heat transfer tube. It flows down the outside of 4. On the other hand, cold water flows into the upper water chamber 14 from the cold water inlet section 6.

The cold water flowing into the water chamber 14 flows into the inside of the heat transfer tube 4. Heat exchange occurs between this liquid refrigerant and the cold water, and the heat is removed from the cold water to cool it. The cold water that has been deprived of heat is sent to the lower water chamber 14.
The cold water flows out from the lower part through the cold water outlet. A space between the upper water chamber 14 and the refrigerant liquid distribution plate 15 is partitioned by an ice chamber partition plate 21 . A slight gap 38 is provided between the refrigerant liquid distribution plate 15 and the heat transfer tube 4, and the refrigerant liquid flows down from this gap.

The gaseous refrigerant having a weight flow rate lower than that of the liquid refrigerant entering from the refrigerant inlet portion 2 is transferred to the evaporation chamber 31 where heat is transferred to the refrigerant distribution chamber 30.
It enters the evaporation chamber 31 through a steam vent pipe 32 connecting the two. In a falling film evaporator having such a configuration, a refrigerant such as fluorocarbon or ammonia, which starts evaporating at a boiling point lower than the temperature of the chilled water, is used as a heat exchange medium.

Refrigerant vapor generated by evaporation of liquid refrigerant and refrigerant distribution chamber 3
0 vapor passes through the vapor vent pipe 32, these refrigerant vapors are mixed in the evaporation chamber 31, and flow to the compressor 11 via the refrigerant gas discharge pipe 35. In addition, there is a liquid return pipe 3 at the bottom of the evaporation chamber.
6 is installed.

A portion of the lubricating oil that lubricates the sliding surfaces of pistons and the like for compressing refrigerant in the compressor 11 circulates through the refrigeration cycle without staying in the compressor. This lubricating oil
It flows down to the bottom of the evaporation chamber 31 without evaporating even at the top, mixes with unevaporated liquid refrigerant, enters the liquid return pipe 36, and flows to the compressor 11.

As shown in FIG. 2, the liquid refrigerant 37 flows down from the refrigerant distribution chamber onto the surface of the heat transfer tube 4 through a refrigerant distribution gap 38.

In addition to this liquid refrigerant, refrigerant vapor flowing into the refrigerant distribution pipe enters the evaporation chamber 31 through the refrigerant vapor vent pipe 32.

The pressure loss in the steam vent pipe 32 and the gap 38 for refrigerant distribution around the heat transfer tubes shown in FIG. 2 must be designed to have the same value. If the pressure loss in the gap 38 for refrigerant distribution around the heat transfer tubes is large, the refrigerant liquid level on the refrigerant distribution plate will become high, and in extreme cases, the liquid refrigerant may not flow from the vapor vent pipe 32 to the evaporation chamber 31. This means that unevaporated liquid refrigerant exists without touching the heat transfer tubes, reducing heat transfer efficiency. Conversely, if the pressure loss in the gap 38 for refrigerant distribution around the heat transfer tube is small, the refrigerant gas flows out from the refrigerant gap in a two-phase flow state, and liquid refrigerant does not constantly flow on the outer surface of the heat transfer tube. When a disturbance is applied, the two-phase flow is disturbed and the heat transfer performance deteriorates. For this reason,
If only liquid refrigerant flows into the heat exchanger tubes from the refrigerant gaps in the heat exchanger tubes, and only refrigerant gas flows from the refrigerant vapor extraction tube 32 to the evaporation chamber 31, high performance can be achieved. There are various ways to estimate the pressure loss, but here the pressure loss was estimated by using a simple liquid refrigerant or refrigerant gas flow path as an in-pipe flow. For the pressure loss inside the pipe, Blasius' equation was used for laminar flow.

f = 16/Re If the compressor is operated at 75Hz using an inverter, the total refrigerant flow rate will be 360kg/h at R22, and from this point on, the dryness will be reduced to 0.
．． 2 as 360kg/h X (1-0,2) = 288
kg/h was defined as the liquid refrigerant flow rate. The average flow velocity is calculated by dividing this liquid refrigerant flow rate by the cross-sectional area of the gap through which the refrigerant passes.

The specific volume is 0.8 (rn'/kg) for liquid refrigerant, R2
v=o, 04(r
n'/kg) was used. Also.

The kinematic viscosity coefficient ν of the refrigerant gas is ν=0 in the case of a liquid refrigerant.
2X10-6 (Bo/S), ν=3 in case of refrigerant gas.
OXX10-6 (/s) was used. As a result, the dimensions of the refrigerant vapor vent pipe 32 and the refrigerant openings 38 are determined as follows, for example. Heat exchanger tubes - In a falling film evaporator with four semicircular refrigerant openings with a curvature of 2rrn, and two heat exchanger tubes, a steam vent tube with a length of 40 mm and an inner diameter of 16.1 mm is used. You will need two. In this way, by optimizing the dimensions of the refrigerant vapor vent pipe in accordance with the pressure loss of the refrigerant distribution hole, it is possible to reduce the amount of unevaporated refrigerant and obtain a predetermined cooling performance of the evaporator.

To design in more detail, proceed as follows.

The flow velocity vi when the circulating liquid refrigerant passes through the gap for refrigerant distribution is determined, and the kinematic viscosity coefficient ν of the liquid refrigerant is determined.

The Reynolds number Ra was calculated using

vtd consideration Re□=ν The Reynolds number of the exposed refrigerant vapor vent pipe was obtained by determining the flow velocity v2 of the refrigerant vapor passing through the vapor vent pipe and using the kinematic viscosity coefficient ν1 of the refrigerant vapor as shown in the following equation.

2dg Reg= ν g In the configuration of the refrigerant distribution plate and steam vent pipe as shown in FIG. 2, the balance equation of the flow pressure loss flowing from the gap between the refrigerant distribution plates is as follows.

PI PO−ρhawkgΔh0 2 ρIVI2JlliV1” δ ρtV1”2
2 where Po: Refrigerant distribution chamber, Pl: Static pressure at the inlet of the refrigerant distribution gap, P1': Static pressure at the refrigerant distribution gap, R2: Static pressure at the outlet of the refrigerant distribution gap, R5 : Static pressure in the evaporation chamber, g: Gravitational acceleration, ρ, : Density of liquid refrigerant,
Δho: Thickness of liquid refrigerant accumulated on the refrigerant distribution plate, vl
: Flow velocity of liquid refrigerant flowing through the gap of the refrigerant distribution plate, δ: Length of the gap of the refrigerant distribution plate, fl : Resistance coefficient of the gap of the refrigerant distribution plate, d: Equivalent diameter of the gap of the refrigerant distribution plate ( Fourth
In a concentric gap as shown in the figure, (Dz-Dt), D
z: outer diameter of concentric circle, Dl = inner diameter of concentric circle), Kl:
loss coefficient at the inlet of the refrigerant distribution gap, Ko: loss coefficient at the outlet of the refrigerant distribution gap.

The balance equation for the flow pressure loss of refrigerant vapor flowing through the steam vent pipe is as follows.

ρvV2” ρvVZ2 where R8: Static pressure at the inlet of the steam vent pipe, R4: Static pressure at the outlet of the steam vent pipe, ρV: Density of refrigerant vapor, R2: Flow rate of refrigerant vapor in the vapor vent pipe, f2: In-pipe pressure loss coefficient of the steam vent pipe, dq: Internal diameter of steam vent pipe, Q: Length of steam vent pipe, K
t”: Inlet pressure loss coefficient of the steam vent pipe, outlet pressure loss coefficient of the Ko1 steam vent pipe.

The pressure loss passing through the gap in the refrigerant distribution plate can be summarized as follows.

Po+pgΔbo+ρgδ=P5+(Δpered+Δ
Pet+ΔPeexp)=P5+ΔPe where ρ1v12 ΔP area=Kt ΔPeexp”K.

2 ΔPe = ΔP area + ΔPef + ΔPeexy.

The formula for the pressure loss passing through the refrigerant vapor vent pipe can be summarized as follows.

Po=P5+(ΔPvrea+ΔPvi+Δpveexp
)” P 5 + ΔPv.

Here, ρ9■22 ΔPvrei=Ki'' ΔP vexp = Koken ΔPv=ΔPvrem+ΔPVI+ΔPvexp.If these equations for the refrigerant distribution plate and the arrangement equation for the refrigerant vapor vent pipe are modified, they become as follows.

ΔPe−ΔPv=ρgΔh Here, Δh=Δho+δ.

Here, Δho is the height of the liquid refrigerant from the refrigerant distribution plate.

If the pressure loss satisfies this equation, the pressure loss passing through the gap between the refrigerant distribution plates and the pressure loss passing through the refrigerant vapor vent pipe will be equal, and the liquid refrigerant will properly flow into the heat transfer tube.
Heat transfer efficiency can be increased. If ΔPe−ΔPv〉ρgΔh, the pressure loss of the liquid refrigerant passing through the gap between the refrigerant distribution plates will increase, the liquid level of the liquid refrigerant will rise, and the refrigerant liquid may overflow from the steam vent pipe. There is sex. However, if the pressure loss is balanced at a position where the liquid refrigerant level height ΔhO is lower than the height S of the vapor vent pipe on the refrigerant distribution plate,
There is no overflow, and the liquid refrigerant can be evenly distributed over the heat transfer tubes. In addition, if ΔPe−ΔPv ρgΔh, the refrigerant vapor will eject not only from the steam vent pipe but also from the gap in the refrigerant distribution plate and flow onto the heat transfer tube in a two-phase refrigerant flow state, resulting in turbulent flow. Heat transfer efficiency decreases. In this case, the liquid level height Δha of the liquid refrigerant is 0 in a state in which refrigerant vapor flows out from the gap in the refrigerant distribution plate to the evaporation chamber.

Modifying the above equation, the conditions for the refrigerant liquid to flow out from the vapor vent pipe into the evaporation chamber are Δp, < ΔPe−ρg(δ+S), and the refrigerant becomes a two-phase flow from the gap in the refrigerant distribution plate and flows into the evaporation chamber. The condition for flowing out to is ΔPv>ΔPe−ρgδ. In other words, the conditions for stably supplying the refrigerant to the heat transfer tube surface without the refrigerant liquid flowing out from the vapor vent pipe into the evaporation chamber and without the refrigerant flowing out in a two-phase flow state from the gap in the refrigerant liquid distribution plate are ( ΔPa−pgcs+S))〈ΔPv＜(ΔPe−pg
If the length Q of the steam vent pipe or the gap size of the refrigerant liquid distribution plate is determined so as to meet the conditions expressed by this equation, δ), heat exchange can be performed with good heat transfer efficiency. However, in this case, if the height S of the steam vent pipe from the refrigerant distribution plate is made too high, the overall length of the falling film evaporator will become longer.
The volume of the evaporator becomes large, making it impractical.

FIG. 3 shows a system diagram of a refrigeration cycle of the present invention. The almost gaseous refrigerant from the falling film evaporator 10 is
The gas enters the compressor 11 and is compressed into a high-temperature gas state. and,
The high-temperature refrigerant gas radiates its retained heat to the outside in a condenser 12, passes through an expansion valve 13, becomes mostly liquid refrigerant, and exchanges heat with cold water in a falling film evaporator. Note that the arrow in the figure indicates the flow direction of the refrigerant.

FIG. 4 shows an example of a refrigerant distribution gap 38, which is a semicircular opening through which liquid refrigerant passes.

In this case, the equivalent diameter d is, if the curvature of the opening is r. d dew”2xr2/(2r+πr) becomes.

FIG. 5 shows concentric gaps 38 for refrigerant distribution.

〔Effect of the invention〕

According to the present invention, pressure loss occurs when the liquid refrigerant passes through the gap between the refrigerant distribution plate and the heat transfer tube of the falling film evaporator, and refrigerant gas passes from the refrigerant distribution chamber to the refrigerant vapor vent pipe of the evaporation chamber. Since the dimensions are determined so that the pressure loss is equal when the heat exchanger is heated, the amount of liquid refrigerant flowing into the heat transfer tubes increases, and the amount of unevaporated liquid refrigerant decreases, thereby increasing the amount of heat exchanged by the falling film evaporator. Furthermore, the cooling efficiency of a cooling device constituted by a falling film evaporator can be increased.

[Brief explanation of drawings]

FIG. 1 is a partial sectional perspective view of an embodiment of the present invention, FIG. 2 is a partial sectional view of the embodiment shown in FIG. 1, FIG. 3 is a diagram showing a refrigeration cycle, and FIGS. FIG. 5 is an enlarged view of the embodiment shown in FIG. 1, where (a) is a top view and (b) is a partial cross section. 1... Evaporator shell, 2... Liquid refrigerant inlet, 4... Heat exchanger tube, 6... Cold water inlet, 7... Cold water outlet, 10... Falling liquid film type % type % continuous chamber, 32. ... Refrigerant vapor vent pipe, 35... Refrigerant gas outlet, 36... Evaporator shell bottom liquid return pipe, 37...
Liquid refrigerant, P. Taku 3 Figure! 3°゛-Kingyaku 1st Cb)

Claims (1)

[Claims] 1. A plurality of heat exchanger tubes are provided in a shell, the inside of the shell is used as an evaporation chamber by a falling liquid film, a liquid film flow of a liquid refrigerant is formed on the outer surface of the heat exchanger tube, and the In a falling film evaporator configured to exchange heat with a fluid flowing in a heat transfer tube, the liquid that passes through a gap between the heat transfer tube and a refrigerant distribution plate that partitions a refrigerant distribution chamber of the evaporator and the evaporation chamber. The dimensions of the refrigerant distribution gap and the refrigerant vapor vent pipe are adjusted so that the pressure loss of the liquid refrigerant and the pressure loss of the refrigerant gas passing through the refrigerant vapor vent pipe that communicates the refrigerant distribution chamber and the evaporation chamber are approximately equal. A falling film evaporator characterized by having set dimensions.