Air handling units (ahu) are often regarded as simple boxes, designed to supply and extract air in accordance with the design conditions. However, the range of alternatives available is very broad.
Many factors must be addressed when selecting an ahu. The optimum design solution involves the selection of the individual components. Consequently, the final design will be the product of a complex balancing act involving performance issues, space requirements and capital cost.
In this cost model the key design criteria is examined, and summary costs provided for alternative specifications. Note that the comparisons given refer to the air handling units only, and do not cover the full ventilation system.
KEY SELECTION CRITERIA
Plant life expectancy, servicing issues and running costs must all be considered when specifying an ahu's components.
The material specification must be scrutinised to identify all areas that can impact on the physical materials of construction and capital cost of the unit. These factors include coil and fan construction, types of motor, air filtration and humidifiers, unit finishes and electrical requirements.
The performance specification must also be checked to determine the working limits of components, such as coil face, filter and fan discharge velocities, duty tolerances, and maximum air and water pressure drops.
If the working limits are beyond the "normal industry standard" then non-standard components will be required. The performance specification may also impose more onerous demands on the rest of the ventilation system.
The space available for plant is always at a premium. The size and layout of plantrooms may affect the configuration of ahus and, if supply and extract units cannot be collocated, may restrict the use of systems such as thermal wheels.
In particular, access needs to be carefully assessed in relation to the maintenance of consumable items such as filters, and the long-term requirement for replacement of heavy items such as motors or coils.
The fitting of unit-mounted acoustic treatment for system and atmospheric noise control will have a significant impact on the overall space requirement of the plant.
The size and configuration of air handling plant will also vary in relation to its location. Externally sited units, designed for lower operating velocities, require a larger louvre surface area than internal units, which are typically fitted with dampers.
Any abnormal specification requirements, which effect capital cost, unit size or performance, need to be identified at an early stage so that cost and performance can be confirmed.
The basic unit requirements of ahus are:
The tailoring of ahus to project set requirements can lead to a significant variation in unit construction, size and specification. Once a complete specification is available, the selection and pricing of the units can be undertaken accurately.
In the absence of full specification details, it is possible to design a set of templates for standard applications, which detail typical solutions for fresh air and return air units, and supply and extract units. These can be used to provide relatively accurate costing for different duties at an early stage of design.
Air handling units are high-energy users and their specification will have a significant impact on the total energy consumption of a building. For example, ahus serving four-pipe fan coil systems, which are typically supplied with minimum fresh air, consume approximately 50% of the total energy requirement of an office building. The consideration of energy reclaim through recuperators is therefore particularly important.
The different types of recuperators (plate heat exchanger, thermal wheel, and run-around coil) vary in size, efficiency and cost. When specifying a recuperator system, the additional energy consumption due to running the system must also be considered.
Additional energy loads include increased fan motor power in the ahu, and increased pump motor power with the run-around coil.
Table 1, above right, shows the average efficiencies of the various types of recuperator based on manufacturer's information. These values equate to the amount of energy reclaimed from a unit of exhaust air. The efficiencies do not take into account the above stated additional energy loads required to operate them.
The selection of the primary heat source for ahus can have a substantial effect on capital costs. The main alternatives are water heated systems based on conventional lthw distribution, or direct gas firing of the ahu.
Gas-fired air handling units are, as table 2, above right, demonstrates, more expensive than their water heated equivalents. However, once the costs of lthw distribution are included, it is evident that the specification of gas-fired units can result in significant cost savings.
CASE STUDY
A simple case study can illustrate the benefits of specifying externally mounted gas-fired ahus. In this example, the potential saving related to the use of gas-fired equipment is £60 000 (over £8/m2 gross floor area).
The example features an office building with a gross floor area of 7500 m² over five storeys, with an air conditioning system based on four-pipe fan coil units. Roof-mounted air handling plant is rated at 15 m³/s and minimum fresh air component based on 2 litres/m² of gross internal floor area. Boiler plant would be located in the basement.
Other advantages related to using gas-fired air handling plant and which need to be considered include greater efficiency, as the energy transfer does not involve any change in medium.
Running costs are also reduced due to increases in efficiency and the omission of pump motor power from the lthw circuit.
The disadvantages of a gas-fired system must also be considered. These include:
Indirect gas-fired air handling units are also available. These have the same running cost advantages of direct units. The disadvantages of these systems include limited controllability, condensate moisture build-up within the heat exchanger, and the limited life expectancy of the heat exchanger (typically ten years).
Source
Building Sustainable Design
