Direct-expansion air cooler coils
We have already seen in chapter 10 how the capacity of an air cooler coil using chilled
Water may be expressed and these considerations also apply when a refrigerant is evaporated directly inside the tubes of the coil, except that the logarithmic mean temperature difference is between the airstream and the evaporating temperature. If it is assumed that the coil is entirely wet with condensate then wet bulbs may be used as an indication of performance since they are almost parallel to lines of constant enthalpy.
Catalogues often rate DX coils for a range of evaporating temperatures and offer a variety of dry — and wet-bulb temperatures for the entering state of the air, against which the condition of the air leaving the coil is quoted. Interpolation is possible to a limited extent and Table 12.1 gives a small part of a typical catalogue, modified to suit the working of examples which follow.
Table 12.1 Typical performance of a direct expansion air cooler coil with aluminium plate fins spaced at 317 per metre and for an entering air state of 23.9°C dry-bulb, 16.7°C wet-bulb (sling)
The mixture of fresh and recirculated air on to a DX cooler coil in an air handling unit is at 23.9°C dry-bulb, 16.7°C wet-bulb (sling). Use Table 12.1 to determine the state of the air leaving a 4-row coil with a face velocity of 2.5 m s-1 when the evaporating temperature is 5.6°C. Check the psychrometry.
By interpolation we have a leaving state of 12.3°C dry-bulb and 11.2°C wet-bulb (sling). Figure 12.3 shows the psychrometry and the practicality of the coil performance: the points MWA lie in a straight line and the saturation curve is cut at A. The contact factor is (23.9 — 12.3)/(23.9 — 9.8) = 0.82, which is appropriate for a four-row coil with a face velocity of
2.5 m s’1.
Assuming the results of example 12.2 and a rise of 1.2 K to cover fan power and duct gain, calculate the supply air quantity and cooling load to maintain a room at 22°C dry-bulb, 8.071 g kg’1 (about 48 per cent saturation) in the presence of sensible heat gains of 31.13 kW, when it is 28°C dry-bulb, 19.5°C wet-bulb (sling), outside.
Supply air temperature = 12.3° + 1.2° = 13.5°C
„ • ~ , • • 31 13 (273 + 13.5)
By equation (6.6) supply air quantity = (22-~13~5) X————— 358——-
= 2.931 m3 s“1 at 13.5°C
Plotting the relevant states on a psychrometric chart (Figure 12.3) and reading off enthalpies across the coil and the specific volume at the supply state we have
Cooling load = 2’^1 x (46.66 — 32.02) = 52.2 kW of refrigeration
U. o 1L
_______ i__ I i_______________ I i________ i______
9.8°12.3° 22° 23.9° 28°
Temperature, °C Fig. 12.3 Psychrometry for examples 12.2 and 12.3.
The performance of the air cooler coil can be expressed graphically and this shows its behaviour at partial load and what happens when it is piped up to a compressor, condenser and expansion valve. Two characteristic curves may be plotted (see Figure 12.4): one for the air-side perfromance and the other for the refrigeration-side behaviour, with the assumption that the coil surface is entirely wet with condensate. Referring to Figure 12.4 we see that the abscissa can be interpreted as wet-bulb temperature or evaporating temperature, as convenient. The point P can then be identified by the design cooling load (52.2 kW) and the wet-bulb temperature leaving the coil under design conditions (11.2°C). If the wet-bulb leaving the coil were the same as that entering, namely, 16.7°C, this would imply no change of enthalpy either and would correspond to zero cooling load. We can therefore plot
Temperature in °C: evaporating or wet-bulb, as convenient
Fig. 12.4 Direct-expansion cooler coil characteristics.
A zero load point, O, at 16.7°C and 0 kW duty. The points O and P lie on the air-side characteristic and may be joined by a straight line for all practical purposes and for simplicity. Reference to Table 12.1 and example 12.2 shows that we can achieve a leaving wet-bulb of 11.2°C when the entering wet-bulb is 16.7°C and we have an evaporating temperature of 5.6°C. Another design duty point, Q, is plotted at a cooling load of 52.2 kW and an evaporating temperature of 5.6°C in Figure 12.4. The zero load point, O, also corresponds to an evaporating temperature of 16.7°C, the same as the on-coil and off-coil wet-bulb. The points O and Q are then joined by a straight line to give the refrigerant-side characteristic for the coil under the same conditions as the air-side characteristic.
If the entering wet-bulb temperature reduces, the pair of lines (air and refrigerant) moves to the left, remaining parallel to the design pair but having a different zero load point, at a value equal to the lower entering wet-bulb. For example, Figure 12.4 shows such a pair starting from a zero load point at 4°C on the abscissa. If the coil is only partly wet or if the face velocity falls, capacity is reduced for a given evaporating temperature, as shown by the pair of broken lines in Figure 12.4.
Posted in Engineering Fifth Edition