Pre-heat and humidification with reheat

Air conditioning plants which handle fresh air only may be faced in winter with the task of increasing both the moisture content and the temperature of the air they supply to the conditioned space. Humidification is needed because the outside air in winter has a low moisture content, and if this air were to be introduced directly to the room there would be a correspondingly low moisture content there as well. The low moisture content may not be intrinsically objectionable, but when the air is heated to a higher temperature its relative humidity may become very low. For example, outside air in winter might be at -1°C, saturated (see chapter 5). The moisture content at this state is only 3.484 g per kg of dry air. If this is heated to 20°C dry-bulb, and if there is a moisture pick-up in the room of 0.6 g per kg of dry air, due to latent heat gains, then the relative humidity in the room will be as low as 28 per cent. This value may sometimes be seen as too low for comfort. The plant must increase the temperature of the air, either to the value of the room temperature if there is background heating to offset fabric losses, or to a value in excees of this if it is intended that the air delivered should deal with fabric losses.

The processes whereby the moisture content of air may be increased were discussed in section 3.5. The actual method chosen depends on the application but, for comfort air conditioning or for any application where people are present in the conditioned space, the use of an air washer or any method involving the recirculation of spray water, or the exposure of a wetted surface area to the airstream, is not recommended. This is because of the risk to health caused by the presence of micro-organisms in the water. The use of dry steam injection is much preferred.

As an exercise in psychrometry, it is worth considering the case where an air washer is used. If 100 per cent fresh air is handled in cold weather it is pre-heated, passed through an air washer where it undergoes adiabatic saturation, and reheated to the temperature at which it must be supplied to the room. Pre-heating and adiabatic saturation permit the relative humidity in the room to be controlled, and reheating allows the temperature therein to be properly regulated, in winter.

Figure 3.12(a) shows, in a diagrammatic form, a typical plant. Opening the modulating valve R1 in the return pipeline from the pre-heater increases the heating output of the battery and provides the necessary extra energy for the evaporation of more water in the washer, if an increase in the moisture content of the supply air is required. Similarly, opening the control value R2, associated with the reheater, allows air at a higher temperature to be delivered to the room being conditioned. Cl and C2 are a room humidistat and a room thermostat, respectively.

EXAMPLE 3.13

The plant shown in Figure 3.12(a) operates as illustrated by the psychrometric changes in Figure 3.12(b).

Air is pre-heated from -5.0°C dry-bulb and 86 per cent saturation to 23°C dry-bulb. It is then passed through an air washer having a humidifying efficiency of 85 per cent and using recirculated spray water. Calculate the following:

(a) The relative humidity of the air leaving the washer

(b) The cold water make-up to the washer in litres s_1, given that the airflow rate

Leaving the washer is 2.5 m3 s"1

(c) The duty of the pre-heater battery in kW

(d) The temperature of the air supplied to the conditioned space if the sensible heat

Losses from it are 24 kW and 20°C dry-bulb is maintained there

(e) The duty of the reheater battery

(/) The relative humidity maintained in the room if the latent heat gains therein are 5 kW

Answer

(a) At state O the moisture content is found from tables to be 2.137 g kg-1. Consequently, at 23°C dry-bulb and 2.137 g kg-1 the wet-bulb (sling) value is found from tables to be 10°C. This state is represented by the point A in the diagram. Assuming that the process of adiabatic saturation occurs up a wet-bulb line, we can establish from tables that the moisture content at state C, the apparatus dew point, is 7.659 g kg-1. If the further assumption is made that the dry-bulb scale is linear, we can evaluate the dry-bulb temperature at state B, leaving the washer, by proportion:

Tb = 23 — 0.85(23 — 10)

= 23-11.05 = 12°C dry-bulb

We know that the dry-bulb temperature at the point C is 10°C because the air is saturated at C and it has a wet-bulb temperature of 10°C.

Pre-heat and humidification with reheat

(&)

Fig. 3.12 (a) Plant arrangement for pre-heating 100% fresh air with adiabatic saturation and reheat.

(b) The psychrometry for Example 3.13.

подпись: 
(&)
fig. 3.12 (a) plant arrangement for pre-heating 100% fresh air with adiabatic saturation and reheat.
(b) the psychrometry for example 3.13.
The above answer for the dry-bulb at В is approximate (because the dry-bulb scale is not linear) but accurate enough for all practical purposes. On the other hand, we may calculate the moisture content at В with more exactness, since the definition of humidifying efficiency is in terms of moisture content changes and this scale is linear on the psychrometric chart.

Gb = 7.659 — (1 — 0.85) x (7.659 — 2.137) = 7.659 — 0.828 = 6.831 g kg“1

We can now refer to tables or to a chart and determine that the relative humidity at 12°C dry-bulb and 6.831 g kg-1 is 78 per cent. Percentage saturation is virtually the same as relative humidity. More accurately, we might perhaps determine the humidity at a state of 10°C wet-bulb (sling) and 6.831 g kg-1. The yield in accuracy is of doubtful value and the method is rather tedious when using tables.

(.b) The cold water make-up depends on the mass flow of dry air. It may be determined that the humid volume of the air leaving the washer at state B is 0.8162 m3 kg-1 of dry air. The cold water fed from the mains to the washer serves to make good the losses due to evaporation within the washer. Since the evaporation rate is (6.831 — 2.137) g kg-1, we may calculate the make-up rate:

, (6.831 — 2.137) x 2.5

Make-up rate = 1000xa8162

= 0.0144 kg s“1 or litres s_1

(c) The pre-heater must increase the enthalpy of the air passing over it from h0 to /ia. Reference to tables establishes that h0 = 0.298 kJ kg-1 and h, d = 28.57 kJ kg-1. Then,

U. 2.5 x (28.57 — 0.298)

Pre-heater duty =————- 08162————

= 3.063 kg s“1 x 28.27 kJ s“1

= 86.59 kW

(d) 3.063 kg s“1 of air diffuses throughout the room and its temperature falls from td to 20°C as it offsets the heat loss. Assuming the specific heat capacity of water vapour is 1.89 kJ kg-1 K_1 and that of dry air 1.012 kJ kg-1 K_1, a heat balance equation can be written:

Sensible heat loss = (mass flow rate of dry air x

Specific heat of dry air + associated moisture x specific heat of water vapour) x (supply air temperature — room temperature)

24 = 3.063 x (1.012 + 0.006 831 x 1.89) x (fd -20)

= 3.063 x 1.025 x (rd — 20) whence rd = 27.6°C

The term involving the specific heat of dry air plus the moisture content times the specific heat of water vapour, is termed the specific heat of humid air or the humid specific heat. In this case its value is 1.025 kJ kg-1 K-1.

(e) The reheater battery thus has to heat the humid air from the state at which it leaves the washer to the state at which it enters the room, namely from 12°C dry-bulb and

6.831 g kg-1 to 27.6° dry-bulb and 6.831 g kg-1.

Reheater duty = 3.063 x 1.025 x (27.6 — 12)

= 49 kW

Alternatively, we can determine the enthalpies of the states on and off the heater, by interpolating from tables or reading directly from a psychrometric chart:

Reheater duty = 3.063 x (45.2 — 29.3)

= 48.7 kW

(/) A mass balance must be struck to determine the rise in moisture content in the room as a consequence of the evaporation corresponding to the liberation of the latent heat gains:

Latent heat gain in kW = (kg of dry air per hour delivered to the room)

X (the moisture pick-up in kg of water per kg of dry air)

X (the latent heat of evaporation of water in

KJ per kg of water)

5.0 = 3.063 x (gr — 0.006 831) x 2454 whence gr = 0.007 496 kg per kg dry air

From tables or from a chart it may be found that at a state of 20°C dry-bulb and 7.496 g

Per kg dry air, the relative humidity is about 51 per cent and, for use in Example 3.14(b), h{ = 39.14 kJ kg“1.

Posted in Air Conditioning Engineering


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