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No Regret - Specify Correct

In any vapour compression refrigeration system, there are four major components-whether it is an air conditioning or refrigeration plant and it is essential that while ordering these components either from manufacturer or through consultant/contactor, the end user customer must check whether the specifications of these four equipment have been correctly specified.

In almost all instances, the supplier makes an offer highlighting only the positive points of his equipment and hides the drawbacks. The buyer, should therefore check all the technical specifications and then make the comparison between two or more offers received. If he does that, then he does not have to regret later on, and his decision then would be based on proper assessment, rather than deciding only for the lowest priced basis.

Here, all the specifications that need to be considered for proper assessment of the major components have been given for the benefit of the buyer/consultant.

The four major components of any vapour compression refrigeration system are
1. Compressor
2. Condenser
3. Evaporators
4. Metering devices.

In this article I am covering specifications required for these components used in refrigeration systems with Ammonia as refrigerant for cold storage applications.

Compressors
The first and most important as well as most expensive component is compressor and normally two types of compressors are used in the systems
1. Reciprocating piston compressors
2. Screw compressors.

We shall start with specifications of reciprocating compressors first. We shall now look at specifications of screw compressors.

Most of the parameter mentioned above also need to be considered, besides which additional points mentioned in Table 2 need to be taken in consideration.

Condenser
Condenser is a device where heat is rejected to the atmosphere from the system. Normally, evaporative condensers are most popular for ammonia refrigeration systems and they can be mounted outside the machine room wherever place is available.

Condenser is the most neglected device in the entire system, since it is generally not in the machine room and one does not know whether it is functioning properly, till the discharge pressures start rising beyond the design limits.

Evaporative condensers are available in many different designs with counter flow or concurrent flow. They can be with separate de-superheat section; Evaporative condensers can also be either entirely plain tubes with either round or elliptical tubes or partially wet pads and partially tubes. New deigns with falling water over the plates are also available. The casing can be either S.S. or Powder coated steel or FRP. Etc.

While asking for the offers from evaporative condoner manufacturer, the end-user should specify weather conditions such as maximum wet bulb temperature during monsoon at the place of erection, altitude and most importantly the quality of water available with water analysis certificate. The performance of evaporative condenser and water bleed rate depends largely on water quality.

The selection of evaporative condenser should be done for maximum wet bulb temperature during the year which is normally in monsoon. Many engineers select condenser for highest dry bulb temperature in summer and then face problems of performance during monsoon as it falls short of capacity. The preferred water quality for evaporative condensers is specified in Table 3.

Evaporator or air cooler
The third most important equipment is evaporator or air cooler for cold storages. The evaporator is the most crucial component within any refrigeration system being responsible for removal of the heat from product and maintaining uniform temperature and air distribution to ensure no stagnant area exist or no appreciable weight loss takes place.

The evaporator is a device for absorbing heat into the refrigeration system. The evaporator receives cool, low pressure refrigerant liquid from the liquid metering device and changes into refrigerant vapour as a superheated vapour.

The heat of substance being cooled is removed by air as stored products come in contact with cold air. Air coolers can be gravity flooded type or pump circulation type. Air cooler specifications and selection is most important as the quality of product stored depends upon selection of air cooler. The product should be able to remain in the cold room without deterioration or without weight loss and hence proper temperature and humidity needs to be maintained in the cold room. Also, it should ensure that there is not more than 2-degree C temperature variation in any part of the cold room.

We have covered specification requirement of major items except the fourth one which is expansion valve. This item falls under the category of controls and hence selection of it depends on various design parameters of the system. The types normally used in ammonia plants are level controllers with had expansion valve, or level controller with motorized expansion valve or high side float for low charge refrigeration systems etc. The specifications for expansion valve are not covered in this document.

Conclusion
The above information gives detailed specifications for ordering out major equipment and for comparison of quotations received from different manufacturers/suppliers. Effort should be made to make comparison from offers received, on equal technical specifications basis and then the price should be negotiated. This would lead to proper equipment selection and efficient operation of plant with no regrets later on.



Ramesh Paranjpey,
Fellow ASHRAE Life Member
Past President ISHRAE Pune Chapter