The choice of outside design conditions

There are three ways of establishing outside summer design states:

(1) Use mean daily and monthly meteorological data in the following way:

(i) Choose the highest mean monthly maximum temperature as the design dry-bulb value.

(ii) For the same month used in (i) couple the mean daily maximum temperature and the afternoon record of relative humidity, if necessary using equation (5.4) to adjust the mean daily maximum to coincide with the time of the humidity value.

(iii) With the temperature and humidity coupled in (ii) determine the corresponding moisture content from psychrometric tables or a chart.

(iv) Use the moisture content from (iii) with the mean monthly maximum temperature to establish the corresponding screen or sling wet-bulb temperature.

As

Dry-bulb temperature, °C

Fig. 5.4 Frequency of occurrence of coincident dry-bulb and wet-bulb temperatures over the total hours in the months June-September at Heathrow. (Reproduced by kind permission of the CIBSE from Guide A, Environmental Design (1999).)

(v) Select the mean monthly maximum temperature (i) and wet-bulb obtained from

(iv) as the outside summer design state.

(2) Choose outside dry — and wet-bulb values in terms of their frequency of occurrence in the four summer months in the UK. Table 5.3 and Figure 5.4 should be referred to. In the past, it has been common to select an outside summer design state on a basis of a 1 per cent occurrence in the four summer months over the nine occupied hours in the day, rather than over the full 24 hours. This has yielded 27°C dry-bulb and 19.5°C wet-bulb (screen), referred to a non-urban location, such as the former meteorological station at Kew. Both Table 5.3 and Figure 5.4 refer to a frequency of occurrence over 24 h. The likelihood of warm temperatures occurring during the occupied period of about eight or nine hours during the day is three times the percentage quoted for the full 24 hours. Hence we should be looking for a figure of about 0.33 per cent, in the vicinity of high dry — and wet-bulb temperatures, as a basis for our outside design state. Possibilities are: 0.31 per cent for 28°C to 30°C dry-bulb with 18°C to 20°C wet-bulb (screen), or 0.32 per cent for 26°C to 28°C dry-bulb with 20°C to 22°C wet-bulb (screen).

Making the actual choice needs further consideration. Global warming forecasts vary but something like 0.25 K per decade seems to be a possible rise in outside air temperature in southern England over the next few years. Accountants appear to allow 60 years for the life of an office block and it is very likely that an air conditioning system would be near the end of its useful life after 20 to 25 years, bearing in mind normal wear and obsolescence. This suggests that half a degree should be added to the dry-bulb with perhaps a similar increase for the wet-bulb, to cater for future possible increases in system performance over the life of the building. However, the choice is open to further thought: choosing a lower outside air temperature has an effect on the capital cost of the proposed design and this must be weighed against some measure of dissatisfaction that might result. Example 7.19 examines some of the consequences of choosing different inside and outside design conditions.

The foregoing leads to a view that, after correcting for the difference between screen and sling wet-bulbs, suitable outside choices for an open area near London might be 29°C dry-bulb, 18.3°C wet-bulb (sling) or 27°C dry-bulb, 20.7°C wet-bulb (sling). These states have corresponding moisture contents of 0.008616 kg kg-1 and 0.01264 kg kg-1. An addition of 0.5 K should be made to cover global warming over the life of the plant to give dry-bulbs of 29.5°C and 27.5°C. It is unlikely that the moisture content of the outside air will be constant because vegetation will dehydrate as air temperatures rise. A decision on this is speculative but one might suppose that the wet-bulb temperatures could increase by about half the rise in dry-bulb temperatures because, as we see on the psychrometric chart, the scale of wet-bulb temperature is about twice that of dry-bulb temperature, in the vicinity of 30°C dry-bulb and 20°C wet-bulb. The suggestion is that the wet-bulbs be increased by 0.3 K and a suitable outside design state for an open area near London could be 29.5°C dry-bulb with 18.6°C wet-bulb (sling), or 27.5°C dry-bulb with 21°C wet-bulb (sling), the corresponding moisture contents being 0.008 746 and 0.012 84 kg kg-1. The matter does not end here because, as we shall see, the presence of buildings in London makes a further rise in the outside design dry-bulb necessary.

(3) Adopt what is chosen by common custom, locally. For London, in the past, a value of 28°C dry-bulb, 19.5°C wet-bulb (sling) has been commonly adopted. This is no longer likely to be satisfactory over the next few decades.

In conurbations the mass of the building absorbs radiant solar heat during the day and releases this into the atmosphere later on. Cities are thus warmer areas than the surrounding countryside (often referred to as ‘heat islands’). According to Chandler (1965) the influences at work are: the climate of the region, local morphology, the thermal properties of the congregation of buildings and surfaced roads: apparently each building in an urban complex has its own micro-climate which, because of local influences, may be significantly different from that of its neighbours. In the middle of London mean annual temperatures are about 1K to 1.5 K warmer than those in the surrounding countryside. Dry-bulb temperatures after sunset in central London may be 5 K above those in the outlying country because of the heat released from the buildings in calm weather.

Bearing in mind the foregoing, the two dry-bulb temperatures considered suitable for an open area near London should be increased by 0.5 K to 30°C and 28°C to make them appropriate for London itself. It is argued that the higher dry-bulb is not likely to raise the moisture content because of the much smaller amount of vegetation in the conurbation, compared with open country. So the two outside states to be considered are 30°C dry-bulb,

0. 008 746 kg kg-1, 18.7°C wet-bulb (sling) and 28°C dry-bulb, 0.01284 kg kg-1, 20.8°C wet-bulb (sling). It is suggested that an arithmetic mean be adopted and a suitable outside summer design choice taken as 29°C dry-bulb, 20°C wet-bulb (sling), for which the moisture content is 0.010 80 kg kg *, the enthalpy is 56.76 kJ kg 1 and the specific volume is

0. 8705.m3 kg“1.

When sizing air-cooled condensers and cooling towers, it is recommended that their selections are based on at least 30°C dry-bulb and 20.5°C wet-bulb (sling), respectively.

The selection of outside winter design temperatures is not made in the same way, the reason being that most people are at home in bed at the time of lowest temperature, about one hour before sunrise. For a building occupied during normal office hours, an outside design dry-bulb of -1°C or -2°C is often chosen and is satisfactory, although it should be noted that for purposes of sizing heater batteries that handle 100 per cent outside air, an air temperature of about -5°C should be adopted because the thermal inertia of the building plays no part in mitigating the effect on a heater battery of a drop in temperature below the design value, as it does for comfort temperatures within a building. The CIBSE Guide A2 (1999) recommends choosing an outside design temperature for winter in terms of the frequency of occurrence of low temperatures and the thermal inertia of the building. Reference should be made to the Guide for the selection of winter design conditions.

According to Chandler (1965) records at Kew show that earth temperatures vary in London from a mean monthly minimum of 0.6°C in January to a maximum of 19.8°C in July, with annual averages in those months of 4.2°C and 17.8°C, at 300 mm below the surface. The corresponding values at a depth of 1200 mm are 5.4°C, 17.0°C (in August) and 15.8°C (in August).

Exercises

1. (a) Briefly explain the causes of outside air diurnal variation of temperature, accounting for the times at which maximum and minimum values occur.

(b) Briefly explain what differences there might be and why they occur, in the daily and annual temperature ranges between two places on the same latitude, one place being classified as a dry tropical region and the other a humid tropical region. Similarly, account for the differences between a temperate coastal region and a temperate inland region.

(c) Tabulate factors affecting the choice of outside design condition for air conditioning a modern building.

2. Briefly describe a simple method of estimating summer outside air conditions for the design calculations of an air conditioning scheme. If a more accurate assessment of heat gains to a building is required, explain what further meteorological information is wanted.

Notation

Symbol	Description	Unit
B	Angle factor	—
C	Cloud cover factor	—
D	Diurnal variation in outside dry-bulb temperature	K
A, W	Long-wave thermal radiation from a surface to the sky	W rrf2
K	Vapour correction factor	—
Ps	Vapour pressure	Mbar
To	Outside air dry-bulb temperature	°C
H	Surface temperature	°C
F15	Outside dry-bulb temperature at 1500 h sun-time	°C

Outside dry-bulb temperature at time 0

°C Degrees H

Acute angle between a surface and the horizontal

Sun-time

Posted in Engineering Fifth Edition