The use of dry steam for humidification

It is only if the air can accept the additional moisture, that humidification may be achieved by the injection of spray water or dry steam. If the state of the air into which it is proposed to inject moisture is saturated, or close to saturation, some or all of the moisture added will not be accepted and will be deposited downstream in the air handling plant or the duct system. It is therefore essential to ensure that the airstream is sufficiently heated prior to moisture injection.

EXAMPLE 3.15

A room is to be maintained at a state of 20°C dry-bulb and 50 per cent saturation by a plant handling 0.5 m3 s’1 of outside air at a state of -2°C saturated. The airstream is heated to

A temperature warm enough to offset a heat loss of 2.5 kW and dry steam is then injected to maintain the humidity required in the room. Calculate the supply air temperature and the heating and humidification loads. See Figure 3.14.

Answer

From psychrometric tables or a psychrometric chart, air at the outside state has a moisture content of 3.205 g kg-1, an enthalpy of 5.992 kJ kg-1 and a specific volume of 0.7716 m3 kg-1. Assuming specific heats of 1.012 and 1.89 kJ kg-1 K-1, respectively, for dry air and water vapour, the humid specific heat of the fresh air handled is

1 x 1.012 + 0.003 205 x 1.89 = 1.018 kJ kg’1 K-1

The heat loss is offset by the supply of air at a temperature fs, warmer than the room temperature of 20°C. Hence

2.5 = (0.5/0.7716) x 1.018 x (ts — 20)

Whence ts = 23.8°C.

From tables or a chart the enthalpy at state B, leaving the heater battery, is 32.10 kJ kg-1. See Figure 3.14. Hence the heater battery load is

(0.5/0.7716) x (32.10 — 5.992) = 16.92 kW

Alternatively

(0.5/0.7716) x 1.018 x (23.8 + 2) = 17.02 kW

The first method is preferred because enthalpy values are based on well-established thermodynamic properties of dry air and water vapour whereas, on the other hand, the values of the specific heats used for the second method may not be exactly correct.

Assuming the change of state resulting from the injection of dry saturated steam is up a dry-bulb line, the state of the air supplied to the room is 23.8°C dry-bulb and 7.376 g kg’1, at which the enthalpy is determined as 42.71 kJ kg-1 from tables or a chart. Hence the humidification load is

(0.5/0.7716) x (42.71 — 32.10) = 6.88 kW

Posted in Engineering Fifth Edition