VAPOR ABSORPTION OR VAPOR COMPRESSION MACHINE ? Lowering the Chilled Water Generation Cost in Chlor-Alkali

This study reports a comparison between vapor absorption versus vapor compression cycle for generation of chilled water. The two principle type of refrigeration plants are found in industrial use are: Vapor compression refrigeration (VCM) and vapor absorption refrigeration (VAM). VCM uses mechanical energy as the driving force for refrigeration, while VAM uses thermal energy as a driving force. VCM and VAM, both accomplish the removal of heat through the evaporation of refrigerant at low pressure and rejection of the heat through the condensation of refrigerant at high pressure. The method of creating the pressure difference and circulating the refrigerant is the primary difference between the two cycles.

This paper is prepared on the basis of actual data and will provide advantages & disadvantages of both the system. This paper also help to select type of refrigeration suitable for provided conditions. The performance of the system were analyzed double stage Vapor Absorption Machine to single stage Vapor compression cycle with the same heat load condition.

Evaluation Criteria

The following heads are considered to compare VAM against VCE

1.     First Cost

2.     Economics

3.     Operating Cost

4.     Maintenance

5.     Advantages & Disadvantages   

First Cost

The first cost difference between both the equipment is not significant difference. We are considering that chilled water requirment at 7°C. The refrigeration screw chiller flooded cost is 1.3 times of vapor absorption machine. The first cost data for both the machine is given below in the table-1. The capital cost of VAM is lower (at least30%) than compression system but when electric services, transformers, switch gear, cabling, space are significant component in VCM. 

Table-1 First cost difference between both the Machines

An economic comparison is made between VAM & VCM. The comparison between these two technologies is carried out based on energy efficiency, cost, complexity, and pay back period perspectives.

The results have indicated that either system, VAM or VCM, is cost effective regarding their energy source & cost. The difference in working shown in figure-1.   

Figure-1 Basic Difference between VAM and VCM

Economics

Selection of the type of refrigeration equipment to meet a given cooling duty depends upon the several variables

1 Refrigeration load

2 Temperature levels

3 Energy source of driving

4 Quantity available & temp. of condensing media

5 Space

The actual operating cost is compared for both the system including actual maintenance cost. Here, we have consider a span of 5 years for NPV calculation.

The critical parameters for both the system are shown in table-2. Cost of Machine per TR is taken from the actual data. The chilled water temperature is designed for 7 & 12°C. The efficiency of VAM is poor compared to the VCM. Hence, it is always beneficial to consider VAM in case of waste heat, process hot water, and low cost fuel like hydrogen in chlor-Alkali. 

In actual operating condition VAM are observed higher energy consumption against the rated. Hence, 7% higher steam consumption is considered for above operation. The actual cost of operation is shown in table-3.

Operating Cost Comparison 

It is visible form the table that annual operating cost at provided energy price have 35 Lacs difference between VCM and VAM. The VAM is energy consumption is higher in all aspects. The equivalent operating cost for both the system at steam cost of 850 Rs/ton, either electricity cost at 8 Rs/unit.

The annual maintenance cost also higher in VAM case. The AMC charges for VAM is 1.3 times of flooded chillers due to the complexity of the system.

The NPV of the system is evaluated and presented for 5 years. Which have a difference 21% operating cost. The VAM machines offer less life of the equipment against VCM. The VCM life is suitable for 20-25 Years whereas the VAM life is 15-20 Years.

In practical operation, it is observed that the LTHE & HTHE, and pump failure is common at 5- 8 years’ time line. VAM is using Li-Br Solution, which is corrosive so inhibitor must be added to protect the system against the metal corrosion. Li-Br is also highly viscous. 

LiBr absorbent is prone to crystallization. If something goes wrong in the cycle. The salt and water would permanently separate and the LiBr will crystallize on the wall of the absorber. As it become solid, it will not flow so the chiller will stop working. The cause of crystallization may be lower cooling water temperature. At a concentration of 65% LiBr solution crystallize at 42 °C, with concentration of 60°C at 17 °C and with concentration 55% lower than 15°C

The advantages and disadvantages for VAM and VCM are shown in the table-4 & 5.

The utility consumption is one of factor which required to consider during purchase time as each utility have important factor site to site. The utility consumption for 300 TR machine is provided in table-6.

Maintenance 

The machine maintenance is critical factor to consider for any site. VAM is a complex machine to maintain. It is made in two part lower and upper shell. Normally opening and closing is not done by the site maintenance team. It is require skill VAM inspection mechanic.

The annual maintenance contract provided by the OEM and it is expensive when compared to VCM maintenance contract. It is 1.4.

The VAM major maintenance is in every 3 -4 years. It is observed the purge pump and HTHE, LTHE failure is very high. The LiBr concentration is critical and crystallization will also cause major maintenance issue.

For VCM, the compressor maintenance and refrigerant leak are the major cost. Normally, it is seen that compressor major maintenance is rare.

The cost of maintenance is 1.6 times against the VCM maintenance cost. For eight year operation, the recurring maintenance cost is shown in figure 2. The 5th, 6th, and 8th year have significant cost difference is due to failure of purge canned pump, LTHE, and HTHE. Whereas the VCM have 6th & 7th year high cost due to refrigerant leak. The maintenance cost of VCM has increased from 5th year onward due to maintain and overhaul the compressor every year to maintain the performance.

Figure-2 Maintenance Cost of VAM versus VCM

Results

It can be seen that each system has some advantages and disadvantages over the other. The total cost of VCM system is about 21% cheaper than the VAM. Although, the payback period is higher for the VCM. The VAM is less noisy than VCM. We would go for an absorption machine only.

• · When excess heat source is available, for, e.g. availability of excess steam in boiler.
• ·      LP steam available for dump condenser, this can be used to run the VAM.
• ·      When heat is ineffectively dissipated into atmosphere, for, e.g. exhaust from DG, steam from turbine outlet, flue gas exhaust, water evaporation unit.
• ·      When the operational expenditure is very much lesser than the VCM, for, e.g. when the industry is facing issue with power supply from the grid.
• ·      When the production cost of steam negligible, for, e.g. use of hydrogen to generate steam in chlor-alkali industry.

The summary table is shown the major factors between VAM and VCM. 

Conclusion

An economic comparison is made between a vapor compression refrigeration system powered by electricity and a vapor absorption system powered by thermal energy. The comparison between these two systems is carried out based on every minor and major aspect like energy efficiency, cost, NPV, maintenance etc.

The data shared in this paper is from actual operating system in India. The results has indicated that either system, VCM and VAM, is cost effective regarding their life cycle cost. Which is critical to consider in any purchase or replacement. However, sometime simple payback period is quite confusing and not provide the real picture.

However, after analyze all of the parameters considered, one would prefer and go for the VCM, although it involves noisy compressors, since it is lower and cheaper operating cost in long run operation. VCM is also less bulky, and easily available in the market, simpler and requires lower maintenance; wide system application in industry and commercial building.

This would also keep in mind that VAM system can used only in case of cheap energy source and waste heat recovery option, in addition to the possibility to combine with domestic hot water system.

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Mayank Shukla is BE (Chemical)-AGM (Process Engineer) and working with Technical services in India largest chlor-alkali business house. He is an Accredited Energy Auditor (Gov. of India). His current role involves process, design, troubleshooting, and project management for Grasim Industries.

He is executed various ENCON, Process improvement, and benchmarking project for ABG Group. Prior to join ABG, he worked in for India Largest refinery, Jamnagar. Technical review for CDU and Utilities off sites. He worked as senior process engineer in Air Liquid INC. (former Lurgi India Pvt. Comp.). Mr. Mayank Shukla has earned his chemical engineering from university of Meerut, India.