Injection Mixing from Thermal Storage an Established Technique That Can Be Used in Renewable Energy Systems

Renewable Heating Design John Siegenthaler, P.E. | [email protected]

Injection mixing from thermal storage An established technique that can be used in renewable energy systems.

any hydronic-based renewable radiators on the second floor. The idea is to valves and are not familiar with variable- energy systems include thermal select a maximum supply water temperature speed injection mixing as an option. And M storage tanks. When the heat input for the entire system based on design-load while mixing valves can certainly accomplish is from solar thermal collectors or biomass conditions and then match the various heat the intended task, there are scenarios where boilers, it is possible for the water tempera- emitters to the room loads based on their variable-speed injection mixing may be less ture in those tanks to reach temperatures of respective heat output rates at the selected expensive because it provides both fluid transport 180-200º F. Thermal storage tanks supplied supply temperature. and mixing, thus eliminating the need for a by electric boilers operating on off-peak elec- The combination of high-temperature circulator in combination with a mixing valve. tricity also can reach these high temperatures water in thermal storage and low-tempera- by the end of the charging cycle. ture heat emitters necessitates some type of Hot in/Cool out Although few would argue a 500 gal. tank mixing assembly between these subsystems. Injection mixing is simply a way to add hot filled with 190º water doesn’t contain a lot of There are several mixing options includ- water to a circulating hydronic distribution sys- energy, the usefulness of that energy depends ing thermostatically actuated and motorized tem while at the same time removing an equal on the supply temperature required by the three-way mixing valves or injection mixing amount of cooler water. Replacing cooler water heat emitters to meet the building’s heating using either a motorized valve or variable- with hotter water means heat is added to the load. The lower this temperature, the greater speed injection pump. circuit. When heat is added to a circuit, its rate the temperature drop the tank can undergo The latter is a technique that, while of heat output increases. The faster hotter water and thus the more useful energy it can deliver. available for more than 25 years and applied is exchanged with cooler water, the greater the in many low-temperature systems supplied by rate of heat output. Aim low conventional boilers, has not found frequent With injection mixing, the two fluid In my opinion, all hydronic heating sys- use in systems supplied by renewable energy streams come together in a tee rather than tems, with or without renewable energy heat heat sources. Many engineers immediately within a valve. This fundamental concept is sources, should be designed to supply design associate the need for mixing with mixing shown in Figure 1. load output using a supply water temperature no higher than 120º. This upper threshold > Figure 1. consistently is attainable by most renewable heat sources such as solar collectors, air-to- distribution system water heat pumps, geothermal water-to-water load heat pumps and biomass boilers. It’s also complementary to systems using high-efficiency mod/con boilers. There are several heat emitter options that supply temperature sensor allow designers to assemble low-temperature purge heating distribution systems. They include complete mixing valve floor, wall and ceiling radiant panels, panel radiators, low-temperature fan coils, and even mixing begins here “low-temp” fin-tube baseboard. closely spaced tees Designers can combine multiple types of low-temperature heat emitters into the same

system. One example would be a heated slab- cooler water out hot water in on-grade radiant panel on the first floor of a

building with individually controlled panel Graphics courtesy of John Siegenthaler

Note: The views expressed here are strictly those of the author and do not necessarily represent pme or BNP Media.

8 02.17 HYDRONICS KNOW HOW II > Figure 2.

load BY JOHN SIEGENTHALER AND BNP MEDIA

The Hydronics Know How II DVD contains all the columns and articles John Siegenthaler has written for purge Plumbing & Mechanical and PM Engineer, from the very ﬁ rst in July 1996 through December 2008. That’s over valve flow restricting 200 articles and columns covering everything from heat loss to hydraulic separation. valve b/w tees This digital collection is a virtual encyclopedia on flow rate modern hydronic systems. No more cutting up magazines – This DVD makes it easy to access all of “siggy’s” articles indicator modulating valve and columns. Better yet, all the information is searchable. Just type in a key word or phase and Adobe Acrobat® will instantly search all the columns and articles for you. Good designers know that it never hurts to go back and review what you thought you completely understood a few years ago. Sometimes a small bit of new information can yield

cooler water out major benefi ts when applied repeatedly in future systems. hot water in The information on this DVD will help you transform hundreds of individual components into smoothly operating hydronic systems. Systems that consistently deliver load what their owners expect. In addition to the columns and articles, the Hydronics Know How II DVD contains the latest information on products, design and installation methods from several leading manufacturers. We are confi dent you’ll fi nd details that will further en- purge closely spaced tees hance the systems you design and install. If you don’t, we valve will refund you in full. provide hydraulic separation b/w distribution circulator SPONSORED BY: and injection pump PLATINUM GOLD BRONZE variable speed injection pump

Any hardware assembly that can regu- Figure 3 (on page 10) shows a typical late the exchange of warmer water for cool- piping detail for injection mixing using a er water is a potential means of injection variable-speed pump along with formulas mixing. Common hardware arrangements that govern the rate of heat transfer. include a modulating two-way valve as well The variable-speed injection pump can be as with a variable-speed pump. These are located in either of the two injection “risers.” shown in Figure 2. It can push heated fluid into the distribution The temperature sensor that provides system or push cooler water from the distri-

feedback on the supply water temperature bution system back to the heat-source circuit. + free to the distribution system should be located The latter keeps the pump slightly cooler. $ shipping downstream of the distribution circulator. The injection risers connect the heat- 99 This ensures complete mixing before the source circuit to the distribution system. The water passes by the sensor. closely spaced tees at these connections pro- TWO WAYS TO ORDER vide hydraulic separation between all three 1 // ONLINE at www.pmmag.com/products The math circulators. This is critically important since 2 // CALL Katie at 248/244.1275 The thermodynamics associated with the variable-speed injection pump is often injection mixing are simple and indepen- operating at a much lower speed than the Visit www.pmmag.com/products to dent of the hardware being used. Further- other two circulators. place your order or to view our entire more, injection mixing is fully “scalable” Hot water passes through the red injec- selection of technical resources. CALL KATIE @ 248/244.1275 WITH

Graphics courtesy of John Siegenthaler from residential systems to large commercial tion riser at a rate determined by the speed ANY QUESTIONS. systems where heat transfer rates can be of the injection pump. Since the distribu- several million Btu/h. tion system is completely filled with fluid,

pmengineer.com 9

PME AECStore_Hydronic Know How II DVD_Promo_0614.indd 1 5/16/16 3:30 PM Renewable Heating Design

> Figure 3. to handle the injection flow rate (fi), rather closely from than the full distribution system flow rate spaced heat return (fd). If the heated water being injected is tees source to heat from the heat emitters, the significantlyinjection flow hotter rate canthan be the quite temperature small relative of to the distribution flow source rate. the water returning from the heat emitters, temperature = Tc temperature = Th from the heat emitters, thethe injection injection flow flow rate ratecan becan quite be quite small small relative to the distribution flow injection rate. relative to the distribution flow rate. risers variable For example: Assume water atFor 180 example: ºF is available Assume from water the atheat 180º source, is and that the speed available from the heat source and that the injection flow rate = fi distribution system serves radiant floor heating circuits that operate with a supply water injection sensor (S1)For example: Assume waterdistribution at 180 ºF system is available serves fromradiant the floor heat heat source,- and that the temperature of 100 ºF at design load conditions, with a corresponding return water temperature circulator temperature = Tmix ing circuits that operate with a supply water purge distribution system serves radiant floor heating circuits that operate with a supply water of 85 ºF. Also assume distribution system flow rate is 10 gpm. Under these conditions the rate valve temperature of 100º at design load conditions temperature of 100 ºF at design load conditions, with a corresponding return water temperature of heat dissipation by the distributionwith a corresponding system is: return water tempera- closely distribution of 85 ºF. Also assume distributionture of 85º. system Also flowassume rate distribution is 10 gpm. system Under these conditions the rate spaced circulator flow rate is 10 gpm. Under these conditions tees of heat dissipation by the distribution system is: the rate of heat dissipation by the distribution = − = heat systemQ 4 is95 shown(10)(1 0in0 Formula85) 71:4,250Btu / hr distribution flow rate = fd emitters Q = 495(10)(100 − 85) = 74,250Btu / hr = − The required injection flow rate to transfer heat to the distribution system at this rate can be 1 Q k( fi )(Th Tc ) found using formula 2 in figureThe 3. required injection flow rate to transfer heat to the distribution system at this rate can Q The required injection flow rate to transfer heat to the distribution system at this rate can be = be found using Formula 2 in Figure 3. 2 fi found using formula 2 in figure 3. k(Th − Tc ) Q 74,250 ⎛ fi ⎞ fi = = = 1.6gpm Tmix = Tc + (Th − Tc ) 3 ⎜ ⎟ k(Th − Tc ) 490(180 − 85) ⎝ fd ⎠ Q 74,250 = = = fIni this scenario the injection flow rate1.6 gonlypm k(Th − Tc ) 490(180 − 85) Q = rate of heat input to distribution system (Btu/hr) is about 16% of the distribution flow rate. Such In this scenario the injection flow rate is only about 16% of the distribution flow rate. Such a fi = injection flow rate (gpm) a small flow rate could easily be handled by a small flow rate could easily smallbe handled circulator by and a small1/2-in. circulator injection riser and piping. 1/2” injection riser piping. fd = distribution system flow rate (gpm) In this scenario the injection flow rate is only about 16% of the distribution flow rate. Such a

Th = temperature of fluid being injected into distributionsmall systemflow rate (ºF) could easilyThermal be handled storage by a small is notcirculator always and hot1/2” injection riser piping. Thermal Storage is Not Always Hot: While high temperature water from the heat source While high-temperature water from the heat Tc = temperature of fluid on return side of distribution system (ºF) combined with a low temperaturesource combineddistribution with system a low-temperature is an ideal conditiondistri- for injection mixing, T = temperature of fluid supplied to heat emittersThermal (ºF) Storage is Not Always Hot: While high temperature water from the heat source mix such conditions aren’t alwaysbution present system when is an injecting ideal condition from afor the injectionrmal storage tank supplied by k = a constant, 495 for water, 450 for 50% propylenecombined glycol withsolution a low temperaturemixing, suchdistribution conditions system aren’t is always an ideal present condition for injection mixing, renewable energy heat sources. such conditions aren’t alwayswhen present injecting when from ainjecting thermal storagefrom a tankthermal sup- storage tank supplied by plied by renewable energy heat sources. renewable energy heat sources. and equal flow rate of cooler water exits the injection flow rate, asFor well example, as the temperature a thermal storage For tank example, supplied a thermal by solar storage collector tanks mightsup- attain a temperature of distribution system through the other (blue) of the fluid being injected, and the temperature plied by solar collectors might attain a tem- 180 ºF after a sunny / mild day, but might only be at 100 ºF a couple of cloudy days later. A injection riser, the heated water enters the of the fluid on the return side of the distribution perature of 180º after a sunny and mild day, For example, a thermal storage tank supplied by solar collectors might attain a temperature of downstream tee in the distribution system system are known. thermal storage tank suppliedbut bymight a pellet only boiler be at might 100º reacha couple 19 0cloudy ºF at the end of a boiler firing 180 ºF after a sunny / mild day, but might only be at 100 ºF a couple of cloudy days later. A and mixes with a portion of the cooler fluid Formula 2 is a rearrangementcycle, but could of Formula drop 1. to It a muchdays later.lower A temperature thermal storage as heattank suppliedis extracted, by but before the boiler that has passed through the upstream tee. can be used to calculatethermal the injection storage flow tank rate supplied a pellet by aboiler pellet might boiler reach might 190º reach at the 19 0end ºF ofat the end of a boiler firing These two fluid streams mix together to required to establishfires a specified for the nextrate chargingof heat cycle.a boiler The firing latter cycle, condition but could is verydrop desirableto a much in that it allows for long cycle, but could drop to a much lower temperature as heat is extracted, but before the boiler determine the mixed fluid temperature sup- transfer into the distributionboiler firingsystem. cycles. lower temperature as heat is extracted, but plied to the heat emitters. Formula 3 can befires used for to the calculate next charging the beforecycle. Thethe boilerlatter conditionfires for the is verynext desirablecharging in that it allows for long The formulas shown in Figure 3 are all based mixed fluid temperatureboiler supplied firing cycles.to the heat cycle. The latter condition is very desirable in on conservation of mass and heat balances. emitters at sensor (S1). that it allows for long boiler firing cycles. Formula 1 can be used to calculate the rate A distinct advantage of injection mixing is These conditions are not a problem pro- of heat input to the distribution system if the the hardware regulating the mixing only has vided the injection flow rate can increase as

10 02.17 Renewable Heating Design

These conditions are not a problem provided that the injection flow rate can increase as the temperature of the water from the heat source decreases. For example, we previously the temperature of the water from the heat > Figure 4. calculated an injection flow sourcerate of decreases. only 1.6 gpm For example,based on we 180 previously ºF water from the heat source. If closely calculated an injection flow rate of only 1.6 gpm distribution the heat source could only supply 105 ºF water, and the same 100 ºF supply / 85 ºF return spaced circulator based on 180º water from the heat source. If the tees temperature were to be maintainedheat source in thecould distribution only supply system, 105º water the requiredand injection flow rate heat would be: the same 100º supply/85º return temperature purge emitters was to be maintained in the distribution system, variable speed injection circulator valve the required injection flow rate would be: w/ internal spring-loaded check valve Size injection risers and pump for injection flow rate when tank is at Q 74,250 f = = = 7.6gpm minimum useful temperature. i − − short header k(Th Tc ) 490(105 85) keep tee as close to tank as possible

The same rate of heat transfer (74,250 Btu/h) The same rate of heat transfercan be(74,250 maintained Btu/hr) under can these be maintaine conditions,d prounder- these conditions, vided the injection pump can achieve a flow provided that the injection pumprate of can7.6 gpmprovide through a flow the rateinjection of 7.6 risers. gpm through the injection risers. spring-loaded check Tank as a heat source valve with forward Tank as Heat Source: Figure 4 shows one way to configure a variable speed injection pump in opening pressure of Figure 4 shows one way to configure a vari- at least 0.5 psi combination with a thermal able-speedstorage tank. injection pump in combination with a thermal storage tank. The thermal storage tank is set up as a “two- [insert figure 4] pipe” configuration. The piping supplying the to / from renewable heat source load tees off from a short and generously sized header entering the upper sidewall connection on the tank. Return flow from the load ties into thermal storage tank a similar header at the lower sidewall connec- short header keep tee as close tion. The combination of these short and gener- to tank as possible ously sized headers along with an extremely low dynamic pressure drop through the tank provide adequate hydraulic separation between temperatures under partial-load conditions. Large commercial systems can use vari- the heat source circulator and the variable- Lower tank temperatures – provided they can able-frequency drives to regulate the speed speed injection pump. create adequate heat output from the distri- of larger circulators with high efficiency AC The injection riser piping connects to the bution system – allow for higher efficiency synchronous motors. distribution system using closely spaced tees. with renewable heat sources. Lower tank These provide – you guessed it – hydraulic temperatures followed by “recharging” to pre- Gang tactics separation between the variable-speed injec- set higher temperatures using biomass boilers It’s also possible to use multiple (and identi- tion pump and the distribution circulator. can provide significantly greater temperature cal) injection circulators in parallel, as shown in A spring-loaded check valve with a forward- swings in thermal storage and thus lengthen Figure 5 (on page 14). opening pressure of at least 0.5 psi is necessary boiler cycles. Longer cycles improve combus- When multiple and identical injection to prevent unintentional flow through the heat tion efficiency and reduce emissions from pumps are configured as shown in Figure 5, source when the load is being supplied solely biomass boilers. only one needs to be speed-controlled. The from thermal storage. This check valve also There are several low-cost controllers cur- others can be controlled as on/off. This array of prevents reverse thermosiphoning from the tank rently available that can vary the speed of wet- circulators would operate like a multiple boiler back through the heat source circuit. rotor circulators with permanent split capacitor system having one fully modulating boiler in (PSC) motors. They are appropriate for residen- combination with additional on/off boilers. The Maintaining control tial and light commercial applications where the variable-speed circulator would ramp up as The controller operating the injection injection circulator can be 1/6 HP or less. the load begins to increase. pump could be set up to provide a fixed sup- Some commercial size wet-rotor circulators Once the load increases beyond what can be ply temperature to the distribution system. It using electronically commutated motors (ECM) supplied through the variable-speed injection could also be set up to vary the supply tem- can be operated at variable speed using stan- pump at full speed, one of the on/off injection perature based on outdoor reset control. The dard 2-10 VDC or 4-20ma control signals or pumps would turn on and the variable-speed latter is preferred because it allows the ther- tied directly to building automation systems circulator would drop back to a low speed. mal storage tank to be discharged to lower using LONworks or BACnet protocol. Assuming the load continues to increase,

12 02.17 Renewable Heating Design

> Figure 5. reduce cost relative to a single larger injection pump with a VFD control. It also can provide variable speed NOTE: All injection on/off circulators have internal injection flow rates lower than what a single on/off check valves larger circulator may be able to achieve based on a typical VFD that only reduces motor speed into the range of 20-30% of full speed. injection mixing supply controller If you are working with renewable energy temperature heat sources that involve thermal storage and sensor low-temperature distribution systems, con- sider use of variable-speed injection mixing to shuttle heat between the tank and loads. It’s a reliable and cost-effective technique.

closely

to / from renewable heat source spaced tees John Siegenthaler, P.E.,

thermal storage tank distribution system is principal of Appropriate purge Designs, in Holland Patent, valves N.Y., and author of the text the variable-speed circulator ramps up again circulators, only one of which is speed “Modern Hydronic Heating.” until two of the on/off pumps can be on and controlled. This technique allows smaller wet- He teaches two, 10-week, the variable-speed circulator drops back down rotor circulators to be used to meet the design- design-focused online courses: Master- to low speed. In theory, this control action load injection flow rates that are beyond the ing Hydronic System Design and Hydronic- could be used with any number of identical range of a single small circulator. It also may Based Biomass Heating Systems.

14 02.17