制冷技术(英文版)Ch11-0906-4

Chapter 11 Condensing Equipment

11-1) Function and Classification of Condensers

(1)Function of Condensers

The heat-transfer process in a condenser occurs in three stages as implicated in Fig.11-1.

1. Desuperheating of the hot vapor.

2. Condensing of the vapor to liquid state and release of the latent heat.

3. Subcooling of the liquid refrigerant.

Fig.11-1, The state and temperature variation inside a condenser

(2)Classification of Condensers

Fig.11-2 Classification of condensers [1]

Types of water-cooled condensers can be classified according to their construction (Fig.11-2). Each has features that make it suitable for certain application.

(1) Double-pipe condenser

Fig.11-3, Double-pipe water-cooled condenser

Fig.11-4 Circular and trombone double-tube condenser [11]

Fig.11-5 Double pipe condenser with cleanable tubes

(2) Shell-and-coil condenser 壳盘管冷凝器

Fig.11-6 Shell-and-coil condenser (vertical type) [1]

(3) Shell-and-tube condenser壳管冷凝器

The shell-and-tube condenser consists of a cylindrical metal shell and a number of straight tubes which are arranged in parallel and held in place at the ends by tube sheets. There are two main types of shell-and-tube condensers: horizontal and vertical.

a) Horizontal shell-and-tube condenser

Fig.11-7 Two-pass shell and tube condenser

b) Vertical shell-and-tube condensers

vapor in

Cooling water in

Cooling

Fig.11-10 Vertical shell-and-tube condenser

(4) Heat transfer in water cooled condenser

The following equation is used to calculate heat transfer in water cooled condenser.

00m Q k t F =? (11-1)

where

0Q : Heat transfer rate, kW.

k : overall heat transfer coefficient, kW/m 2 K 0F : Heat transfer area, m 2

m t ?: mean effective temperature difference (METD), K The overall heat transfer coefficient:

1

r w f l

k r R R R R =

?+++

(11-2)

where

r R : thermal resistance in refrigerant side w R : thermal resistance of tube wall

f R : thermal resistance of water fouling

l R : thermal resistance in water side

r : ratio of outside to inside tube surface area

Since the temperature of cooling media is constantly changing in a condenser, the temperature difference between the the refrigerant and the cooling media is not constant, as seen in Fig.11-11. A mean temperature difference between them must be determined for Equation 11-1. It has been found that the Mean Effective Temperature Difference (METD) is the best way to represent this mean temperature difference, which can be obtained from the following equation:

ln A B

m A B

TD TD t TD -?=

(11-3) where

A TD : temperature difference at one end of the condenser, K

B TD : temperature difference at other end of the condenser, K

liquid out

refrigerant

vapor in

water out

water in

t c '

t h ''t c t h '

Fig. 11-11 Temperature profile in a condenser

As we can find from Equation 11-1, increasing m t ?, i.e., METD, is one of the ways to enhance heat transfer for the condensers. The mean temperature,m t ?, can be increased by the flow arrangement in the condensers. Counterflow and parallel flows are two common arrangements applied to shell-and-tube heat exchangers. For a given set of required conditions, the counterflow arrangment gives a greater METD than the parallel flow arrangement does. However, the METD is affected by the flow arrangement when the refrigerant is condensed from saturated vapor to saturated liquid because the the

temperature of the refrigerant is constant in this process (Fig.11-11).

In order to increase the heat transfer performance of condensers, the overall thermal resistance,1k, should be as small as possible. Of the four elements involved in Equation 11-2, the thermal resistance of metal tube wall resistance Rw is the least significant and can be ignored. usually. Thus, the k of a condenser is determined primarily by the other three factors and usually the fouling resistance and the liquid film resistance are the most significant. Table 11-1 compares the thermal resistances in a water cooled condenser with the ammonia as refrigerant.

*:This is special for ammonia. Some of the refrigerants do not have this type of resistance because the lubrication oil can dissolve in these refrigerants.

(1) Types of Cooling Towers

Fig. 11-13 Mechanical Draft Cooling Tower [13]

(2) Cooling Tower Performance

Fig.11-14 Range and approach of cooling towers

/()

=+(11-4)

Cooling Tower effectiveness Range Range Approach

In cooling towers, evaporation and water droplets carried away by the air flow are the two causes of

water loss. Also, water in the tower basin should be regularly removed to prevent the accumulation of the dissolved solids. For these reasons, it is essential to supply enough make-up water to the cooling tower.

The evaporated water does not carry away the solids, so that the concentration of the solids would reach a level at which they would precipitate out of the water on surfaces as a coating, or scale. This will

reduce the effectiveness of the condenser.

Fig.11-15 Make-up and purging of a cooling tower

, ,c

m

concentration of solids in circulation water S r concentration of solids in make up water S =

-, (11-5)

And the make-up water has thus to be:

()11

m d d

r

G G w G r =+=- (11-9)

Fig11-16, Air-cooled condenser (natural draft type with wire fins) [15]

Fig.11-17 Typical forced draft air condenser arrangement

R e c i r c u l a t e d w a t e r

Make-up water

plates Sprays liquid out

vapor in

Fig.11-18 Schematic diagram of evaporative condenser

References

1. Dossat R. J., Principles of Refrigeration, Third Edition, Prentice-Hall, Inc.1991.

2. Wang S. K. Handbook of Air conditioning & Refrigeration, McGraw-Hill, 1994.

3. Gosney M.W., Principles of Refrigeration, Cambridge University Press, 1982

4. Source: Perry’s Chemic al Engineers Handbook (Page: 12-17)， McG raw-Hill,1997

5. Bureau of Energy Efficiency, Ministry of Power, India. Cooling Towers. In: Energy

6. Efficiency in Electrical Utilities. Chapter 7, pg 135 - 151. Edward, 2004

7. Zhu P.G. The principle and calculation of heat exchangers. Tsinghua Pub. Comp., Beijing. 1987.7 8. Wu Y.Z. The principle and equipment of refrigeration. Xi an Jiaotong Press.,Xian, 1998.1 9. Wu Y.Z. Design of refrigeration system. Mechanical Press.Beijing,1988

10. Zheng X.D. The principle and equipment of refrigeration. Mechanical Press.Beijing, 1988

11. https://www.360docs.net/doc/a417306298.html,/2725490/product-5868091/photo-7619329/Coaxial-Coil-Used-for-He

at-Pump-Water-Heater.shtml

12. https://www.360docs.net/doc/a417306298.html,/techdb/manual/cooltext.htm 13. https://www.360docs.net/doc/a417306298.html,/A2/A2.htm

14. https://www.360docs.net/doc/a417306298.html,/71026/product-2323161/photo-4484628/Condenser.shtml