Air Conditioning: Local vs Central Systems

PAUL HAY Capital Projects

www.phcjam.com

Air-Conditioning, part 1 of 2

Author: Paul Hay

e-mail:              paul.hay@phcjam.com

profile:             www.linkedin.com/in/phcjam

1.0       LOCAL SYSTEMS ARE USED IN SMALL BUILDINGS

1.1     A major part of energy consumed in Jamaican buildings is used for cooling and fans.

Figure 1: Itemized energy end-use for a typical large office

1.2       Small one or two storey office buildings may be served by packaged air-cooled units installed on the roof slab.

            1.2.1   Equipment cost and installation is low; but

            1.2.2   This lacks flexibility.

1.3       Local systems are simple and require less space.

            1.3.1   There is no need for large mechanical rooms

            1.3.2   Ductwork is minimized; and

            1.3.3   Controls are simple

1.4       Roof locations are beneficial.

            1.4.1   The roof facilitates easy access to outdoor air for fresh-air supply; and

            1.4.2   Headroom is unlimited.

1.5       The system is sized to cope with the maximum design load of the building.

1.6       Direct Expansion (DX) systems are simple, economical and energy-efficient.

            1.6.1   Refrigerant passes through an expansion valve to the evaporator.

            1.6.2   Air is directly cooled at the evaporator; but

            1.6.3   Precise temperature control is difficult to achieve.

1.7       All-air single-zone systems transport air to conditioned spaces by one duct and has one room thermostat.

1.8       Direct refrigerant systems reduce the distribution tree by locating the evaporator adjacent to the conditioned space.

1.9       The Coefficient of Performance (COP) is one measure of A/C efficiency:

            COP   =          Cooling output to conditioned space       [1.1]

                                          Energy input to equipment

Table 1: Coefficient of Performance [COP] for typical A/C units

2.0    LOCAL SYSTEMS ARE UNSUITABLE FOR LARGE BUILDINGS

2.1       Large commercial buildings have at least two independent thermal zones:

            2.1.1   The perimeter zone is predominantly affected by external heat-gains.

                        2.1.1.1            This zone is adjacent to the roof or up to 6 m in from external walls.

                        2.1.1.2            If the building is up to 12 m wide, this can be further divided into two sub-zones: each representing two different wall orientations;

                        2.1.1.3            If the building is wider, the zone can be divided into at least four sub-zones: each representing one wall orientation.

            2.1.2   The core is the zone located at the centre of the building at least 4.5 m in from external walls, which is predominantly affected by internal heat-gains.

2.2       Sub-zones can also be produced based on differences in the schedules of use.

2.3       The Jamaican Energy Efficiency Code gives specifications for zones as follows:

            2.3.1   Each zone shall have independent thermostat controls; and

            2.3.2   Zones not operating concurrently for more than 750 hours/year should have separate A/C systems.

2.4       Local systems are awkward in the thermal core.

2.5       DX systems are unsuitable for installations in the perimeter, because it requires multiple fan coil units.

2.6       Maintenance of units is difficult because of the distribution of the units.

3.0   SPACES NEED TO BE PLANNED FOR CENTRAL SYSTEMS

3.1       Central Systems comprise one or more large mechanical spaces.

            3.1.1   The roof is a favoured location, however

            3.1.2   Equipment rooms occupy entire floors in very tall buildings.

                        3.1.2.1            There should be access to fresh air;

                        3.1.2.2            Ceilings should be high; and

                        3.1.2.3            Structure should be designed to support the weight.

            3.1.3   These spaces generate heat, moisture, vibration, noise and air motion.

            3.1.4   Multiple cooling units are recommended by code wherever loads exceed 500 kW.

3.2       Systems are sized to cope with the maximum design load of the zones served.

3.3       Central Systems are generally DX or chilled water systems.               

            3.3.1   Chilled water systems are marginally more efficient than DX systems.

            3.3.2   Chilled water systems cool water, instead of air, and passes it through heat exchangers to cool the air.

            3.3.3   They permit use of multiple coil-units and offer more precise temperature control.

            3.3.4   Chilled water systems can be either air-and-water or all-water systems.

3.4       Complex control systems are utilized.

3.5       Sizable distribution trees are used:

            3.5.1   Chilled water systems have smaller distribution trees.

                        3.5.1.1            All-water systems have the smallest distribution tree; but

                        3.5.1.2            Ventilation has to be dealt with separately.

                        3.5.1.3            Air can be exhausted locally; or

                        3.5.1.4            Exhaust can also be transported through return-air ducts.

            3.5.2   Vertical chases reduce the amount of space that can be rented and may limit the flexibility to partition the space.

            3.5.3   Horizontal runs influence the ceiling height.

            3.5.4   Maintenance is difficult in concealed distribution trees that are not properly detailed, but there are advantages.

                        3.5.4.1            There are less surfaces to clean;

                        3.5.4.2            Appearance of walls and ceilings are controlled; and

                        3.5.4.3            Noise is reduced.

3.6       Central DX cooling and local air-distribution takes advantage of benefits attributed to both the central and local systems.

3.7       Drawbacks of central systems are:

            3.7.1   Equipment failure can affect the entire building or multiple zones; and

            3.7.2   An entire system may have to be running to serve one zone or a section thereof.

                       

4. 0  SYSTEMS USED VARY WITH BUILDINGS TYPE

4.1       Speculative office buildings have shifting tenancy.

            4.1.1   They typically require many independent zones.

            4.1.2   Air-and-water systems called induction-type air terminals may be used in the perimeter zones.

            4.1.3   All-water fan-coil units can also be used

            4.1.4   All-air systems called variable air-valume units can be used for the core.

4.2       Shopping Centres have transient occupancy.

            4.2.1   Air distribution is less critical because shoppers are moving about;

            4.2.2   Large shopping malls may use all-water fan-coil units in individual stores.

4.3       Hotels have variable occupancy.

            4.3.1   Central systems are usually desired in public areas alone, but

            4.3.2   Induction-type terminals or fan-coil units are used in individual rooms.

4.4       Apartment blocks have variable usage schedules and require individual control.

            4.4.1   They have as many zones as individual apartments.

            4.4.2   Apartment blocks over three-storeys high can use chilled-water systems with individual fan-coil units.

4.5       Hospitals need special temperature and humidity control.

            4.5.1   Operating Theatres do not use re-circulating air.

            4.5.2   All-air systems with adequate filtration may be used; but

            4.5.3   Induction terminals are often used in closed wards.

FURTHER READING

            Mechanical and Electrical Equipment for Buildings, 8^th edition, Benjamin Stein, John S. Reynolds, John Wiley & Sons Inc., USA, 1992

Construction Materials & Processes, Don G. Watson, McGrawHill Book Co., USA, 1978;

            Ramsey/Sleeper Architectural Graphic Standards, A^.I.A., Robert T. Packard (ed), John Wiley & Sons Inc., USA, 1981;

Architectural Handbook, Alfred M. Kemper, John Wiley & Sons Inc., USA, 1979

Jamaica National Building Code, Volume 2: Energy Efficiency Building Code, Requirements and Guidelines, 1994, Joseph J. Deringer (ed.), Jamaica Bureau of Standards, Jamaica, 1995.

Daylighting: Design and Analysis, Claude L. Robbins, Van Nostrand Reinhold, N.Y., 1986.

Water Supply: Pipe Size Calculation

  

PAUL HAY Capital Projects

www.phcjam.com

Pipe Size Calculation

Author:          Paul Hay

e-mail:            paul.hay@phcjam.com

profile:           www.linkedin.com/in/phcjam

Problem:-  Find service pipe size given the following:

                       Street main pressure [E]               = 350 kPa

                       Height of uppermost fixture

                       (above street)                              = 10 m

                       Uppermost fixture                       = WC with flush valve
                       Total fixture units in the system^a   = 85

a. See table 1 for supply fixture units of individual plumbing fixtures.

                       Developed length (DL) of piping (to

                       highest and most remote fixture)   = 30 m

                       Predominant flushing mechanism   = Flush valves

Table 1: Water Supply Fixture-Unit Values [source:- International Residential Code]

Solution:-  Calculation of Service Pipe Size

                       1.            Find the pressure required in the system to provide the minimum fixture pressure [A] for uppermost fixture^b:

b. see value for Aflush valve for closet in table 2

                     A         = 103 kPa

Table 2:  Minimum Pressure & Flow required for typical plumbing fixtures   [source:- Mechanical & Electrical Equipment in Buildings]

                      2.            Calculate the Static Head [B]:

                   B         = 10 kPa/m x Height of uppermost fixture

                  = 10 x 10 m

                  = 100 kPa

Table 3:  Minimum pipe sizes for typical plumbing fixtures  [Source:- National Building Code of Jamaica 1983]

Figure 1:  Demand Load for predominant (1) flush valves & (2) flush tanks  [source:- Mechanical & Electrical Equipment in Buildings]

Figure 2:  Flowchart for typical pipes  [Source:- Mechanical & Electrical Equipment for Buildings]

                      3.            Using total fixture units, determine the demand load^c for the relevant flushing mechanism from figure 1.

c. 3.78 L/s demand, for 85 FU.

                       4.            Using demand load [3], determine an approximate pipe size^d from fig. 2 that is closest to the flow velocity of 3 m/s.

d.  40 mm dia. pipe, for 3.8 L/s demand.

                     5.            Using demand load [3] and approx. pipe size [4], determine the pressure loss in the water meter [D] from fig. 3:

                 D         = 62 kPa

Figure 3:  Pressure losses in Water Meters  [Source:- Mechanical & Electrical Equipment in Buildings]

                    6.            Calculate the maximum frictional loss [C] that can be tolerated in the service pipe:

                 C         = E - (A + B+ D)

                 = 350 - (103 + 100 + 62)      = 350 - 265

                = 85 kPa

                    7.            Calculate the Pipe length equivalent of fittings

                 [DL'] (estimated at 20 % of DL):

                 DL'      = 0.2 x DL = 6 m

                    8.            Calculate the total equivalent length (TEL) of the piping:

                TEL      = DL + DL'    = 30 + 6          = 36 m

                    9.            Calculate the unit-frictional loss of the pipe:

                 100 x C/TEL              = 100 x 85/36            = 189 kPa

              10.         Using the demand load [3] and the unit frictional loss [9], determine the pipe size^e from fig. 2

e.  50 mm dia. service pipe size

Table 4:  Maximum allowable pipe sizes  [Source:-  National Building Code of Jamaica 1983]

Domestic Hot Water Systems: Heat Sources, Methods, Systems, & Distribution

  

PAUL HAY Capital Projects

www.phcjam.com

Hot Water Systems

Author:            Paul Hay

e-mail:              paul.hay@phcjam.com

profile:             www.linkedin.com/in/phcjam

1.0       INTRODUCTION TO HOT WATER SYSTEMS

1.1       Domestic Hot Water (DHW) is needed for comfort and a degree of sterilization particularly in Laundries and Kitchens.

1.2       In Jamaica, the need for hot water is greatest in hotels, hospitals, pharmaceutical manufacturing and food processing facilities.

1.3       DHW Systems vary depending on (a) heat source, (b) Method of heating water, (c) local versus central equipment and (d) respective distribution trees.

2.0       HEAT SOURCES

2.1       Solar Energy is the only renewable source of energy used.

2.2       Natural gas and electricity are also used as energy sources.

2.3       Otherwise, heat-recovery devises can be used to heat water.

3.0       METHODS OF HEATING

3.1       Water is heated to a maximum temperature of 60 deg. C.

3.2       On-demand heaters do not use no storage tanks:

            3.2.1   Heaters rapidly heat water to the desired temperature and immediately distributes it;

            3.2.2   Heater capacity is equivalent to the peak demand.

3.3       Storage tanks hold approximately one-third the daily cold water consumption:

            3.3.1   A pressure-relief valve is placed on storage tanks as a safeguard against excess temperature and pressure.

            3.3.2   Direct heating involves (a) the use of submersible electrical heating elements, or (b) water is first passed through coils which are subsequently heated by solar radiation, fire, or hot gases;

         3.3.3   Indirect heating involves (a) the use of heat-extracting devices, such as incinerators, not primarily meant to heat water, or (b) passing steam or hot liquids through submerged heating coils.

.

Figure 1: Recommended Capacity of Hot Water Tanks [source:- Journal of Light Construction Field Guide (vol. 2)]

4.0       LOCAL & CENTRAL SYSTEMS

4.1       Local systems are recommended where distance between areas of use exceed 15 m;

4.2       Central systems should be located nearest to fixtures which utilize the most hot water (eg. dish-washers and washing machines.)

Figure 2: Central Hot Water Distribution [source:- Journal of Light Construction Field Guide (vol. 2)]

5.0       DISTRIBUTION TREES

5.1       EEBC-94 requires all exposed piping and storage tanks to be insulated.

5.2       A check valve is located on the cold water intake pipe to prevent hot water entering the cold water distribution tree.

5.3       If hot water storage tanks are located indoors, the pressure relief valve should be connected to a drain.

5.4       A non-circulating system has a distribution tree directly from the heater to individual plumbing fixtures.

5.5       A circulating system has a return line to the heater:

            5.5.1   This system is used in large residences or systems;

            5.5.2   Time for delivery of hot water to fixtures is reduced;

            5.5.3   A circulating pump can be used for constant supply of hot water.

6.0       INSTALLATION

6.1       Gas-burning equipment should be installed in well ventilated areas.

6.2       Exposed open-flame equipment should not be installed in bedrooms, enclosed garages, or rooms primarily intended for storage.

6.3       Where more than one fixture is supplied by a single gas line, each fixture shall receive an independent lock-off valve.

6.4       Equipment should be accessible for servicing and repair.

_________________________________________________

FURTHER READING

Mechanical and Electrical Equipment for Buildings, Benjamin Stein & John S. Reynolds, John Wiley & Sons Inc., U.S.A.

Construction Materials & Processes, Don G. Watson, McGraw-Hill Book Co., USA.

                        Jamaica Energy Efficiency Building Code [EEBC-94], Jamaica Bureau of Standards

Water Distribution: pumping systems, pipes, fittings & valves

  

PAUL HAY Capital Projects

www.phcjam.com

Subject:         Water Distribution

Author:          Paul Hay

e-mail:            paul.hay@phcjam.com

profile:           www.linkedin.com/in/phcjam

1.0       INTRODUCTION   

1.1       An adequate supply of potable water shall be provided to each plumbing fixture;

1.2       Water shall be distributed in a manner that ensures it is kept clean and sanitary;

1.3       Good design requires sufficient fittings and valves for ease of maintenance; and

1.4       There should be minimum interference with the architectural form.

2.0 DISTRIBUTION SYSTEMS

2.1       Up-feed distribution involves the distribution of water under the pressure available at the water main or from pressure tanks fed by pumped wells:

            2.1.1   Up-feed systems are primarily used in low-rise buildings;

            2.1.2   Water pressure available at the main must be greater than 103 kPa (15 psi);

            2.1.3   Otherwise, pressure available at main shall exceed losses due to (a) flow through meter, (b) fiction in piping, and (c) height of water column in order to provide proper flow pressure to the highest fixture.

Figure 1: Upfeed Distribution System [source:- Journal of Light Construction Field Guide (vol. 2)]

2.2       Pumped Up-feed Distribution utilizes pumps to supply the additional pressure required:

            2.2.1   Water must first be collected in low-level storage tanks (because direct connection of pumps to the water main is illegal in Jamaica);

            2.2.2   Pumped up-feed systems are primarily used in medium-rise buildings;

            2.2.3   Back-up generators must be provided to allow distribution of water during a power outage.

2.3       Down-feed distribution involves the pumping of water to upper level storage tanks which gravity feed to plumbing fixtures:

            2.3.1   Static Pressure is the pressure exerted by a column of water by virtue of its depth below its stationary head;

            2.3.2   Tanks shall be elevated in order to provide the static pressure needed on the floor immediately below;

            2.3.3   With insufficient head, tanks can be pneumatically charged with an air compressor to attain the required pressure:

                        2.3.3.1            Air-charged pneumatic pumping systems result in greater levels of dissolved air which may result in an undesirable “fizz” on discharge;

                        2.3.3.2            Dissolved oxygen increases corrosiveness of the water supply.

            2.3.4   Water distribution for high-rise buildings should be divided into zones up to 45 m (150 ft.) high:

              2.3.4.1          Static pressure beyond this height can damage plumbing fixtures;

              2.3.4.2            Each zone must be served by independent distribution systems and plumbing.

3.0       PIPES & FITTINGS

3.1    Water supply pipes and fittings can be brass, cast iron, malleable iron, galvanized wrought-iron, galvanized steel, or several types of plastic:

            3.1.1   Copper does not corrode and is durable:

                      3.1.1.1            Copper is subject to electrolytic attack if connected to a dissimilar metal without dielectric separation;

            3.1.1.2            Fittings can be (a) by flared compression joint,  (b) solder-type or brazing fittings;

                        3.1.1.3            Copper’s smooth interior permits the use of smaller sized pipes; and

                        3.1.1.4            Copper pipe should not be used for hot water systems where temperature exceeds 60EC (140E F).

            3.1.2   Galvanized Steel has a long service life:

                        3.1.2.1            Galvanized pipes are dimensionally stable and strong;

                        3.1.2.2            Pipes used in water distribution are joined with screwed fittings;

                        3.1.2.3            Pipes may be used for both cold and hot water distribution, and

                        3.1.2.4            Water Treatment may be required against corrosiveness of water.

         3.1.3   Rigid Plastic Pipe is light-weight offering exceptional resistance to (a) chemicals, (b) impact, and (c) pressure:

                        3.1.3.1            Rates of expansion exceed those of copper and steel;

                        3.1.3.2            Special adapters are available for joints with metal pipes;

                        3.1.3.3            Fittings are typically solvent-welded to the pipes;

                        3.1.3.4            Polyvinyl Chloride (P.V.C.) is one of the common types;

                        3.1.3.5            Polyvinyl Dichloride (P.V.D.C.) Should be used to carry hot water no hotter than 82E C (180E F).

3.2       Except for valves and similar devices, fittings used must be the same material as the pipe;

3.3       Pumps shall be connected with unions;

3.4       Cross connection is an arrangement of piping or connections that allows contamination:

            3.4.1   Syphoning of contaminants occurs when there is a difference in pressure, or a vacuum, in the water distribution system;

            3.4.2   Air gap or Air break is the vertical separation between a pipe or faucet and the flood-level rim of the receptacle;

            3.4.3   Back flow or Check valves allow water to flow in only one direction.

Figure 2: Drainpipe Fittings [source:- Journal of Light Construction Field Guide (vol. 2)]

4.0       VALVES & CONTROLS

4.1       Pressure Regulators should be used in cases when (a) water pressure in mains is excessive, and (b) with problems of variable pressure at different floor levels;

4.2       Valves shall be used (a) on each riser, (b) on each branch to bathrooms, kitchens, etc. and (c)  run-outs to individual fixtures.

_________________________________________________

FURTHER READING

Mechanical and Electrical Equipment for Buildings, Benjamin Stein & John S. Reynolds, John Wiley & Sons Inc., U.S.A.

Construction Materials & Processes, Don G. Watson, McGraw-Hill Book Co., USA.