Sensible heating and cooling

Sensible heat transfer occurs when moist air flows across a heater battery or over the coils of a sensible cooler. In the heater, the temperature of the medium used to provide the heat is not critical. The sole requirement for heat transfer is that the temperature shall exceed the final air temperature. In sensible cooling there is a further restriction: the lowest water temperature must not be so low that moisture starts to condense on the cooler coils. If such condensation does occur, through a poor choice of chilled water temperature, then the process will no longer be one of sensible cooling since dehumidification will also be taking place. This complication will not be discussed further here but is dealt with in sections 3.4 and 10.6.

Figure 3.3 shows the changes of state which occur, sketched upon a psychrometric chart. The essence of both processes is that the change of state must occur along a line of constant moisture content. The variations in the physical properties of the moist air, for the two cases, are summarised below:

Dry-bulb Enthalpy Humid volume Wet-bulb

Percentage saturation Moisture content Dew point Vapour pressure

Sensible heating

Increases

Increases

Increases

Increases

Decreases

Constant

Constant

Constant

Sensible cooling

Decreases

Decreases

Decreases

Decreases

Increases

Constant

Constant

Constant

EXAMPLE 3.3

Calculate the load on a battery which heats 1.5 m3 s-1 of moist air, initially at a state of 21°C dry-bulb, 15°C wet-bulb (sling) and 101.325 kPa barometric pressure, by 20 degrees. If low temperature hot water at 85°C flow and 75°C return is used to achieve this, calculate the flow rate necessary, in kilograms of water per second.

Heating medium in

Sensible heating and cooling

Cooling medium out

подпись: cooling medium out
 
Fig. 3.3 Psychrometry for sensible heating and cooling.

Answer

Heating load = , _ _

подпись: heating load = , _ _( mass flow of moist air expressed ^ in kg s"1 of associated dry air

(increase in enthalpy of moist air expressed^

^ in kJ per kg of associated dry air J

From CIBSE tables of psychrometric data (or, less accurately, from the CIBSE psychrometric chart), the initial enthalpy is found to be 41.88 kJ kg-1, the moisture content to be 8.171 g kg-1 and the humid volume to be 0.8439 m3 per kg of dry air. Since the air is being heated by 20 degrees, reference must now be made to tables in order to determine the enthalpy at the same moisture content as the initial state but at a dry-bulb temperature of 41 °C. By interpolation, the enthalpy of the moist air leaving the heater battery is found to be 62.31 kJ per kg of dry air.

Heating load = (0M9 ) x (62’31 — 4L88) = 363 kW

Flow rate of LTHW = (85o _37y} x42 = 0.864 kg s’1 where 4.2 kJ/kg K is the specific heat capacity of water.

EXAMPLE 3.4

Calculate the load on a cooler coil which cools the moist air mentioned in example 3.3 by 5 degrees. What is the flow rate of chilled water necessary to effect this cooling if flow and return temperatures of 10°C and 15°C are satisfactory?

Answer

The initial enthalpy and humid volume are the same as in the first example. The final dry — bulb temperature of the moist air is 16°C but its moisture content is still 8.171 g per kg of dry air. At this state, its enthalpy is found from tables to be 36.77 kJ per kg of dry air.

Cooling load = ^Qg4539 j x (41-88 — 36.77) = 9.1 kW

Chilled water flow rate = ^ ^ = 0.433 kg s"1

It is to be noted that, for sensible cooling, the selection of cooler coils and the choice of chilled water flow temperature require some care. See section 10.6.

Posted in Air Conditioning Engineering


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