MASTERCLASS – AIR CONDITIONING TECHNOLOGY

Volume 48 – Fan Coil Units

In last month’s article (VOL 47) we commenced a simple introduction to Fan Coil Units.  We now continue with this subject with the intention of providing a detailed overview of this important approach to air conditioning, which is currently realising increasing popularity.

Fan-Coil Units

Fan Foil Units (FCU’s) provide a very effective solution to cooling of multiple room building situations and are therefore extensively used for hotels, large office blocks, and office situations in other types of buildings such as factories.   FCU’s can also be applied to hospitals and numerous specialist applications.  There are also larger capacity, industrial Fan Coil Units that are used, for example, in factory production areas.  These are usually more rugged and do not offer the options of attractive casing for example, as is the case with typical smaller ranges.   They are of considerably higher capacity the FCU’s used for comfort air conditioning.

Clearly, the first step in selecting Fan Coil Units for an air conditioning system is to determine the cooling and heating loads for each of the room(s) to be served.

The heat gains must be conducted using calculation methods of the highest degree of accuracy, since under-sizing of the fan coil unit will result in the room design condition not being attained under maximum summer ambient conditions and minimum winter ambient temperatures when heating is required.  Over-sizing, either by incorrect calculations or deliberately, to be “on the safe side”, is also undesirable since this will lead to excess unnecessary expenditure on not only the Fan Coil Units, but also the associated pipework, pumps and control valves and probably the central station chiller(s).  Temperature control within the conditioned area may also be erratic.

Costly mistakes can be avoided by conducting heat gain and loss calculations in accordance with the CIBSE Design Method (if you know how and have the time to do this!) or by using respected software.   Advanced products such as these are very simple and fast to use, handle multiple room buildings and, since they are a relatively low cost product, are an ideal solution to the massive problem of correct equipment sizing for both small and large costly installations.

Having completed the heat gain and heat loss calculations, the appropriate unit(s) of the correct capacity can be selected.  The conditioned space can served by one or more FCU’s and the number of FCU’s required to provide the specified conditions will be based upon adequate air distribution to provide even temperature and comfort whilst avoiding dead zones.  FCU’s must also be located so as not to create uncomfortable draughts.

Having decided upon the size and number of FCU’s, the absorbed fan power must be considered.  The specified noise level and required thermal capacity will be used to determine the fan speed to be used since both increase as the fan speed is raised.  Many FCU’s are fitted with what is termed an Auto-Transformer.  This is simply a transformer which takes 240 Volt A.C. mains supply and provides multiple tappings of different voltages to enable the fan to be run at, for example, 6 alternative speeds.  This is also termed a Multi-Tapping Transformer.  Tappings 1 to 6 can then be used to govern the motor speed.  If tapping 2 is used, for example, and it transpires that more or less capacity is required or the fan noise is a little too high, then the fan speed can be raised or reduced using tappings 1 or 3 instead.  The loads and application for the room in question may also change in the future and this facility can again be used to good advantage to adjust the capacity to meet the new thermal load.

Absorbed Fan Power

The fan motor fitted to a specific Fan Coil Unit must be sized and rated (in kW or horsepower) sufficiently to deal with the internal resistance of components (coils, filter, casing, etc.).  The fan motor will also have to handle the required external static pressure where the FCU has to supply air to flexible ductwork and distribution air diffusers for example.  However, at lower fan speeds, the internal and external resistance will fall and the actual power absorbed by the motor will reduce.   The sensible heat energy released by the fan motor is a function of the absorbed power, not the stamped overall power rating of the motor.

Accordingly, the amount of heat energy released by the fan-motor assembly into the air stream (and consequently the room being conditioned) needs to be established and this figure must then added to the sensible gains for the conditioned space.  Alternatively, this heat energy can be deducted from the sensible cooling capacity of the FCU.

Another approach is to include an estimated allowance for fan motor heat gain in the original load calculations.  In some cases, the FCU manufacturer will declare the net cooling performance at different fan speeds/external static pressures, having deducted the fan motor heat gain from the actual cooling coil performance.

It is interesting to remember that the fan motor heat gain works of course to our advantage when the FCU is running in heating mode and there is no reason why this benefit should not be taken into account when selecting the appropriate FCU(s) for the application.

While calculating the cooling loads for the conditioned space, the Sensible Heat Ratio of the room will have been established thus identifying the cooling characteristics of the room.  With this information, the optimum coil selection can be specified which will result in minimum system operating costs.  For example, if the cooling characteristics of a coil result in excessive moisture removal, it would then be necessary to run the humidifiers within the Fresh Air AHU constantly, with a severe penalty on operating costs and GWP.  According to the type of humidifier, this may result in further sensible heat being added to the room.

Conversely, if moisture removal were inadequate, increased humidity within the room could make occupancy conditions unpleasant and may even cause damage to the fabric of the building.  In order to reduce humidity levels, the central plant would have be able to operate in dehumidification mode.  This would probably require a lowering of the temperature of the cooling medium in order to increase the rate of condensation from the cooling coils.  Unnecessary dehumidification and humidifying represent excessive running costs.

FCU’s can utilise either Direct Expansion (DX) coils (evaporators) or water-cooled coils (heat exchangers).   Each Fan-Coil Unit can be a part of a multiple FCU installation connected to central cooling plant.  This could be a water chiller or a large DX system.

Each system type has its pro’s and con’s which we endeavour to list below.  The strength of each argument in the pro’s and con’s list will, to a large extent, depend upon the level of expertise in each of the disciplines involved, together with the resources available within a company who plan to install the system.

The Pro’s & Con’s of DX based Fan-Coil Systems

The Pro’s

The primary advantage of the DX system is that it is thermodynamically more efficient than a chilled water system.  This is essentially due to the absence of a refrigerant-to-water heat exchanger which experiences heat exchange losses and requires pumping energy for the water.  The heat exchanger and all the flow and return chilled water pipework, whilst insulated, also experience continual heat gain form the surroundings.  DX system liquid refrigerant pipework requires no insulation, loses heat energy and actually benefits from this situation.

However, while this appears to confer a significant theoretical advantage over the chilled water system, in practice, the advantage is reduced by virtue of the a greater heat exchange efficiency achieved by utilising all the heat exchanger internal surface area on water cooled FCU’s. Whereas the need for a DX system to use an thermostatic expansion valve requires between 4°K and 6°K of superheat to be generated within the evaporator resulting in only 80% of the internal surface of the evaporator being used efficiently.

An advantage of the DX Fan Coil however lies in the requirement for a smaller cooling coil (say 1 row).  This is due to the cooling power of the evaporating refrigerant and the fact that the refrigerant evaporates at nearly a constant temperature throughout the coil leading to a large TD (Temperature Difference) between the entering air and the coil surface.  Chilled water however may require 3 cooling rows, since the water can rise in temperature by only a few degrees (5 – 7 K) and in so doing, reduces the overall TD between the air and the coil surface temperature.  A greater mass of water is also required for the same cooling heat exchange than that required by refrigerant.

The capital outlay for a DX multiple FCU system is usually less than a FCU system that uses chilled water, particularly when a small number of FCU’s are required.  For a small system of up to three FCU’s for example, chilled water systems are very rarely viable.

The forms of heating available on DX based FCU’s are the same as on Water Cooled systems.  Both systems can use electrical resistance heaters or low pressure hot water (LPHW) heating coils.  However, the DX system has a major advantage in that it can also use reverse cycle mode (Heat Pump).  As is generally well known, this mode of operation does not necessitate any additional pipework although the controls will be more complex.  There are of course, Reverse Cycle Heat Pump Water Chillers which can bring major heating efficiency advantages to water based systems.

Dehumidification is more easily built into a DX system.  Very often, all that is required is either a dual function suction pressure regulator or a by-pass around a single function suction pressure regulator thereby allowing the system to reduce the evaporation temperature by a couple of degrees.

The Con’s

A significant detractor for using DX FCU’s is the potential for refrigerant leakage and the larger the system and number of joints, the greater the potential.  This potential for leakage requires greater levels of skill in piping the system due to the greater pressures within a DX systems and the fact that the refrigerant is very fine from a molecular viewpoint and will easily leak from any joint with the tiniest flaw.  The refrigerant is also in vapour form in a good part of the circuit and can easily leak form defective joints in these regions.  The ability to find leaks is significantly reduced if any of the FCU’s and pipework is concealed behind decorative or architectural features.

Most DX systems are not as flexible as water-cooled systems in terms of load variation or from the point of view of changing the locations of FCU’s.  This is due to pipe sizing considerations, oil return and system load balancing.  There are sophisticated, multi-split DX based systems available which address these problems adequately, albeit at a cost.  These will be covered in great detail later in the Masterclass Series.

On small to medium size installations, load variations between FCU’s can be difficult to accommodate due to the need to match compressor capacity to evaporator load.  On small compressors, capacity control is very often achieved by discharge gas by-pass (Hot Gas By-Pass) while on medium size installations, the steps of capacity control are likely to be 50% or 33% and may have to be augmented with discharge gas by-pass.  Any system that utilises discharge gas by-pass on a frequent basis is inefficient and very costly to run.

Large systems comprising multiple compressors which incorporate cylinder unloading, can adjust their capacity to match load variations by a much greater range of capacity steps, possibly down to 6% of total plant capacity.   However, even this range of capacity control may not be sufficient on a critical system application since 6% of plant capacity may be significantly greater than the capacity of one or more FCU’s.

The Pro’s & Con’s Water Cooled Fan-Coil Systems

The Pro’s

A major advantage of water-cooled FCU’s is their flexibility in terms of layout and ease of relocation.   This allows an office layout to be easily revised where partitioning may have to be relocated to suit a change of use or the requirements of new building occupants.  A single system can have a number of FCU’s located in a large open plan office and individual FCU’s from the same system can also be used in a single occupancy offices.

Each FCU can be individually controlled by it’s own return air thermostat and the resultant load variation can be easily matched without the need to resort to discharge gas by-pass as is required in the case of the DX approach.

On medium to large systems, two water circuits are used.  The primary circuit is between the refrigerant to water heat exchanger (Water Chiller Unit) and a weir-buffer tank, while the secondary circuit is between the buffer tank and FCU’s.  A degree of thermal inertia is conferred by the buffer tank which absorbs small fluctuations in the load from the FCU’s and prevents short cycling of the central station Water Chiller(s) which is unsatisfactory for the operation of the central DX refrigeration plant.

Generally speaking, a medium to large installation will be more reliable than a DX system.  This is primarily due to the most vulnerable elements of the installation, (the refrigerant pipe work, components and joints) being limited to a packaged item of DX equipment, safely located in a central plant room.  Leak checking, detection and repairs are also easily carried out and the original assembly will have been conducted at the manufacturer’s plant under controlled conditions and strict quality control.

The Con’s

Chilled Water FCU’s are not economically viable on small applications.  Apart from the extra cost of incorporating a water chiller with the refrigeration system, the efficiency losses represent a significantly greater percentage of the overall operating costs.  The overall coefficient of system performance is less than a DX system due to the need for water pumps and the additional heat exchange losses.  Greater plant-room space is required to accommodate water tanks, pumps and associated controls.

Although water leaks are more easily found than refrigerant leaks, the consequential damages resulting from a major water leak can be far greater than for a refrigerant leak, since refrigerant simply evaporates without little trace.  A serious refrigerant leak in liquid form would of course be physically dangerous to occupants nearby.  A major refrigerant leak in vapour form which might allow the entire system refrigerant charge to escape into a single room whilst occupied must also be considered dangerous.

The capital cost for a Chilled Water FCU installation will be greater than a DX system.  This is due to the additional components required such as water tanks, water pumps, water to refrigerant heat exchanger, larger pipe sizes together with the extra pipe insulation required, bearing in mind that both the flow and return chilled water pipework between FCU’s and buffer tank have to be insulated.

The ability to provide dehumidification with Chilled Water FCU’s is considerably more difficult and costly.

NEXT MONTH:           VOL 49 – Fan Coil Units – Continued

DISCLAIMER:

Whilst every effort is made to ensure absolute accuracy, Business Edge Ltd. will not accept any responsibility or liability for direct or indirect losses arising from the use of the data contained in this series of articles.