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.