Air conditioning in Telecom buildings

Air conditioning in Telecom buildings

           Air conditioning requirements of Telephone exchanges changed drastically over the years with change in Exchange technology. In Initial days Strowger exchanges in 70s then X-bar exchanges in 80s and after introduction of electronic exchanges, mobile server rooms, and data centers recently.

Following is over view of changes that took place.

   Prior to 1985,  Strowger  & X-bar Exchanges have following  features

Switch rooms are very large in size for 10K ( 16 mtr X 30 mtr ) & ceiling height 5 mtrs, Temp required 24⁰ +/- 2⁰C.

Numbers of technicians, engineers working in switch room was more & also large space (volume ) hence have fresh air requirement was large and must.

Chilled water type / DX type central AC plants (n+1) having (1+1) AHU’s serving Switch rooms , meter room & MDF were installed for Air conditioning of these exchanges.

   After 1987, With the introduction of E-10B local ,D-Tax ,SPC-Telex electronic exchanges having  following features as

Switch rooms size reduced by great extent 20K main E-10B exchange with OMC & Exploitation ( 12 mtrs X 16 mtrs) with bottom feed ,false flooring & false ceiling arrangement.

 Stringent Temp requirement 22+/-2 C, RH 50%+/-10%, Air flow 60 LPS.

 Occupancy also reduced limited to exploitation area only. Fresh air requirement also reduced as less occupancy & small space. The exchange load to a large extent is sensible load.

Ø    To start with existing central AC plants , AHU’s as main & water cooled package ac units as standby.

Ø    In 1990 as more n more electronic exchanges were coming up, central ac pants phased out for following reasons.

Difficulty in maintenance, Installation time more, large space requirement for chillers, pumps, cooling towers , more accessories etc., and Stringent Temp & humidity requirement for electronic exchanges.

Less requirement of space, easy & quick installation , easy argumentation package ac units were preferred with (n + n) configuration and for first time national level rate contract was framed for water cooled & air cooled package ac units with design specifications as follows

Table 1

Package units rating	Water cooled package ac units were rated corresponding to SST 7.20C  , SDT  43.30C	Low SST for given compressor capacity is less, High SDT this also makes less capacity. Units were rated for extreme conditions (which are going to come for very small time period) thereby making them inefficient in view of energy consumption. In design more stress was given on operation in extreme condition which occurs very few hours in a year.
	Air cooled package ac units were rated corresponding to SST 7.20C  , SDT  58.60C. At ambient temp 43.30C.
Evaporator coil	Evaporator coil face area designed liberally for SHF  upto 0.95, air flow across coil 120 to 180 m/m.	Though SHF required up to 0.95, CFM @ 350 per TR.
Condenser	Designed to rated capacity for entering DB temp 43.30C.
Blower fan	Static pressure not mentioned, Blower fan motor 2HP for 10TR,1HP for 7.5 TR provided.	External booster fan- tube axial fan or centrifugal fans were provided additionally.
External accessories	Externally Re heater , Pre heater for humidity control, Micro wave filters in plenum.

Ø    Due to water scarcity, air cooled package ac units were mostly used , and in subsequent year use of water cooled package ac was stopped.

               

              With large requirement and more reliable development of package ac units plant configuration modified to (n+1) for n up to 3 & (n+2) for n more than 3,high static pressure blower fans introduced in place of booster fans the design specifications were as follows.                       

Table 2

Package units rating	Air cooled package ac units were rated corresponding to SST 90C  , SDT  560C. At ambient temp 43.30C.	For given compressor capacity is less for High SDT. Units were rated for extreme conditions (which are going to come for very small time period) thereby making them inefficient in view of energy consumption. In design more stress was given on operation in extreme condition which occurs very few hours in a year.
Evaporator coil	Evaporator coil face area designed liberally for high SHF  , air flow across coil 120 to 180 m/m.	Though high SHF required, CFM @ 350 per TR.
Condenser	Designed to rated capacity for entering DB temp 43.30C.
Blower fan	Static pressure 50 mm of WG, Blower fan motor 3HP for 7 TR provided.	External booster fan not required.
External accessories	Externally Re heater , Pre heater for humidity control, Micro wave filters in plenum.

Ø    In year 1998 , for first time tenders were called for high sensible precision  package ac units in Lucknow  &  Ahmedabad Divisions .M/s Tata libert and M/s Hiross Batliboi offered units for prototype ,of which units of M/s Tata libert were tested and got approved.

Table 3

Package units rating	10+/-1 TR Air cooled package ac units were rated corresponding to SST 90C  to 100C , SDT  530C. At ambient temp 430C.Return air temp -250C Supply air temp-160C	Double circuit  with Scroll compressor
Evaporator coil	Evaporator coil face area designed liberally for high SHF  minimum 0.95, air velocity across coil less than 2.5 m/sec.	Minimum 600  cfm per TR., Dehumidification arrangement by reducing effective coil area by solenoid valve to limit exchange surface area of evaporating coil ,thus providing a lower evaporating temperature, And Reheat coil
Condenser	Designed to rated capacity for entering DB temp 430C.	Speed control for condenser motor depending upon discharge pressure.
Blower fan	Static pressure 35 mm of WG, shall capable of withstanding 50 mm of WG , Blower fan motor 3HP 2 nos. for 10 TR provided.	External booster fan not required.
Internal accessories	Heater humidity control, Micro wave filters in return air path.

   And based on interaction with M/s Tata libert and others specification for high sensible package ac units without precision controls were finalized which were as follows.

Ø             Double circuit per unit (n+1) configuration was used without pre and re heater arrangement the salient features as under.

Present package units design specifications

Table 4

Package units rating	7 or 10+/-1 TR           Air cooled package ac units were rated corresponding to SST 90C  to 100C , SDT  530C. At ambient temp 430C.	Double circuit  with reciprocating compressor
Evaporator coil	Evaporator coil face area designed liberally for high SHF  minimum 0.95, air velocity across coil less than 2.5 m/sec.	Minimum 600  cfm per TR.,
Condenser	Designed to rated capacity for entering DB temp 430C.	Speed control for condenser motor depending upon discharge pressure.
Blower fan	Static pressure 35 mm of WG, shall capable of withstanding 50 mm of WG , Blower fan motor 3HP for 7 TR ,5HP for 10 TR provided.	External booster fan not required.
Internal accessories	Dehumidification arrangement by reducing effective coil area by solenoid valve to limit exchange surface area of evaporating coil ,thus providing a lower evaporating temperature, And Reheat coil Heater humidity control, Micro wave filters in return air path.

Observations and Suggestions:-

1.    For High sensible BSNL package ac (1999-00) with static pressure 35mm of WG blower fan capacity is 3 HP for 7 TR, 4200 CFM & 5 HP for 10 TR, 6000 CFM.

2.    For DOT package ac (1994-95) with  static pressure 50mm of WG blower fan capacity is 3 HP for 7 TR  3750 CFM.

3.    Though motor capacity is 3 HP for 7TR ,air handled is more, i.e. current actually drawn is more in case of High sensible BSNL package ac. Forward curved fans has property of self loading i.e. power rises continuously with flow, damper control for regulation is a limited adjustment option which energy efficient. Now a day’s more efficient backward curved fans getting popular with EC DC.

4.    Provision of package units in conditioned space will reduce requirement static pressure of blower fan drastically resulting in savings. We may option for high static pressure package unit also.

Design of package units with option for downward / upward flow so as to keep Shortest air circulation path / route with minimum static pressure, Reduce mixing and short circuits of air path.

5.    Similarly condensing temperature also have major effect on package unit capacity we may go for 35⁰C  and 43⁰C options depending upon location of installation ( maximum Ambient temperature).

6.    The seasonal test is conducted in hot n humid condition i.e. 1^st April to 30^th September each year. Air cooled condenser capacity dependent upon entering dry bulb temperature, hence air cooled condenser capacity, its actual performance can be assessed only in hot condition i.e. in summer 1^st April to 30^th June. The temperature of entering to  air cooled condenser must be at temperature @ 43⁰C.

7.    We may also specify sub cooling required @ 15⁰F along with super heat 6⁰C.

8.      Liberally designed condenser with subcooling  @ 15⁰F may operate erratically in lower temperature applications for which we may use two speed control of condenser acting on discharge pressure, balance port thermostatic expansion valve for better operation.

9.    It is observed that thermostats provided with package units are not accurate , imported carel /evco/siemens make temperature controllers suitable for group operation may be introduced.