[Home]	Search:
Theme: Select one Steely Old Modern Midnight Ultramarine Swiss Chocolate Traditional

Issue : October-December 2001

One Building Experiences Three Types of Chillers

By M.H. Lulla
Consultant, Chennai

A 1996 engineering graduate from Annamalai University he worked 8 years with a contracting company before setting up practise as an HVAC consultant. Teaches at the Anna University School of Architecture and is an ISHRAE member.

TIAM HOUSE Chennai is the corporate headquartersof the Murugappa Group, once known as the TI Group. Built forty years ago it commands an imposing view of the harbor and the sea front. The building has extensive windows, but these are shaded copiously, externally, by “sun shade” devices which not only make the windows “energy efficient” but also provide for adequate glass cleaning facility. The building is oriented with the long walls facing East/West which is a site constraint. A typical floor measures 140 ft × 60 ft. The building is 8 floors high and has a basement. The total floor area is 70,000 ft2.

The building when originally completed was cooled by a Worthington centifugal chiller of 300 ton capacity, working on R-11 in conjunction with 9 air handling units – one per floor. Each floor was virtually treated as a single zone. Condenser heat rejection was into a special masonry shell cooling tower with wood fill from Marley USA and wood bladed fans from the same source. Thirty years after its installation, the centrifugal water chiller was replaced. The replacement was desired as it was difficult to obtain spares since Worthington, an American company had closed down. The Montreal Protocol with its soon-to-be-imposed restriction on the use of R-11 refrigerant was also an issue.

The centrifugal chiller was replaced with three R-22 reciprocating chillers, 100 tons each, consisting of a Kirloskar recip compressor, a water cooled shell and tube condenser and a DX shell and tube chiller. The original plant room which housed the Worthington machine was barely able to accommodate the large slow speed reciprocating chillers, making maintenance difficult. Plant room ventilation became inadequate and increasing air supply was a problem due to the nature of the building construction.

Another 10 years later the Kirloskar compressors were experiencing large down times, and in peak summer even with all the 3 machines running, there were times of inadequate cooling. The management instituted detailed investigations, which indicated a 360 ton peak load requirement. This meant that there was an increased cooling requirement perhaps due to larger internal loads and increased provision for fresh air (20 CFM per person) to meet better IAQ standards. A detailed study also brought out alternate equipment capabilities as shown below.

Use of screw compressors would result in an annual power saving of close to Rs. seven lakhs per year for the 360 ton plant . Even though such chillers would cost roughly Rs. twenty lakhs more than recips, the annual saving in power would mean a simple pay-back period of only three years. In light of this a decision in favour of screw chillers was finalised and offers invited from different manufacturers. The order was placed for 2 × 186 ton units each with twin circuit screw compressors at a price close to the budget level in June 2000 with a commitment to have all equipment at site by early December 2000.

The existing layout had the 3 × 100 ton chillers working with three condenser pumpsets and three chiller pumpsets, with each bank of pumps having two running and one standby. In the new system each 186 ton chiller would have a 100% pump running with a 100% pump standby for both condenser and chiller duty. The dedicated pump circuits were used keeping in mind the load cycle. The work of changing “common pump sets” to “dedicated pump sets” was a part of the replacement. The cooling towers were also to be replaced as part of the equipment change.

Now began some serious planning to install the new equipment, piping and wiring with minimum disruption in the comfort and working of the office staff and executives inside the building.

During initial planning a provision was made for obtaining a shut down of nine days, i.e. five week days and two weekends. Later an ambitious exercise on reducing the down time was started. Common headers for the condensers and chillers and common cooling towers for the plant were identified as the chief road blocks in achieving the replacement without any downtime. Improved delivery schedules and co-operative workers enabled the planning team to aim for the change over without any shut down of the air conditioning during normal working hours.

The most critical item was replacing the 2 x 150 ton existing cooling towers with a new 400 ton cooling tower using the same sump. The innovative building design with light weight material did not permit an alternate location for the cooling tower. The original masonry sump of the cooling tower was the only tower location possible and had to be re-used. After careful study of the various popular models of towers available a particular model (Mihir Ecocell) was identified as the tower with the smallest foot print.

Over a weekend the existing tower was pulled down and a new tower put in place. Components of the new tower had been laid out battle style on the roof. The exercise of putting together the tower was preceded by minor civil works. The tower assembly proceeded hand in hand with the exercise of relaying the piping headers for the new configuration of towers. A nightmare scene of welding being done with FRP/PVC components lying all around was unavoidable. The fire marshal and security men were on their toes armed with shielding plates and fire extinguishers. The entire work which should have been done over a long, three day weekend, did not fall in place as planned and a days over-run was necessary before the entire exercise was completed. The one day over-run become necessary due to some unforeseen problems with newly welded pipe joints, and some FRP jointing on the tower.

In the plant room one of the three recip chillers was identified for dismantling first and the selection of this machine was made as its particular location permitted placement of one new “200” ton chiller in the same place. The old chiller was dismantled – its huge 200 cft foundation broken – a new base cast and the new chiller placed in position. This exercise was carried out in a weeks time, work was done during normal working hours and nights as well. During a weekend the new machine was piped into the headers and the headers modified for dedicated pumping circuits. For the first time on that Monday the building had 400 tons of cooling – 200 from the new machine and 200 from the two existing reciprocating chillers. Amazingly the old chillers were not required to come on and the single new 200 ton chiller was able to meet the January, off peak load, fully. Electrical readings led to the following conclusion:

“Each old 100 ton recip chiller drew 160 amps and hence two chillers put together drew 320 amps whereas the single 200 ton screw machine was drawing only 180 amps.”

Spurred by this encouraging picture of power saving and afraid that the summer was just around the corner, the balance two 100 ton recip chillers were quickly dismantled and the new screw chiller installed. This work took another two weeks with work sequences exactly like the one used for chiller 1. The building air conditioning system was kept fully operative with the new screw chiller. In 15 days time from the commissioning of the first screw chiller the second chiller was also on line. Ambient temperatures had by this time started climbing up, and one 200 ton new machine was barely able to maintain the desired 7°C water temperature. The second 200 ton chiller was initially required only for peak loads and for the pull down load. In other words, it was being brought online only for an hour in the morning and for two small stretches of time in the forenoon and afternoon.

TIAM HOUSE has a full fledged data collection system for its electrical equipment and there is also a fair level of automation on the Maximum Demand control, PF control, etc. A print out of the power consumption figures is shown in Table 1. The notes at the bottom of this table need careful reading.

On an average, the saving in power is about 20,000 units per month or Rs 1,00,000/- per month based on an approximate power cost of Rs. 5/- per unit including Maximum Demand charges. Overall, the building is much better cooled after the replacement than before.

A closer study of Table 1 can lead one to the following conclusions:

1. Difference in power consumption between peak summer and coolest winter = 50,000 units
2. Likely load factor for a typical office building in Chennai = 0.5 (Based on statistical research)
3. Peak A/c power consumption may be 50,000/0.5 = 1,00,000 units
4. Peak monthly A/c cost of the building HVAC system, 360 ton = Rs. 5,00,000/- (for 70,000 ft2 area)
5. Likely average hours of operation per month = 250 hour
6. Hourly peak power cost of A/c alone = Rs. 2000/- per hour
7. Peak monthly cost of A/c per sq. ft. 5,00,000/70,000 = Rs. 7.15

Conclusion

Within a period of forty odd years TIAM HOUSE has experienced centrifugal, reciprocating and screw chillers in their air conditioning system that not many other buildings can talk about, if they could. The building has kept up with the times and the present installation of two screw chillers, high efficiency heat exchangers and microprocessor control panel has led to a large saving in power consumption over the earlier installation of reciprocating chillers.

[top]