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.

Precision air conditioner / CRAC

1) Twin Compressor with single/Twin Ckt. ( for more reliability)

• Capacity of air conditioning plant  is worked out based on peak outside conditions prevailing in that area, heat load, lighting load ,occupation in room ,heat gain from surroundings.
• It is worthwhile to note that both the operating conditions and the capacity of a refrigerating system change as the load on the system changes and also depending upon heat gain from surroundings.
• Equilibrium maintained between vaporizing and condensing sections depending upon internal load and heat gain from surroundings affects rated capacity at designed operating conditions of air conditioner.
• When the load on the system is light, the space temperature will be lower than the average design space temperature, the evaporator (Δt) will be less than the design (Δt) and the suction temperature will be lower than the design suction temperature. Therefore, the system operating conditions will be somewhat lower than the average design operating conditions and the system capacity will be somewhat less than the average design capacity.
• Good practice requires that the system be designed to have a capacity equal to or slightly in excess of the average maximum sustained load. This is done so that the system will have sufficient capacity to maintain the temperature and humidity at the desired level during periods of peak loading. Obviously, as the cooling load decreases, there is a tendency for the system to become oversized in relation to the load.
• The degree of variation in the length of the on and of cycles well depend on the degree of load fluctuation.
• The design conditions occur may be, for example, during only 1% of the total time the equipment is in operation throughout the year. 

·        Multiple-System Capacity Control is one of the methods of controlling capacity.

• With Double independent refrigerant circuit, capacity control of 50% and 100% is possible with blower fan common for total capacity , is essential as our load is predominately sensible load.
• It is desirable to use of Double independent refrigerant circuit where the refrigeration load is variable. ( Less capital cost and easy maintenance and more reliability )
• For Heat load  10 TR to 20 TR  -- 7  TR  with n+1 Configuration
• More than 20 TR -- 14 TR   with n+1 configuration. For n >6  n+2 configuration
• For heat load calculation fresh air requirement, duct heat & return air heat gain not to be considered. ( Fresh air arrangement not required, infiltration of air during opening of door is sufficient to make up oxygen as it unnecessarily increase latent load .) (Shortest air circulation path / route., Arrange racks in hot aisle / cold aisle, Matching server air flow by aisle., Reduce mixing and short circuits., Provide isolation between hot and cold spaces. And free return air path., Best option is to install units in area to be conditioned) 

2) Electronic Expansion valve.

                 The conventional TXV is controlled by springs, bellows, and push rods. The spring force is a closing force on the TXV. The evaporator pressure, which acts under the thermostatic element's diaphragm, is also a closing force. An opening force is the remote bulb force, which acts on top of the thermostatic element's diaphragm.

There is also a liquid force from the liquid line, which acts on the face of the needle valve and has a tendency to open the valve. However, this force is cancelled out when using a balanced port TXV. Working together, these forces maintain a constant evaporator superheat in a refrigeration system. There are no electronic devices associated with a conventional TXV.

The electronic expansion valve (EEV) operates with a much more sophisticated design. EEVs control the flow of refrigerant entering a direct expansion evaporator. They do this in response to signals sent to them by an electronic controller. A small motor is used to open and close the valve port. The motor is called a step or stepper motor. Step motors do not rotate continuously. They are controlled by an electronic controller and rotate a fraction of a revolution for each signal sent to them by the electronic controller. The step motor is driven by a gear train, which positions a pin in a port in which refrigerant flows. Step motors can run at 200 steps per second and can return to their exact position very quickly. The controller remembers the number of step signals sent by the controller. This makes it possible for the controller to return the valve to any previous position at any time. This gives the valve very accurate control of refrigerant that flows through it. Most of these EEVs have 1,596 steps of control and each step is 0.0000783 inches.

Sensors

The electronic signals sent by the controller to the EEV are usually done by a thermistor connected to discharge airflow in the refrigerated case. A thermistor is nothing but a resistor that changes its resistance as its temperature changes. Other sensors are often located at the evaporator inlet and outlet to sense evaporator superheat. This protects the compressor from any liquid floodback under low superheat conditions. Pressure transducers can also be wired to the controller for pressure/temperature and superheat control. Pressure transducers generally have three wires. Two wires supply power and the third is an output signal. Generally, as system pressure increases, the voltage sent out by the signal wire will increase. The controller uses this voltage to calculate the temperature of the refrigerant with the use of a pressure/temperature table programmed into the controller.

The benefits derived from the installation of electronic expansion valves are as follows:

• Improved control of liquid refrigerant flow to evaporator. The evaporator is always optimally filled with refrigerant. Even with large load variations, which means an extremely wide range of partial-load operating conditions, exactly the right amount of refrigerant can be injected.

• Improved heat transfer, since more evaporator surface area used for boiling liquid ,less superheat.

• Raised evaporating temperature and higher suction pressure, reducing energy use by 2% to 3% per 1ºC in evaporating temperature.

• Reduced risk of liquid carry-over to compressor, reducing risk of compressor

damage.

• Avoids need for constant pressure drop across expansion valve.

• Allows condensing temperature (discharge/head pressure) to reduce at times of low ambient temperatures

          Thereby making air conditioning unit energy efficient. Hence it is recommended to use Electronic expansion valves.

3) Direct driven fan motor assembly for evaporator.

Backward curved freewheel fans Direct driven by electronically communicated motors are recommended as they save upto 30% as compared to forward curved centrifugal fans having dampers for cfm control.

Backward curved centrifugal fans characteristics are Energy efficient as no transmission loss, easy maintenance, less noise , high static pressure ,high flow ,power reduces as flow increases beyond point of highest efficiency. Where as forward curved fans are self loading type power rises continuously ,dip in pressure curve moreover damper control is not particularly energy efficient method of air flow control.

 4) SMPS power supply for control ckt.

The Precision air conditioning units are used in telephone exchange building having their own transformer substation, having reasonable voltage stability .Requirement of SMPS power unit for may differ manufacturer to manufacturer depending upon their microprocessor controller to make specification generic ,SMPS power supply unit may not insisted.

Provision of Precision air conditioner as replacement to existing package units in existing setup is not recommended.

Following are additional comments on precision air conditioning specification:

1) Precision AC Unit designed for COP 2.9.

• The following definitions are taken from ASHRAE Standard 90.1-1999 (2001).

• Coefficient of performance (COP) – cooling: the ratio of the rate of heat removal to the rate of energy input, in consistent units, for a complete refrigerating system or some specific portion of that system under designated operating conditions.

• Energy efficiency ratio (EER): the ratio of net cooling capacity in Btu/h to the total rate of electric input in watts under designated operating conditions.

• Integrated part-load value (IPLV): a single number figure of merit based on part-load EER, COP, or kW/ton, expressing part-load efficiency for air-conditioning and heat pump equipment on the basis of weighted operation at various load capacities for the equipment.

• As per ASHRAE 90.1-2004 $ 6.4.1 & ECBC 2007

      Unitary Air Conditioning Equipment

Equipment class	Min COP	Min IPLV	Test Standards
Air cooled chiller <530 kw (<150 tons)	2.9	3.16	API 550/590-1998
Air cooled chiller >530 kw (>150 tons)	3.05	3.32	API 550/590-1998
Centrifugal water cooled chillers  <530 kw (<150 tons)	5.8	6.09	API 550/590-1998
Centrifugal water cooled chillers  >530 kw  and < 1050 kw (>150 tons and <300 tons)	5.8	6.17	API 550/590-1998
Centrifugal water cooled chillers  >1050 kw (>300 tons)	6.3	6.61	API 550/590-1998

• BSNL specification requires COP 2.9, at designed conditions i.e. SST 9 to 10 ⁰C & SDT  53 ⁰C for ambient temperature of 43 ⁰C.

• Considering designed condition and 35 mm static pressure evaporator fan, it is not possible to achieve required COP.

2) Check for good installation of Air cooled Condenser.

• Proper operation and giving rated capacity at designed conditions / peak conditions of PAC is mainly depend upon efficient working of Air cooled condenser.

• In case of water cooled system, checking of cooling tower efficiency by web bulb approach was invariably done during AT.
• However in case of air cooled system there is  no check of efficiency of air cooled condenser which is solely depend on entering and leaving dry bulb temperature.

• Seasonal  test shall be conducted only in summer months April to June  and  October.

      Air entering condenser shall be at ambient temperature.

               Or

• During testing ambient temperature shall be more than 40 ⁰C.

Capacity calculation by Enthalpy method (Evaporator side ), Condenser capacity    and Capacity from compressor manufacturer capacity chart at working conditions shall give clear picture about ENERGY EFFICIENT INSTALLATION OF AC UNITS.

3) Electrical console for Package AC units.

• In coming FP MCB ,insulated busbar for distribution ,Motor Protection Circuit Breakers (MPCB) ,suitable size contactor and overload relays shall incorporated to avoid fire hazards , as these units are preferably installed in area to be conditioned.

• Provision of wet floor sensor to indicate water leakage problem.

• Provision of Electrical control panel at entrance air lock lobby with two incomings electrically & mechanically interlocked through shunt trip coil and AC cutoff in case of Fire.

4) Insulation of refrigerant piping / drain pipe  inside the conditioned space.

5) Provision of false floor height 600 mm ,avoid refrigerant / drain piping in path of supply air. 

5) Use of site suitable package unit variants.

Different models suitable for site requirement to be selected as follows

v    Low static pressure (Installation inside switch room is preferred). / High static pressure ( Ducted units)

v    Condenser suitable for 35⁰C & 43⁰C ambient temperature ( shall reduce initial cost ).

v    Upward and downward flow.

v    Condenser top and side throw.

Calculation of Carbon footprints

Augest 2012

Calculation of Carbon footprint:-

Following methodology helps you to calculate your carbon footprint resulting from the use of Electricity, Petrol, Diesel and LPG.

 Step 1- Data collection;

1. Electricity: Collect data on your annual electricity bills. One can find number of  units consumed in establishment from the monthly electricity bills issues by State Electricity Board/ Distribution/Collection companies.
2. Petrol/Diesel:  Add number of liters of petrol/diesel you used in DG set in a year.
3. LPG: Generally one LPG cylinder has around 14 kg of liquefied petroleum gas. Multiply number of cylinders used in a year by 14 and add the resulted value in the calculation.    

Step 2 – Calculation Methodology;

1. Electricity : Input value (in KWh/Yr) X 0.00085 (Emission Factor) =  Output value in (Kg of CO₂)
2. Petrol: Input Value(In Litres/Yr) X 2.296 (Emission Factor) =  Output value in (Kg of CO₂)
3. Diesel: Input Value(In Litres/Yr) X 2.653 (Emission Factor) =  Output value in (Kg of CO₂)
4. LPG: Input Value(In Kg/Yr) X 2.983 (Emission Factor) =  Output value in (Kg of CO₂)
5. Your Carbon Footprint :  Add (1+2+3+4) = Output value in (Kg of CO₂)

Divide final value (no 5) with 1000 so that you get total carbon footprint in ton of CO_2.

Final Carbon footprint should be in tons of CO₂ (tCO_2.).

Know more about the source of emission factors;

• Electricity = 0.00085 kg CO₂ per KWh, Source: CO₂ emission factor database, version 06, CEA (Government of India), http://www.cea.nic.in/reports/planning/cdm_co2/cdm_co2.htm 
• Motor gasoline/ Petrol = 2.296 kg CO₂ per liter, Source: Emission factors are taken from the file “Emission factors from across the sector -tool”,  extracted from http://www.ghgprotocol.org/calculation-tools/alltools
• Diesel= 2.653 kg CO₂ per litre, Source: Emission factors are taken from the file “Emission factors from across the sector -tool”,  extracted from http://www.ghgprotocol.org/calculation-tools/alltools 
• Liquefied petroleum gas (LPG) = 2.983 kg CO₂ per kilogram, Source: Emission factors are taken from the file “Emission factors from across the sector -tool” extracted from http://www.ghgprotocol.org/calculation-tools/alltools

Carbon footprint calculation For Year : (Telecom application )

= Electricity use + Diesel use

Electrical  use

= Emission factor X  Electricity consumption /Year

= 0.00085  X  ( Average MD  X  Load factor  X  365  X  Average EB Hour  per day )

  Note -- 

Average MD  is  average  of  actual MD recorded in year.

Load factor  will vary  As 50% load of  telephone exchange load is air conditioning load which is depend on ambient temperatures during a day.

It observed  that 

BTS Indoor                          0.85                     AC without FCU   4.8 KW

BTS Indoor                          0.7                       AC with FCU        4.8 KW

BTS Outdoor                       1                           Without AC          2.4 KW

For Telephone Exchange    less than  0.65

Administrative buildings     0.7

Average EB Hour is Average Electricity Board supply available in one day (24hour).

Diesel use

= Emission factor X  Diesel Consumption /Year

= 2.653 X  ( Diesel Consumption in ltr   X  365  X  Engine Run Hour per day )

Diesel Consumption = SFC  X  bhp   /  0.83 X 1000   Liter

SFC will vary depending upon loading of DG set For Full load  167 gms/hp-hr approx.

As per Recommendations

on Approach towards Green Telecommunications

April 12, 2011 

by   TRAI  ( Available on TRAI website )

Refer page 28

If the consumption of power of the Network element, in KW (including Air

Conditioning etc) is P , the Grid power is for ‘x’ hrs , the power from ‘z’

KVA DG is for ‘y’ hrs and the efficiency of the generator is ‘η’ then

C = 0.365 [0.84Px + (0.528 yz /η) ] in Tonnes

Similarly carbon footprints for each of the network elements are to be

calculated. The detailed calculation of footprints of various network

elements is at Annexure –I

page no 30

sample calculation for estimation carbon footprint

2.24 Calculation of carbon footprints towards the conclusion of the peak

season – of winter and summer – may be desirable. Hence the telecom

service providers can declare their Carbon footprints twice in a year.

2.25 All service providers should declare to TRAI, the carbon footprint of

their network operations in the format provided in Annexure -II.

This declaration should be undertaken after adopting the formulae

and procedures mentioned under para 2.20 and at Annexure -I. The

Declaration of the carbon footprints should be done twice in a year

i.e. half yearly report for the period ending September to be

submitted by 15th of November and the succeeding half yearly report

for the period ending March to be submitted by 15th of May each

year.

2.25 All service providers should declare to TRAI, the carbon footprint of

their network operations in the format provided in Annexure -II.

This declaration should be undertaken after adopting the formulae

and procedures mentioned under para 2.20 and at Annexure -I. The

Declaration of the carbon footprints should be done twice in a year

i.e. half yearly report for the period ending September to be

submitted by 15th of November and the succeeding half yearly report

for the period ending March to be submitted by 15th of May each year.

Certain assumptions has been for calculating  carbon footprint for Diesel use.

Also efficiency in case of DG set  is taken as 1 in Annx –I.