reduce costs and improve control with enviro-tec® ssr based, time ...

VOLUME 1SPRING 2003PX-00-0045F OR HVAC DESIGNERS & FACILITIES M ANAGERSREDUCE COSTS AND IMPROVE CONTROL WITH ENVIRO-TEC ®SSR BASED, TIME PROPORTIONAL ELECTRIC HEAT CONTROLToday’s higher outdoor airquantities demand larger electricheaters to meet design heatingloads. The traditional method ofstep control results in eitherincreased first costs and maintenancecosts if many steps areused, or increased energy costsif few steps are used.Many designersdo not considerdischarge airtemperature controlwith staged electricheat. Now, however,time proportioningthe power to the electricheater essentially providescontinuously variable heat outputto exactly meet the needs of thesystem. Each of these points willbe discussed in this article.HOW DOES TIMEPROPORTIONING WORK?Essentially, the power to theelectric heater is switched on andoff rapidly. The higher the ratioof “on” time to “off” time, thehigher the discharge temperatureof the air leaving the heater.For the purpose of providing anillustration, let’s assume this “on”and “off” switching is done basedon one hundred cycles of theincoming AC power. For example,if very little heat is required, theheater would be energized for onecycle and offElectric heaters are a factory mountedand wired option on ENVIRO-TEC ® airhandling units. Single point powerconnections are standard; dual pointpower connections are available. Airhandler and heater are cETL listed asan assembly. The solid state relayprovides stable space temperatureunder widely varying load conditions.for ninety-nine cycles; if fifty percentof the capacity of the heaterwere required, the heater wouldbe energized for fifty cycles, andoff for fifty cycles; and so forth.Of course, if this rapid switchingwere done with an electromechanicaldevice such as a relayor contactor, the device would beextremely noisy and wearout rapidly.WHAT IS AN SSR?Early versions oftime proportionedheater controls usedSCR’s or triacs,which are types ofelectronic switches.These devices avoided theproblem of mechanical wear,but, due to the nature of theirsmall packages, were unable todissipate heat well, causingreliability problems. The SSR (SolidState Relay) was designed to solvethis problem. Although it usesessentially the same technologyas the other devices, the SSRfeatures silicon wafers up to ten

times larger than the older deviceswith equal current ratios, andheat dissipating areas over twentytimes the size of the olderdevices. The latest versions ofSSR’s feature three phase, 480volt control in a single package,greatly reducing the number ofconnections required, whichfurther enhances reliability.Another advantage of the SSR iszero voltage switching. In otherwords, the power to the heater isonly switched on and off when theAC waveform is crossing zerovolts.This switching method not onlygreatly reduces stress on systemcomponents, it also avoids voltagetransients which are generatedwhen the power is switched on oroff at the peak of the AC voltage.Most older time proportionedcontrols, and many current ones,do not use this switching method.DOES TIME PROPORTIONEDHEAT REDUCE COSTS ANDIMPROVE CONTROL?With only a limited number ofsteps of electric heat, chances arethat the heating requirements ata given point in time will notmatch the incremental capacity ofthe number of stages available. Forinstance, in the case of a twostep heater, if the actual outputrequirement of the heater is sixtypercent, the second stage willneed to be switched on (onehundred percent output) and off(fifty percent output) in an effortto maintain the desired temperaturecontrol. Consequently, thespace or discharge temperature(depending on the heater application)is mostly either too warmor too cool. Not only is energywasted when the space is toowarm, but, since it takes moreenergy to heat a space to a desiredtemperature than to maintain itat the temperature, energy iswasted when the space is reheatedafter cooling. This energy losscan be reduced by switching thesteps on and off more rapidly,but, as discussed early, this reducesthe life of the system components.Also, occupants may findthe “clicking” associated withenergizing and de-energizingmagnetic contactors annoying.VERTICAL HI-RISE FAN COIL ELECTRIC HEATElectric heat is available in a wide variety of kWand voltage configurations. Options include doorinterlock disconnects and low voltage controls.Controls and electric heat components on allENVIRO-TEC ® products are factory mounted andwired, and ETL listed as an assembly.DIRECT DRIVE BLOWER COIL ELECTRIC HEATPremium quality materials such as stainless steelelement hardware, Ni-Chrome wire elements, andhigh temperature rated wiring exceed typicalindustry quality standards. Control and electricheat configurations on all ENVIRO-TEC ® productsare with single point power and are ETL listed asan assembly.Specifier’s Guide Volume 1 • ©2003 Environmental Technologies, Inc.

Adding more steps will also helpreduce this energy loss, but at agreater first cost and increasedmaintenance due to an increasednumber of components. The costof time proportioned control isactually less than four steps ofcontrol using magnetic contactors,or two steps of control usingmercury contactors. Also, mostDDC controllers cannot handlemore than two or three steps ofelectric heat on fan poweredterminals.options include wall or ductthermostats, or inputs for controlby others. Input options include0-135Ω, 4-20 mA, 2-10 VDC,6-9 VDC or PWM (pulse widthmodulation).For further information on this orany other product, contact yourENVIRO-TEC ® representative or visitwww.enviro-tec.com.WHAT OPTIONS DOESENVIRO-TEC ® OFFER WITH ITSTIME PROPORTIONEDELECTRIC HEAT?ENVIRO-TEC ® Time ProportionedElectric Heat is available in eithersingle or three phase with linevoltages up to 480 VAC. InputSSR ELECTRIC HEAT CONTROLLER AND RELAYHORIZONTAL FAN COILELECTRICAL ENCLOSUREThe bottom hinged electricalenclosure provides access toa spacious electrical compartment.This compartmenthouses all electric heat andcontrol components. Terminalstrips are furnished for simplepower andcontrol wiringconnections.Multipleknockouts allowwiring entriesfrom eitherside of thecompartment.Specifier’s Guide Volume 1

ENVIRO-TEC ® S INGLE Z ONE E LECTRIC H EAT VAVC ORRECTS C OMMON I NDUSTRY H EATER P ROBLEMSThe SDR-EH breaks new ground in singleduct VAV electric heater design. Thepatented FlowStar TM sensor permitsmodulation to lower airflow levels than allother sensors in the industry. This minimizesthe energy expended for heat inmany applications. The FlowStar probeprovides a control signal that is 2.75 timesthe signal provided by the traditional Pitottube sensor.Historically, heater elements placeddownstream of a VAV damperhave experienced two majorproblems:• Elements fail prematurely due tohot spots resulting from anuneven air velocity profile overthe heater face• Heaters suffer rapid nuisancecycling of the contactors andelements because the airflowswitch probe is located on thelow pressure (downstreamside) of the VAV damperENVIRO-TEC‘s unique electric heatVAV terminal, the SDR-EH, solvesthese problems. The heaterelements are located midwaybetween the air inlet and thedamper. This design providesuniform airflow over the face ofthe electric heater at all damperpositions. Element life is extended,reducing repair cost,discomfort, and inconvenience.With the heater elements locatedon the high pressure side ofthe VAV damper, the airflowpressure switch receives areliable pressure signal even atminimum damper positions. Thisarrangement provides greatersafety, as well as enhancedreliability.The SDR-EH design permitstremendous flexibility whenselecting KW, voltage, phase,balanced or unbalanced circuitingand method of control.With over 100,000 of these electricheat terminals installed, thisdesign concept is “tried and true.”The SDR-EH from ENVIRO-TEC ®raises the standard of excellencein electric heat VAV design.The FlowStar TM probe is visible in the inlet of theSDR-EH. The elements, partially removed for thisphoto, are midway between the inlet and damper.The outlet end of the SDR-EH reveals the VAVdamper. Heater elements are well upstream of thedamper in an area of uniform airflow.Specifier’s Guide Volume 1 • ©2003 Environmental Technologies, Inc.