CONDENSER

A first guess of a condensing temperature has already been taken as a guide. Users should be aware of the wide difference in owning costs arising from the choice of condenser, so the options should be compared on this basis. Certain machines, such as the centrifugal compressor, are very sensitive to high con­densing conditions, and the correct choice (in this case, of a cooling tower) can give a considerable gain in COP.

Catalogue ratings show heat rejected at a stated condensing temperature and related to the following:

— Ambient dry bulb temperature for air-cooled condensers

— Available water temperature for water-cooled condensers or

— Ambient wet bulb temperature for evaporative types

Choice of equipment based on first cost only will almost certainly result in an undersized condenser and a high head pressure.

Example 10.4

An application requires a cooling capacity of 218 kW and the running time is 2000 h/year at an electricity cost of 8 p/(kWh). In order to achieve the condensing temperature of 30°C the condenser would cost Ј14000, while a smaller condenser for a temperature of 35°C would cost Ј8500. Estimate the pay back time if the larger condenser is fitted.

Condensing temperature

30°C

35°C

Rated capacity of plant (kW)

242

218

Running time for 218 kW x 2000 h

1802

2000

Compressor electrical input power (kW)

60

70

Electricity cost per year (Ј)

11533

14933

Electricity saving per year (Ј)

3400

N, 14000 — 8500 1 C

Break-even time =—————— = 1.6 years

3400

This is a very approximate calculation, based on direct capital cost and not on interest rates, and needs to be analysed in terms of the general plant econom­ics. Also variations due to seasonal air temperature changes are not accounted. It should also be borne in mind that this is based on present-day electricity costs, and a greater saving will be made as fuel costs rise. Tendering contractors and prospective users should make themselves aware of alternatives of this sort.

In most climates the wet bulb temperature is well below the dry bulb tem­perature and there is an advantage in using water or evaporative cooling for larger plant. The present concern over spray-borne diseases may indicate a preference for air cooling but the alternatives should be considered. Table 10.2 , based on the tentative temperature differences of 15 K and 12 K as given in the table, shows that such figures need to be reconsidered in extreme cases. For example, if it is necessary to use an air-cooled condenser, there will be con­siderable economy in over-sizing the condenser to reduce the condensing tem­perature from a first guess of 62°C down to, possibly, 56°C.

Table 10.2 Typical condensing temperatures for air-cooled and evaporative condensers in various locations

Climate

Air-cooled

Evaporative

Dry bulb

(°C)

Condenser

(°C)

Wet bulb

(°C)

Condenser

(°C)

South UK

27

42

21

33

Scotland

24

39

18

30

Mediterranean

32

47

24

36

Desert

47

62

24

36

Tropical humid

33

48

28

40

The maximum design condensing temperature will only apply when the ambient is at its hottest, and full advantage should always be taken to allow this temperature to drop at cooler times, down to its minimum working limit. Systems should be allowed to drop to a condensing temperature of 25°C when the cooling medium permits this, and some systems can go lower. A true esti­mate of total owning cost should take this into account.

The performance of condensers with a compressor-evaporator system can be shown graphically as in Figure 10.6 . The curves are the rejected heat from the compressor, i. e. cooling duty plus compressor power. These are plotted against the basic performance of the condenser. A development of graphical methods for matching of components is given by Sulc (2007). Some condenser manu­facturers provide rating curves based on the cooling capacity of the compressor

CONDENSER

Condensing temperature (°C)

Figure 10.6 Balance of heat rejection with condenser

And using typical factors for the power. Where there is significant non-useful cooling as in Example 10.3, the heat rejection load will include this additional capacity. The sub-cooling may occur after the condenser in which case this load can be deducted.

Air-cooled condensers require a large air flow for a given heat rejection, and the ability to locate them where this air flow can be obtained without re­circulation may limit their use. Water or evaporative cooling should always be considered as a possibility, except for smaller sizes or where using packaged condensing units.

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