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What ails our industry – Part IV

In this, the fourth of a six-part series, George Berbari focuses on the air side of things, highlighting the typical design and manufacturing mistakes related to FCUs and AHUs, and how they are impacting energy efficiency and indoor environmental quality (IEQ) in the GCC region

| | Jul 9, 2015 | 9:55 am
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– By George Berbari

CEO, DC PRO Engineering

George Berbari, CEO, DC PRO Engineering

The indoor chilled water Fan Coil Unit (FCU) design has not changed much since 1960; in fact, it stagnated until the new efficient EC motor came into fashion.

For long, the industry in the GCC region has persisted with a low-cost application of a basic FCU unit with on/off control valve and three-speed thermostat. A traditional FCU electric motor consumes 100 watts per tonne for low static and around 200 watts per tonne for high static at high speed.

One of the common mistakes the industry has made is to select the FCU on medium speed, leading to oversizing, higher noise and greater indoor humidity in an already oversized situation. FCUs should always be selected at high-speed value to allow the user the flexibility of a good step-down range. This ought to be the norm, especially when the selection is done at extreme design conditions of 45 degrees C, which occurs for less than 35 hours in a year.

Another common mistake is to combine the kitchen with the living room area, limiting the kitchen to a minimum supply of less than 100 CFM, which is not sufficient to cool the kitchen, particularly when 1.5 to 6 kW of heat is generated by the stove and kitchen oven. It is always recommended that every kitchen has a dedicated FCU for proper temperature control and proper ventilation, which would allow 50 CFM to be exhausted from the space and for it to be under negative pressure to prevent the smell emanating from cooking to spread to the other parts of the house without compromising on the internal comfort.

A new trend that is elevating energy efficiency and internal comfort is the EC motor FCU. EC stands for Electronically Commutated, which basically means it is a fan with a brushless DC motor, with the variable speed drive integrated with the motor. EC motor FCUs consume less than 80 watts per tonne at full speed and offer further saving at partial speed. And when used along with modulating chilled water valve, they can offer unrivalled indoor comfort levels from temperature, humidity and noise perspectives, in addition to one of the most efficient air movement conditions in the industry.

When we come to real practice, however, only one main manufacturer offers affordable EC motor FCUs. Manufactured in China, they come at an estimated price of USD 200 per tonne, or USD 0.5 per CFM, compared to USD 120 or USD 0.3 per CFM for traditional units. However, the motor is noisy and the control is not up to the mark. Instead of fully varying the speed, it selects a lower speed and acts as a two-speed FCU. The true full variable speed EC motors are offered by some local and European manufacturers, at a higher price, when compared to the Chinese unit.

I expect the industry in the GCC region to develop and hope that all major manufacturers offer affordable Chinese-origin FCUs that will be challenged by many Middle East-based manufacturers to offer equivalent units. However in these changing times, I advise every consultant and contractor to buy a sample unit and controls to test them prior to placing the final order.

Hotels and bedrooms, in particular, present themselves as great applications for EC motor FCUs; in fact, many international hotel chains have started to specify them as the only acceptable option.

Hotels also have sophisticated room management systems that control the access to the room, the internal lights, the FCU fan speed and temperature, as well as the curtain. A room management system integrates very well with the EC motor technology, as it can send a 0-10 V signal and modulate the fan speed.

Office buildings can integrate EC motor FCU with thermal non-electric-operated diffusers with special controls that can measure the main supply duct pressure as well as supply and return air temperature, and modulate the fan speed as well as the modulating control valves without any wall thermostat. They can carry out the function by using just one comptroller per office to control multiple FCUs through a Human Machine interface (HMI), installed in one location.

EC motor FCUs, with thermal diffusers, can offer a lower capital cost and much lower operating cost when compared to offices cooled through central air handling units (AHUs) with variable air volume (VAV) diffusers.

The AHUs are specified with extensive ductwork and complex communicating controls, such as VAV boxes with measuring stations, which raise the cost of installation to over USD 5 per CFM, or three to five times that of EC motor and traditional FCUs.

Their power consumption is also unattractive, running as they do at 200 to 300 watts per tonne at design flow; and as they run for extensive hours, I have come across two buildings, where the annual consumption is a staggering 1.0 kW/tonne.

Some of the shortfalls of current AHU designs are as follows:

• None of the AHUs have a BTU meter on chilled water and a VFD drive with a Kwh meter linked to an intelligent control system that can provide year-round performance.

• Hydraulic chilled water flow that uses high authority combination control and pressure independent balancing motorised valve, which are not meant for variable chilled water flow and which can simply be replaced with an equal percentage two-way control valve with 35 to 40% authority and double regulating valve to save on pressure drop and, hence, pumping energy. The variable chilled water flow strategy should follow a completely different pass, where an industrial-grade differential pressure transmitter, placed at three-fourths of a distance downstream, can be used with PLC-based controller that varies the set point between, for example, 1 Bar Gauge at a design ambient of 45 degrees C linearly down to 0.1 Bar at mild ambient temperature of 25 degrees C.

• AHUs with mixing boxes that mix return air with treated fresh air, which uses VFD associated with VAV boxes, may compromise fresh air delivery at part load or reduced speed. We need to have dedicated outdoor treated fresh air delivered directly to the spaces to avoid that.

• VAV boxes have stagnated in development, with the exception of dynamic flow measurement and sophisticated controls and communications that are very difficult to commission, to operate and to maintain. It is time we looked for simpler alternatives, such as non-electric self-powered thermal diffusers with dedicated bypass connection for fresh air, and avoided any centralised communication protocols.

• I see a big focus on AHU body strength, leakage, thermal bridging and filter leakage, with the internal body pressure varying between -1,000 and +1,000 Pa for strength and from -400 to + 700 Pa for leakage, in compliance with Eurovent EN 1866. Nothing is wrong in that, but we should minimise external pressure from the typical 500 to 750 Pa, to less than 250 Pa, which will decrease the AHU as well as the duct leakage situation and, further, reduce the fan motor power. This can be achieved by selecting the right size and right location of AHU, to minimise ducting; we must also select a low duct velocity of less than 5.0 m/sec.

CPI Industry accepts no liability for the views or opinions expressed in this article, or for the consequences of any actions taken on the basis of the information provided here.

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