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By Sunil Khatwani
Asst. General Manager
Voltas Ltd., Mumbai
Sunil Khatwani is a mechanical engineer from R.E.C. Jaipur with a masters in marketing management from Jamnalal Bajaj Institute of Management Studies, Mumbai. He has a total of 17 years experience with Voltas Ltd. and he is a member of both ASHRAE & ISHRAE Mumbai chapter.
Packaging a product provides a great convenience. Thus we have packaged foreign tours and packaged holidays which take care of all the hassles of visas, hotel reservations, airline bookings, special food and sight seeing.
Packaged air conditioners and ductable splits provide a similar convenience for engineers involved in designing comfort air conditioning systems where control of temperature and good air distribution are the main objectives. Engineers need not concern themselves with selecting a compressor, matching its capacity with a suitable condenser and evaporator, sizing the refrigerant piping, choosing an adequate size blower and providing electric wiring. All of that has already been worked out by a team of experienced engineers and a neat bundle of various components put together to make a package that can be installed as a single unit for a small office, retail shop or restaurant or in multiples for bigger offices, larger shops or restaurants.
Standard comfort package ACs and split ACs are available from many companies and our company can offer a variety with reciprocating (both hermetic and semi hermetic) as well as scroll-compressors, 4-row deep evaporator coils and an intelligent management system comprising a programmable microprocessor controller that can communicate with a PC through a master controller.
Some unique installations of comfort ACs have been carried out in shopping malls, auditoriums, multiplexes, entertainment centers, showrooms, server rooms and operating theatres and these will be described in some detail for the benefit of those young engineers who are entering the growing market for packaged and ducted split air conditioners.
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Packaged and ductable split ACs have become very popular for this application. Their usage involves placing the air conditioners in designated rooms, strategically located on the floor in the case of packaged units, or on the loft above the toilets or above the false ceiling in the passage areas, where splits are involved. A few precautions at the design stage will ensure a satisfactory installation and these are:
Mall builders or construction companies prefer packaged / split ACs for the following reasons:
The "Mega Mall" in Oshiwara, near Lokhandwala complex, Mumbai has installed 250 ductable split ACs with a total capacity of 1885 tons and is the largest single installation of ductable split units in the country.
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Such applications are characterized by large occupancy levels and require special care in selection of cooling coils. Low to medium occupancies, say upto 350 persons, can be handled by normal comfort units with 4-row evaporators as a standard, except that some precautions for low noise levels are required for applications like multiplexes, theatres and auditoriums. Ensure that:
As an added precaution where standard packaged units are used for such applications, the compressor can be shifted to inside the outdoor condenser unit and this will help to further reduce the noise level in the air conditioned area.
Where high occupancy levels, above 350 persons are encountered the normal comfort AC cannot be used since the 4-row cooling coil is not capable of handling the high latent load.
The best method of air conditioning such areas is by using air handling units (AHUs) with one or two sections of 5 or 6 row DX coils. Each section can then be connected to the standard condensing unit of ductable splits. The air handling units can be single skinned or double skinned and with regular or low noise blowers, depending on the noise level requirement.
Other norms mentioned earlier like acoustic insulation of ducts, lower blower fan speeds, grille selection for lower NC levels etc. are equally important here.
A large number of multiplexes and theatres have been air conditioned with such units, quite successfully.
Such built-up systems offer the following advantages:
In earlier days, servers required very low temperatures of 18–20°C with 5 micron filtration of air and air supply from below the false flooring.
With servers becoming sturdier day by day, only temperature control with 15-20 micron filtration and air supply from the top of the area is being increasingly accepted, thereby making standard packaged and ductable split units very popular for such applications.
Such areas need high air quantity from the AC units due to the high sensible loads and can work without any fresh air intake since they are occupied by humans for very brief intervals only.
Standard packaged units can be used for such areas by locating the units inside the air conditioned area in between the server racks. A plenum on top of the packaged unit discharges supply air in 3 or 4 directions, as desired. Due to the absence of ducting, the air quantity delivered by these units is higher.
Only temperature control (± 1°C) is possible with such systems.
One variation in the air conditioning system for such areas is the use of standard packaged units by locating the units inside the air conditioned area.
These critical applications involve maintenance of temperature, humidity, cleanliness, desired number of air changes and air velocities. Let's examine the two types of systems involved viz. systems with specified fresh air changes and those with 100% fresh air applications:
Specified fresh air changes (say 4-6 per hour)
Standard packaged units can be used for such applications with the following changes:
A better method is the use of single or double skinned AHU, with a high static blower design, connected to condensing units. The fine filters can be located in the plenum at the outlet of the evaporator blower. The HEPA filters can be located in the plenum itself or at the terminals, depending on the cleanliness levels desired. Laminar flow can also be used with these systems.
100% fresh air
There are two methods of air conditioning involving 100% fresh air.
The bypass factor for a 13 FPI, 4-row DX coil with 600 fpm face velocity is about 12%. This is effectively reduced and can be calculated approximately as under:
This lowered bypass factor of 5% can then be used in the heat load calculation to arrive at the improved ADP.
This is a simple empirical method compared to working out of the inlet conditions of the mixed air passing over the evaporator coil.
A large number of operation theatres, in hospitals of the Municipal Corporation in Mumbai, have such systems and have been running successfully for many years.
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Areas with high sensible loads, due to the presence of a large number of PC's or areas with large, exposed glass windows or skylights or areas with large equipment loads, require a higher amount of cooled and dehumidified air quantity as compared to typical comfort applications.
The air quantity requirements are about 500 cfm per ton as compared to about 400 cfm per ton for standard comfort applications.
The system in earlier days involved the installation of one or two additional packaged units to match the higher cfm requirements, making the system unnecessarily expensive with a larger space requirement.
The word 'standard' has been redefined now with the introduction of high air quantity packaged units to cater to such specific requirements.
A standard 8.5 ton packaged unit delivering 3000 cfm is fitted with a 5.5 ton compressor, all other components remaining the same. Thus, the unit now delivers 3000 cfm and 5.5 tons i.e. about 545 cfm/ton.
The additional benefits are:
The demand for operation and monitoring of equipment from a single point, like a PC, is growing day by day. A number of building management systems (BMS) are available in the market to cater to this requirement.
These systems are effective but expensive. A very cost effective solution (VSCADATM) has been introduced indigenously by our company to meet international standards in controlling and monitoring HVAC installations.
The system comprises of a programmable microprocessor controller installed on each packaged or ductable split unit panel which communicates with a master controller. The master controller in turn communicates with the PC through a data converter.
A modem connected to the PC makes remote monitoring possible.
Main Features:
General Features:
Standard packaged air conditioners for comfort are gradually eating into the share of central DX and chilled water system. Several medium sized office buildings have also been fully air conditioned with packaged ACs. For reasons explained in this article “builders” prefer such units because of their low cost and speed of installation.
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