E ngi neers News l e t t e r providi ng i nsight s fo r t oday’s hva c sys tem des igner © 2012 Tr ane, a business of I ngersol lR and. Al l r i ght s r eser ved. 1 vol um e41 –3 Condenser Water System Savings Optimizing flow rates and control onl y save instal l ati on co st but w ill us uall y save energy over the cou r se of the ye ar . ” • Tayl or 3 states, “ C al cu late t h e co nden ser w at er f l ow rate f or al l pi pe sect i on s assu mi ng a r ang e o f 15°F ( 8°C ) . ” Fr om a desi gn par am et er s p er spe ctive, r educi ng condenser w at er flow r at es by i nc r ea si ng t he de sign t em pe rature di f f er ence is ad van tag eous si nce it r educe s b ot h i nst all ed and o perating co sts.In ad di t i on, usi n g vari ab le- sp eed dri ves on cooling t ow er fan m otor s is be ne f i ci alduring t i m es w he n t he he at r ej ecti on load i s l ow er. To unde r stand t hi s i ndustr y g ui dan ce , t his ne w sl et t er cove rs the ef f ect of con de ns er w at er f l ow r at es and t em perature on syst em de sign and op er ati on . System Design W hy do ASH R AE, Kel l y and Chan,and Tayl or r eco m m end f low r at es t hat ar e l ow er t ha n t he A H R Ist an da r d r at ing co nd i t ions? To an sw er t hat qu est ion, l et ’ s e xam i ne t he r el ati on shi p bet w een the con de nse r w ater f l ow r at e an d o t her system vari ab l es ( see t he si deba r on p.7 for a b r i ef ove r vi ew ) .W e no t e t ha t as t he con de ns er w at er f low r at e decreases: • Th e chiller l eavi ng co ndenser w at er tem per at ur e an d chi l l er pow er rise. • The condenser w ater pum p pow er drops. • I f the sam e co ol i ng t ow er i s u se d,its approach t em pe rature decreases. • I f the pi pe size is reduced and so m e of the pi pe cost savi ngs i s u sed to o ve r si ze the co ol ing t ow er ,t he t ow er f an po w er can al so b e red uce d. Optimized flow rates in the condenser water system provide installed and operating cost savings in chilled-water systems. This EN provides guidance with respect to flow rate selection and offers project teams options to consider for implementation in specific circumstances. Rat he r t ha n rel eg at ing co nd en ser w at er syst em de si gn t o st andards devel op ed 50 yea r s ag o, conside r taking a se co nd look. R ece nt de vel op m ents such as i ncreas ed ch i l l er ef f ici ency, t he escal ati ng c ost of m ater i al s, an d the i ncr ease d use of vari ab l e- sp eed dri ve s on p um ps and co oli ng t ow er fans pr ovi de si gni f i cant oppor t unities to the ow ner and desi gn er of condenser w at er sys t em s. Taking ad van t ag e of t he se opp ort un i t i es r equires d eter m i ning t he op t i m um con denser w ater f low rat e. P r ese nt i nd ustr y g ui dan ce has sh i f t ed f r om t he “st an dard”3 gp m / t on f l ow rat e ( an d o f t-assu m ed 10°F ∆T) to l ow er f l ow rates and l arge r∆Ts as evidenced bel ow . • From t he AS H R A E G r eenG uide: 1 “ Th e C ool Tool s C hi l l ed W at er P l ant Desi gn and P er f or m ance Speci f i cat i on Gui de recom m ends a de sign m ethod t h at st arts w ith a condenser w at er t em perature dif f erence of 12°F to 18 °F ( 7 °C t o 10 °C ) . ” • K ell y and C han 2 state, “ I n m ost cases, l arge r∆Ts and the associated low er f low r ates w i l lnot *A t t he A H R I S tandard fl ow rat e o f 3 g pm / t on , us i ng t od ay’ s eff i ci en t ch i l l ers, the∆T i s 9.3° F rat her t han t he 10°F oft en assum ed. Table 1. Summary of selection results for example condenser water systems for 700-ton building load AHRI Standard 550 / 590 9.3ºF T* ASHRAE GreenGuide 14ºF T, same tower, smaller pipes ASHRAE GreenGuide 14ºF T, oversized tower, smaller pipes C W f l ow r at e 2100 1400 1400 C W pi pe size 10 8 8 C W system P D 30 30 30 condenser PD 24 11 15. 6 t ow er st atic l i ft 12.3 12.3 19. 2 chi l l er power 398. 9 410.7 411.9 C W pump power 37.6 20.2 24.5 t ow er fan power 32.1 32.1 16. 0 t ot al( kW ) 468. 6 456.7 452. 4
