3.4.1 An air distribution diagram in the form of a single line diagram shall be prepared for supply and return air circuits for each of the air-conditioned areas, which are grouped together.
This diagram is an internal document and not part of the basic study which is circulated to other departments for review or submitted to client for review and approval. This air distribution diagram shall include all the elements like Air Handling Units (AHUs), supply air ducting with air outlets, return air ducting with return air inlets, supply or return air plenums, volume control dampers in supply and return air ducting for unit isolation and air flow adjustment. Elements like filters, cooling coils, heating coils, supply and return air fans located within the AHU shall also be indicated in the air distribution diagram.
3.4.2 Typically, an air-conditioning air circuit is a closed loop system with air being routed from the AHU to the air-conditioned area by supply air ducting and then routed back to the AHU inlet by return air ducting. Supply air and return air ducting is generally routed to and from the air-conditioned areas, above a false ceiling provided for aesthetic reasons. When a false ceiling is provided, return air may be routed back to the AHU between the ceiling or roof and false ceiling. In such cases the return air ducting may be eliminated.
The air-conditioned area is generally maintained at an over-pressure, i.e. a pressure slightly above the ambient. As a result of this over-pressure, air will leak out of the air-conditioned area through cracks and gaps between door and window frames and minimise outdoor or un-conditioned air ingress into the area. Fresh air is taken into the system in the return air path at the AHU inlet and this completes the circuit.
The most optimum design of an air-conditioning system attempts to minimise the loss of the conditioned air from the room.
3.4.3 The air distribution diagram shall also indicate local exhaust or return air collection points.
Local air collections points may be provided for the following typical cases:
(a) Local Exhausts from Equipment
Such air collection points are provided to minimise the exhaust air so as to minimise the loss of conditioned air from the room and consequently the cooling load.
(b) Local Return from Equipment
For equipment having high or concentrated heat dissipation loads, local return air collection points are provided. These local return air collection points allow a higher return air temperature than the room temperature. While this does not reduce the total heat dissipation from the equipment, the heat dissipation to the room from the equipment will reduce. Consequently the supply air flow rate required to be supplied to the room will be lower.
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The measures described above will reduce equipment size and therefore the cost of the system.
3.4.4 Based on the air distribution diagram, an air pressure diagram shall be prepared indicating the air pressure at the inlet and outlet of each of the element in the air circuit as given below.
(a) Start from the air-conditioned area for which the room over-pressure is defined as per the input data collected or as per appendix 4.
(b) Proceed upstream along the air circuit towards the AHU. The pressure shall increase progressively as the pressure drops in the individual elements in the air path. Indicate the air pressure at the inlet and outlet of each of the elements in the air path. The procedure shall be continued until the air circuit is completed. The pressure drops to be considered in each of the elements are listed below. These pressure drops are preliminary and shall be reconfirmed during further engineering, with firm data from bidders and vendors. The pressure drop across the plenum being negligible, may be ignored for the purpose of preliminary estimation.
(i) Volume control dampers : 3 mm WG
(ii) Filters in clogged condition
Pre-filters* : 12 mm WG
Fine filters* : 18 mm WG
HEPA filters* : 75 mm WG
(iii) Cooling coils* : 20 mm WG
(iv) Heating coils* : 10 mm WG
(v) Supply and return air ducting : 0.084 mm WG
per metre of duct length
(vi) Supply air outlets : 3 mm WG
(vii) Return air inlets : 2 mm WG
(viii) Pressure in area to be air-conditioned : 1 mm WG (ix) Pressure in AHU room or at AHU inlet : (-) 3 mm WG
The differential pressure across the AHU inlet and outlet is the External Static Pressure (ESP) for the AHU.
* Filters, cooling coils and heating coils are normally located within the AHU. Hence pressure drops in these elements shall be considered for estimation of fan total static pressure and motor rating. Pressure drops across these elements shall not be considered for estimation of ESP for the AHU.
3.5 The guidelines for selection of suitable air-conditioning system for a particular requirement or duty are given in the following paragraphs:
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3.5.1 Window Air-Conditioners
Window air-conditioning units are available in capacities of 0.75, 1.0, 1.5, 2.0 and 2.5TR.
Window units are generally preferred for comfort air-conditioning applications where control on temperature within (±) 2 0C is acceptable and no RH and dust control are required. The units require access to an external wall for the air-cooled condenser cooling and therefore for proper air distribution to be achieved, the units have to be located such that a uniform distribution of air can be achieved in the room. Multiple units may be used for larger rooms. These units are designed to provide a throw of about 8 metres when mounted at an elevation of about 3 metres, which normally is the bottom of beam elevation for buildings. Considering this limitation window units may be recommended for installation where the room widths are restricted to 8 metres with one external wall or with about 16 metres with units mounted on external walls on opposite sides.
3.5.2 Split Air-Conditioners
Split air-conditioning units are available in capacities of 1.0, 1.5, 2.0, 2.5, 3.0, 4.0, 5.0, 7.5 and 8.5 TR. These units are air-cooled and are available as ductable or non- ductable units. These units may be recommended for installation where the temperature control between (±) 2 0C is acceptable. The ductable units generally have dehumidified air blower with ESP of 15 to 20 mmWG and their use for applications requiring fine filter installation, which require higher ESP of about 35 to 40 mm WG, is not recommended. The non-ductable units are available as decorative floor mounted or decorative or non-decorative ceiling suspended or wall mounted units. These have in-built pre-filters and cannot be used for applications requiring fine filters. Sufficient care shall be taken in selecting these units and their installation to limit the refrigerant piping lengths to about 15 metres and ensuring proper drainage of cooling coil condensate from the air-conditioned areas. To prevent condensation on it’s surface, the condensate drain pipe shall be thermally insulated.
3.5.3 Packaged Air-Conditioners
Packaged air-conditioning units are available in nominal capacities of 5.0, 7.5, 10 and 15 TR. Nominal capacity is the Net Total Cooling Effect of a unit at temperature conditions defined in IS 8148. These are ductable units and are available as water-cooled or air-cooled units. Water-air-cooled units may be selected in case cooling water is available.
Alternatively, if cooling water is not available but make-up water is available, a dedicated cooling water system comprising of cooling tower, cooling water pumps and inter-connecting cooling water piping, may be selected. In case water is not available, air-cooled units shall be selected. Refer relevant guides for the selection of the cooling tower and cooling water pumps for the system.
These units may be used for applications having inside design conditions of temperature between 220C and 280C with a tolerance of (±) 20C and RH control between 50% to
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70% with tolerance of (±) 5%. These units are not suitable for temperature below 220C and RH below 50%.
Indigenous units available are generally for a dehumidified air blower ESP of about 25 mmWG. If specifically asked for, an ESP of 35 to 40 mmWG may also be available.
Considering this limitation, these units may be considered for applications requiring fine filter installation, where duct lengths are restricted to match the available unit ESP. Special imported precision type units are available with higher dehumidified air flow per TR than those for standard applications. These units are intended for high sensible load applications, e.g. computer and control rooms requiring stringent temperature and RH control. These units are either in top or bottom discharge configuration. Special units are available for capacities up to 25 TR and are equipped with in-built electric heaters and steam humidifiers. Soft water is to be made available to the units for the steam humidifier.
The units are of standard construction, factory assembled and tested, mass-manufactured and have shorter delivery periods, making them suitable for projects having short schedules. The unit installation is relatively simple as skilled personnel are not required.
Packaged units are also available with in-built micro-processor based control panels which display and monitor parameters such as current drawn, power consumption, temperature, RH etc.
Multiple packaged units installation may be considered for cooling requirements up to 100 TR. Higher cooling capacity requirements may result in large number of units and consequent higher installation and operating costs.
3.5.4 Central Direct Expansion (DX) Plant
DX plants comprise of a skid mounted compressor with drive motor, condenser interconnected by refrigerant piping with an Air Handling Unit (AHU) located near the area to be air-conditioned. The primary refrigerant expanding in the cooling coil, cools the air which is supplied to the area to be air-conditioned, hence the name Direct Expansion (DX).
These systems may be either water-cooled or air-cooled and may be used wherever the cooling capacity requirements are beyond the range of packaged units with respect to DBT, RH and ESP. The capacity of the condensing units ranges between 7 and 100 TR for indigenously available equipment having single refrigerant circuit. The units may be installed for low temperature applications like cold storage etc., with room temperatures of about (-) 250C. High ESP is generally not a limitation with these units as these units are specially assembled for a specific application.
Multiple unit installations though technically possible may be avoided due to maintenance and operation difficulties. The units along with the AHUs are generally assembled at site.
Special care has to be taken during design and installation of the refrigerant circuit so as to ensure proper distribution to each of the units (AHUs or DX units) installed in parallel.
Unless proper suction and oil pressure equalisation is ensured, oil and refrigerant suction
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gas flooding or starving of the individual DX unit compressors of oil and refrigerant suction gas may lead to major damage to the compressors.
3.5.5 Central Chilled Water or Brine Systems
The plant comprises of the water or brine chilling unit (hereafter referred as chillers), chilled water or brine pumps, and chilled water or brine piping inter-connecting the chilled water or brine AHUs. The AHUs may be located adjacent or near the area to be air-conditioned while the chillers may be installed in the plant room located conveniently in the utility area for ease of maintenance and operation. Two categories of chillers are commercially available as described below:
(a) Vapour Compression Chillers
These systems comprise of compressor, condenser, evaporator and inter-connecting refrigerant piping. The chillers with evaporator having water or brine on the shell side is called a DX chiller, whereas those with water or brine on the tube side is called a flooded chiller. These chillers use primary refrigerants like R22, R134a, R123 or R717 (ammonia), to chill a relatively cheaper and dispensable secondary refrigerant like water which is circulated to the cooling coils in the AHUs to cool the air supplied to the air-conditioned area. Refrigerants R11 and R12 which were commonly used and are still in operation, are Chloro-Fluoro-Carbons (CFCs) and their use is being restricted as per Montreal Protocol.
These chillers are available in capacities of 5 TR and higher. However, it is preferred to recommend these units for cooling loads of 100 TR and higher or where the loads are distributed over a number of areas requiring individual air- conditioning units.
Vapour compression chillers are available with the following types of compressors:
(i) Reciprocating compressors (ii) Screw compressors (iii) Centrifugal compressors
Reciprocating compressors for a capacity range of 5 to 150 TR, screw compressors from 100 to 500 TR and centrifugal compressors from about 100 to 2,500 TR are indigenously available. These chillers are available in water-cooled or air-cooled construction.
Centrifugal chillers are not recommended for installation where load fluctuations cause low loads below 40% for extended periods. These chillers reach the surge limit at about 40% of the rated capacity unless a hot gas bypass arrangement is provided. This arrangement provides an artificial load on the chillers, limiting the lowering of load on the chiller to about 40%. This method is not energy efficient and may be avoided as far as possible.
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The units are available in open type or in hermetic construction i.e. a hermetic motor drives the compressor. In case the temperature to be maintained in the air-conditioned areas requires a secondary refrigerant at a temperature lower than 50C a pure liquid having a low freezing point or solution of water and brine is used as a secondary refrigerant.
Screw chillers with lower specific power consumption at part loads as compared to reciprocating or centrifugal chillers are available. However, these chillers may be more expensive and the economics have to be studied before recommending for installation.
(b) Vapour Absorption Chillers
Vapour absorption chillers or Vapour Absorption Machines (VAM) as they are commonly known, are available between a capacity of 100 to 2,500 TR. VAM may be steam, hot water or direct-fired machines operating on fuel oil or fuel gas. Steam fired VAMs are available in double effect, requiring saturated steam at pressures of 3 to 8 Kg/cm2 (g), or in single effect construction requiring steam pressures in the range of 0.5 to 3 Kg/cm2 (g). Direct oil fired chillers are available for capacities ranging between 30 to 500 TR.
(c) A comparison of the various types of compressors available in these categories is given in appendix 5.
(d) Types of Brines
Various types of brines normally used are given in the following table.
MAXIMUM CONCENTRATION
LOWEST OPERATING TEMPERATURE SL.
NO.
NAME OF BRINE
% BY WEIGHT 0 C
REMARKS
1. Sodium chloride 23.00 (-) 25.0
2. Calcium chloride 29.87 (-) 50.0
3. Aqueous ethylene glycol 60.00 (-) 43.0
4. Aqueous propylene glycol 60.00 (-) 45.0 5. Methylene chloride Used as pure liquid (-) 96.7 6. Trichloroethylene Used as pure liquid (-) 86.1 7. Polydimethylsiloxane Used as pure liquid (-) 73.3
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The operating temperatures for sodium chloride brine, calcium chloride brine, aqueous ethylene glycol and aqueous propylene glycol are 50C higher than their freezing points. For concentrations exceeding 60% aqueous propylene glycol does not have a freezing point but forms a glass (being an amorphous, under-cooled liquid of extremely high viscosity having all the appearances of a solid). For further details of the brines refer ASHRAE Handbook, Fundamentals may be referred.
The selection of brine shall be optimised wherever possible considering the process requirements and the installation and operating costs. It is recommended that nitrogen blanketing is provided for methylene chloride and trichloroethylene. Any other specific requirements shall be examined and clearly indicated in the basic study.
(e) Various configurations of chilled water or brine systems are indicated in appendices 6 to 10. The relative merits of these configurations are given in appendix 11. The configuration of the system shall indicate the number of pumps, their flow and head, tanks and piping. The selection of the configuration shall be based on the relative merits for the particular application.
3.6 Based on guidelines given in para 3.5 above, select a suitable system for the various areas.
The number of units may be selected considering the continuous minimum cooling load available and also the economy of providing a standby unit. If more than one system could be considered, carry out a brief economic analysis. Some of the features which may be examined in the economic study to minimise installation and operating costs are as follows:
(a) Minimisation of the air-conditioning and refrigeration cooling load by use of better insulation materials or building materials of construction, energy efficient lighting systems etc.
(b) Optimum sizing of the equipment considering the maximum continuous total load on the system rather than the total of peak individual loads which may not be simultaneous. The optimum sizing of equipment may also be effected by use of chemical dehumidifiers (for low RH applications so as to reduce the latent cooling loads on the cooling coils), thermal storage systems like ice banks or chilled water or brine storage tanks, to meet peak loads in a 24-hour cycle, etc. The thermal storage arrangement is especially effective in case electrical tariffs are different during day and night.
(c) Optimum selection of equipment considering the utilities available for the total project configuration, so as to match the project philosophy. e.g. sizing of chillers so as to restrict the motor rating within limits of 415 Volts power supply. This has to done in consultation with the project electrical engineer.
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(d) Use of enthalpy control systems, which utilise the low outdoor air conditions (temperature and enthalpy of air) to meet the refrigeration loads
(e) Use of heat-recovery systems, e.g. heat recovery wheels, variable speed drives to minimise operating loads
A clear recommendation shall be made highlighting technical and commercial advantages of the system proposed.