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Rumi H. Bharucha graduated from California State College with air conditioning engineering as a major and has 32 years of consulting experience. He has designed the HVAC systems for many restaurants and is a member of ISHRAE.
It is said that when people in Mumbai are not working, they are eating. The growing middle class, bulging purse strings, especially amongst the youth, have led to more and more new restaurants, coffee shops and fast food joints around every corner. Although the quality of food and service has improved due to increased competition and the entry of Western style foreign chains, these stand-alone restaurants lack adequate facilities including well-designed air conditioning systems, for various reasons.
Space is the biggest constraint in a congested city like Mumbai where real estate prices are amongst the highest in the world. It is only natural for owners of new restaurants not only to select good locations in the most crowded areas with high human traffic but also to design for maximum seating capacity, leaving completely inadequate space for the AC plant.
Left to their own fancies and often due to compelling reasons such as a very cramped area with a mezzanine seating space thrown in, the only way out is to install several through - the - wall window ACs or at the best some ductless split ACs. Lack of fresh air, that cannot be introduced with standard window ACs and cold air drafts due to the tightness of seating space and low ceiling heights just cannot be avoided and must be tolerated. Consultants have little or no part to play in such cases.
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Larger restaurants in less congested localities generally opt for ducted split ACs, either with ceiling suspended or floor standing evaporators and air cooled condensers. The basic system design should take into consideration 15-20 cfm of fresh air per person for the restaurant area and 30 cfm per person for a bar or where heavy smoking may occur. This volume of fresh air can only be handled by ducted split ACs.
Locating the unit properly so that it is convenient to service and at the same time the operating noise does not interfere with the ambience of the restaurants, calls for some negotiating skill with the interior designer as well as the owner, both of whom have other priorities to worry about. With a horizontal ceiling-suspended evaporator unit, the first thought of the interior designer is to install it above the seating area and maximize the number of tables and seats, without realizing the difficulty of access for service, the strong possibility of high operating noise level and chances of water leakage from a choked drain pan. The floor mounted vertical evaporator unit requires a separate room close to the seating area, but is a far better alternative than the horizontal unit. The only precaution required is that the owner should not turn this room into a storage area which can lead to unsanitary conditions later.
The area around the evaporator unit should be acoustically treated to contain the operating noise of the blower, motor and drive. Fibre glass insulation – 50 mm thick and 32 kg/ m3 density, covered with R.P. Tissue and perforated aluminum sheet is an ideal treatment. In some installations, I have observed that R.P. Tissue has been omitted and this can lead to health problems with glass fibres passing through the perforated aluminum sheets into the return air stream, through the evaporator and into the lungs of restaurant customers and staff.
The outdoor air-cooled condensing unit requires an unobstructed flow of cooling air to the condenser coil to keep the operating head pressure as low as possible and cooling efficiency at its peak. Interior designers once again enter the scene and insist on concealing the condensing unit with architectural louvers, which not only obstruct the free airflow required but can also make it very difficult to service the unit in case of a breakdown or routine maintenance. Make sure you insist on a liberal free area across the architectural louvers. The noise level of the standard condensing unit must be carefully considered and the enclosure acoustically treated, if necessary to prevent any disturbance to the neighbors.
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It is impossible to maintain any standard of IAQ when window ACs or ductless splits are installed but quite often one can sense the same poor quality of IAQ even when ductable split ACs are installed and positive fresh air intake ducts provided. This occurs when untrained plant operators or simple electricians who perform the same job decide to close the fresh air damper, merely because it is a hot summer day and they figure "why burden the cooling plant with this hot air intake?"
To prevent this from happening and affecting the quality of IAQ, I have successfully used controlled ozone generating systems which have led to a relatively smoke-and-odorfree atmosphere within the restaurant in spite of the whims of the plant operator. I am not, for a moment, advocating that fresh air requirements be replaced by the installation of an ozoniser but merely saying that ozonisers do help eliminate smoke, food and musty carpet odors, so that when you come out from a restaurant you don't smell like the food you ate.
While refurbishing existing restaurants I have come to realize that while the interiors may look relatively clean and decent, the space above the false ceiling, used as a return air plenum, is a shattering health hazard - with dust, construction dirt and rat droppings all spread out over the false ceiling. Out of sight, out of mind, as the saying goes!
To take care of this, McDonald restaurants, several of which were designed by us, made it a standard practice to have a substantial amount of removable tiled ceiling, which can be periodically opened and vacuum cleaned.
With duct cleaning services now commonly available (though at unreasonably high prices) it is easier to maintain a better standard of hygiene above false ceilings as well as inside supply air and exhaust air ducts, provided access panels in ceilings and access doors in ductwork are provided at strategic locations. The exhaust duct, in particular, requires frequent cleaning, in spite of the grease filters installed in the hoods.
The typical kitchen exhaust system consists of one or more exhaust hoods equipped with grease filters and welded black iron ducts to transport the exhaust air from the hoods to the exhaust fan and then to the outdoors.
Cooking methods vary with the cuisine, but under-the-hood capture velocity requirements remain practically the same. Because of the high density of cooking, the earlier 100-120 fpm hood face velocity is barely effective, thus requiring an increase to 150 fpm face velocity. This, at times, can be dangerous especially with high capacity burners leading to a "flash" in the hood. A fire quench system installed in the hood is highly recommended.
The kitchen area must be maintained at negative pressure with respect to the restaurant so that cooking odors do not leak out to the restaurant. Adequate fresh air must be introduced through the air conditioning equipment to compensate for the air that is being exhausted plus some excess to properly pressurize the restaurant. The kitchen area must have at least 850 m3/ hr (500 cfm) negative pressurization with respect to the restaurant area. The toilets must also have negative pressurization. The air balance diagram Figure 1 illustrates the correct airflow and balancing required to achieve these levels of pressurization.
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Propeller fans are widely used, or shall I say misused, for kitchen exhaust. They cannot provide the static pressure required to overcome the pressure loss of the grease filters, exhaust hood and ductwork and if used for general ventilation (exhaust) only without being connected to the hood, they will soon clog up with grease and stop working. A SISW centrifugal fan is the only type of fan recommended for use in restaurant kitchen exhaust as the motor and V-belt drive are placed outside the grease-laden air stream from the hood. Other important issues connected with the exhaust fan are:
Exhaust ductwork should be made of all-welded black steel with a minimum duct velocity of 8 m/sec (1500 ft/ min) and sloped towards the exhaust hood. Access doors on the side of the ducts of adequate size and number must be provided to provide access to all parts of ductwork for cleaning. A grease collector fabricated out of 1.6 mm thick black iron steel, as shown in Figure 2, should be installed horizontally in the duct after the fan outlet before the air is discharged into the atmosphere, and this discharge should be higher than the highest neighboring building.
A display kitchen, surrounded by a glass enclosure, is very popular for tandoori restaurants where the chefs are on display showing their skills in making chicken tikkas and nans. Naturally these displays should send out the right signals to the customers who should go away impressed not only with the good food and service, but also with a neat and cool chef who shouldn't be seen sweating. Spot cooling in the display kitchen should therefore be carefully planned so that its discharge can be adjusted to suit the chef's convenience.
A dining experience must be pleasant in all respects and the owner who proudly asserts on a discreet signboard near the entrance "I also eat here" should be made to understand that air conditioning is not just about cooling but the right ambience and indoor air quality which includes other issues such as odor-free air, draft-less air distribution and noise-free surroundings.
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