Warm air supply temperatures

Air density is inversely proportional to its absolute temperature (see section 2.5). As a consequence, if the temperature difference between the warm supply air and the room air is too great, stratification will occur. The layer of air beneath the ceiling will be higher than the room temperature, under heat loss conditions, and the air in the occupied part of the room will be uncomfortably cool. This places an upper limit on the supply air temperature when it is used to offset heat losses in winter. Although supply air temperatures of more than 60°C have been used in the past, with direct-fired warm air heaters, these were with high discharge air velocities (10 m s_1) from the supply openings and were for industrial applications.

It is recommended that, for comfort air conditioning in commercial applications, the maximum supply air temperature should not normally exceed 35°C, when room temperatures are about 20°C. The absolute limit should be 40°C. The risk of stratification is made worse if the face velocity at the supply grille is low and also if the extract or recirculation grilles are at high level. Large floor-to-ceiling heights, with supply grilles at high level, increase the chances of cold discomfort from undesirable stratification.

A consequence of this is that achieving a boosted heating capacity during pre-heat periods with intermittently operated systems is not always easy when warm air is used, supplied at high level.

1. A room is to be maintained at a condition of 20°C dry-bulb and 7.376 g kg-1 moisture content by air supplied at 15°C dry-bulb when the heat gains are 7 kW sensible and 1.4 kW latent. Calculate the weight of air to be supplied to the room and the moisture content at which it should be supplied. Take the latent heat of evaporation at room condition as 2454 kJ kg’1.

Answers

1.365 kg s"1 dry air, assuming a value of 1.026 kJ kg-1 K~] for the specific heat of humid air; 6.958 g kg-1.

2. Briefly discuss the factors to be considered when selecting

(i) a design room temperature,

(ii) the temperature differential between supply and room air.

3. An air conditioning plant comprising filter, cooler coil, fan and distributing ductwork uses only fresh air for the purpose of maintaining comfort conditions in summer. Using the information listed below, choose a suitable supply air temperature, calculate the cooler coil load and determine its contact factor.

Sensible heat gains to conditioned space: 11.75 kW.

Latent heat gains to conditioned space: 2.35 kW.

Outside design state: 28°C dry-bulb, 19.9°C wet-bulb (sling).

Inside design state: 21°C dry-bulb, 50 per cent saturation.

Temperature rise due to fan power and duct heat gains: 1°C. A psychrometric chart should be used.

Answers

Suitable supply air temperature is about 11°C: corresponding cooler coil load is 33.1 kW; corresponding contact factor is 0.88.

Notation

Symbol	Description	Unit
A	Surface area	M2
C	Specific heat capacity of air	KJ kg"1 K"1 J kg’1 K"1
G	Moisture content	G per kg dry air
G a	Moisture content at the apparatus dew point	G kg"1
§0	Moisture content of outside air	G kg 1
8r	Room air moisture content	G kg 1
Gs	Supply air moisture content	G kg-1
H&	Enthalpy of air at the apparatus dew point	KJ kg"1
Hз,	Enthalpy of the outside air	KJ kg-1
/iw	Enthalpy of the air leaving the cooler coil	KJ kg-1
/Zfg	Latent heat of evaporation	KJ kg-1

M mass flow rate of dry air and its associated water vapour kg s’1

TOC o "1-5" h z n air change rate per hour h~’

Q rate of production of heat kW or W

T absolute dry-bulb temperature K

T temperature °C

Rchw chilled water flow temperature °C

T0 outside dry-bulb temperature °C

TT room dry-bulb temperature °C

Ts supply air temperature °C

Tw dry-bulb temperature of air leaving the cooler coil °C

U overall thermal transmittance W m-2 K“1

V volume m3

V specific volume m3 per kg dry air

V volumetric flow rate of air m3 s“1 or litres s’1

Vt volumetric flow rate of air at temperature t m3 s~’ or litres s_1

3 Contact factor —

P density of air kg m-3

Pt air density at temperature t kg rrf3

Po standard air density kg m-3

T) total fan efficiency %

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