EVAPORATOR

Once the evaporating temperature has been provisionally decided, an evapor­ator can be selected from catalogue data or designed for the purpose. The rat­ing of an evaporator will be proportional to the temperature difference between the refrigerant and the cooled medium. Since the latter is changing in tempera­ture as it passes over the cooler surface, an accurate calculation for a particular load is tedious and subject error.

To simplify the matching of air-cooling evaporators to condensing units, evaporator duties are commonly expressed in basic ratings (see Figure 10.1),

Air in

EVAPORATOR

Figure 1C.1 Basic evaporator rating and LMTD

In units of kilowatts per Kelvin. This rating factor is multiplied by the AT between the entering air and the refrigerant.

This factor, the basic rating, is assumed constant throughout the design working range of the cooler and this approximation is good enough for equip­ment selection. The basic rating will change with fluid mass flow and, to a lesser extent, with working temperature. It may change drastically with fluids such as the glycol brines, since the viscosity and hence the convection heat transfer factor alter at lower temperatures. In unusual applications, the supplier should be consulted.

Example 10.1

An air-cooling evaporator has a mass air flow of 8.4 kg/s and a published ‘rating’ of 3.8 kW/K. What will be its rated duty at -15°C coldroom temperature with refrigerant at -21°C? What is the true LMTD?

Entering air temperature = -15°C Refrigerant temperature = -12°C ‘Rating’ AT = 6K Rated duty = 3.8 x 6 = 22.8 kW

22 8

Reduction in air temperature =——- :—— = 2.73K

1.006 x 8.4

Air leaving temperature = -15-2.73 = -17.73°C LMTD = 4.5K

There would be an error at other conditions since the basic rating is only accurate at one point, so this short-cut factor must only be used within the range specified by the manufacturer.

The method of balancing such an evaporator with a condensing unit is graphical. The condensing unit capacity is shown as cooling duty against evap­orator temperature, line CD in Figure 10.2. The coil rating is plotted as the line AB, with A at the required coldroom (or ‘air-on’) temperature, and the slope of the line AB corresponding to the basic rating. The intersection of this line with the condensing unit curve CD gives the graphical solution of the system balance point. Similar constructions for higher condenser air conditions (EF, GH) or different room temperatures (A1B1) will show balance points for these conditions.

EVAPORATOR

&

CO

A.

Ra

O

Cn

G

15

O

O

Evaporating temperature (°C) Figure 10.2 Graphical Balance of evaporator with a condensing unit

The graph also indicates the change in evaporating temperature and coil duty when the ambient is lower or higher than the design figure. This will show if there is any necessity to control the evaporating temperature in order to keep the correct plant operation.

Frequently, coil data will be available for a design air flow, but the system resistance reduces this flow to a lower value. There is a double effect: the lower­ing of the LMTD and lower heat transfer from the coil by convection.

The outer surface coefficient is the greatest thermal resistivity (compared with conduction through the coil material and the inside coefficient), and a rough estimate of the total sensible heat flow change can be made on the basis of:

H = constant X (V)0 8

Example10.2

An air-cooling coil extracts 45 kW sensible heat with air entering at 24°C and leaving at 18°C, with the refrigerant evaporating at 11°C. Estimate the cooling capacity at 95, 90 and 85% mass air flow.

Design mass air flow =——————— = 7.35 kg/s

1.02 X (24 — 18)

138 Refrigeration and Air-Conditioning An approximate analysis gives the following result:

Air flow (%)

100

95

90

85

Mass air flow (kg/s)

7.35

6.99

6.62

6.25

Air temperature on coil (°C)

24

24

24

24

AT for 45kW (K)

6

6.3

6.7

7.1

Air temperature off coil (°C)

1 8

17.7

17.3

16.9

LMTD, refrigerant at 11 °C (K)

9.7

9.5

9.2

9.0

H, in terms of design (from V0.8) (%)

100

96

92

88

Capacity (45 x h x LMTD)/9.7 (kW)

45

42.3

39.3

36.7

With all calculations involving convective heat transfer, it must be remem­bered that the figures are predictions based on previous test data, and not precise.

Posted in Refrigeration and Air Conditioning