Presentation on theme: "Heat Exchangers By Dave Miller Copyright Dave Miller 2010. May not be reproduced in any form without Permission of the author."— Presentation transcript:
Heat Exchangers By Dave Miller Copyright Dave Miller 2010. May not be reproduced in any form without Permission of the author.
Heat Exchangers for the Home Brewery What they do How they do it Why wort cooling is important Why you may want a better heat exchanger Thermodynamics of counterflow heat exchange Heat exchanger operation and maintenance
Basics of heat exchange 1. Usually involves fluids 2. Same or different, but they are not mixed 3. Liquids or gases 4. May involve transition from one state to another 5. Only universal is that both fluids change temperature 6. Heat gain by one = heat loss by other Wort cooling is the simplest kind of heat exchange
The Basic Mechanism Conduction In heat transfer, conduction (or heat conduction) is the transfer of thermal energy between neighboring molecules in a substance due to a temperature gradient. It always takes place from a region of higher temperature to a region of lower temperature, and acts to equalize the temperature differences.
Wort Cooling Depends on the conductance of a metal surface which separates a warm liquid (wort, which is basically water) from a cooler liquid, which may either be water or a water/glycol mixture.
Why it must be done: because boiling wort kills yeast Why it should be done quickly: Minimum time in the “red zone” where yeast cannot survive, but bacteria can – and can multiply! Maximum cold break = cleaner clearer beer
Heat Exchanger Designs Parallel flow – least efficient, never used Cross flow – more efficient “Immersion chiller” Counter flow – most efficient Tube in shell (often called “counterflow”) Plate Brazed plate Plate in frame (commercial breweries)
A: Counter Flow B: Parallel Flow Red: Hot fluid Blue: Cold Fluid Temperature gradient for the two designs shows the difference in efficiency. Note the “crossing” of hot and cold fluid temperatures in A.
Comparison of Cooling Times 25 ft “immersion” – constant stirring 80 F tap water – 5 gal. wort to 90 F in 20 min. 40 F chilled water – to 70 F in 10 min. Home built counterflow tube-in-hose 5 gal. wort to 70 F – 10 min. Heat exchanger capacity – 1 quart Actual cooling time 10 min. / 20 = 30 sec. Plate heat exchanger – same parameters Heat exchanger capacity (est.) – 10 oz. Actual cooling time < 10 sec.
Pale ale wort Pre-pitch, 70 F. Cold break This is why I wanted a counterflow heat exchanger.
Full disclosure – the case against counterflows 1. Operation complex, may require a pump 2. Costly 3. Cleaning difficult, requires a pump All true to some degree. Cleaning is biggest issue. If you use a counterflow you must have a pump. If you already have a pump the other objections are outweighed by the advantages.
The Master Equation Q = TC x LMTD Q: Cooling Power in Kcal/hr TC: Transfer Coefficient LMTD: Log Mean Temperature Difference TC is fixed by the the fluids involved in the transfer, and by the physical properties of the heat exchanger, such as the transfer surface material, thickness, and area, and by the turbulence of flow. In practice it must be determined by testing a working unit. LMTD is Log Mean Temperature Difference between the coolant and the liquid being cooled. Think of it as “thermal pressure.”
Calculating Q: example 19 L boiling wort to 20 C. in 2.5 min. Water flow rate = 5 gpm (19 Lpm) Total water = 19 x 2.5 = 47.5 L Water temp = 15 C Total heat loss/gain = 80 (cal/gm) x 19,000 = 1520Kcal Q = 1520 x 60 / 2.5 = 36480 Kcal per hour
Calculating LMTD: same example Water entry temp = 15 C Heat gain = 1520 Kcal / 47.5 L = 32 C Water exit temp = 47 C Wort entry = 100, Wort exit = 20 D1 = Wort entry – water exit = 100 – 47 = 53 D2 = Wort exit – water entry = 20 - 15 = 5
The payoff: Calculating TC Q = TC x LMTD Q = 36480 Kcal/ hr; LMTD = 20.3 C 36480 = 20.3 x TC TC = 36480 / 20.3 = 1797 If we know TC, we can determine whether a desired cooling job is within the capabilities of the heat exchanger. Example: can we cool 19 L of boiling wort to 20 C in 5 minutes, using only 19 L of chilled water at 5 C?
To answer the question, we must calculate Q and LMTD for the new set of conditions. 19 L boiling wort to 20 C in 5 min. Water flow rate = wort flow rate = 3.8 Lpm Water volume = wort volume = 19 L Water temp = 5 C Total heat exchanged is as before, 1520 Kcal Q = 1520 x 60 / 5 = 18240 Kcal /hr
When wort flow and water flow are equal, the equation is greatly simplified. D1 is equal to D2 and to LMTD. D2 = Wort exit – water entry = 20 – 5 = 15 D1 = D2, so water exit temp = 100 – 15 = 85 C. D2 = D1 = LMTD = 15 Q / LMTD = 18240 / 15 = 1216 TC must be 1216 or higher. We previously calculated it as 1797, so the heat exchanger is easily equal to the task. In fact the knockout would have to be faster, or the water rate would have to be be slowed down, to meet the desired temperature.
Another example: how long would it take to cool 19 L of boiling wort to 10 C using the same cooling water? To solve this we must calculate LMTD and then solve the equation for Q. D2 = 10 (wort exit temp) – 5 (water entry temp) = 5 D2 = D1 = LMTD = 5 Note water exit temp is 95 C! Q = TC x LMTD = 1797 x 5 = 8985 Kcal / hr. Heat to be removed = 90 x 19L = 1710 Kcal 1710 / 8985 =.19 hr. = 11.4 min.
Two Stage Heat Exchangers If you cannot afford a monster heat exchanger, you can use two smaller ones. This will be slower and use more water. Example: two home built units w/ TCs of 170 & 340. Larger unit (30 ft. of 3/8 copper): city water Smaller unit (15 ft.): cold liquor Larger unit cools wort to city water + 10 C Smaller unit cools wort to pitching temp Total cost of 2 units approx. $80
Notes on the photo Heat exchangers stacked vertically, tap water unit on top. Wort line connects the two units. Hot wort is pumped into upper unit, cold wort exits from lower unit into fermenter. CO2 pushes cold liquor from cold liquor back (keg) through lower unit.
Operating the Heat Exchanger 1. Make all connections. 2. Prime pump. 3. Start tap water flow @ approx. 2 gpm. 4. Start pump, set flow rate @ approx..5 gpm. 5. Start cold liquor, adjust flow to achieve correct wort knockout temperature. No problem, IF you have calibrated the flow rates! Disclaimer: these settings work for ales (65 – 75 F.)
CPVC thermowell on wort outlet of second (cold liquor) heat exchanger. 1/8 in. compression fitting = McMaster Cat. # 5533K433. 1/2 x 1/8 in. stainless Bushing = McMaster Cat. # 4464K264. Thermometer = Harbor Freight.
To stay out of trouble: CIP heat exchanger before first use Calibrate wort, cold liquor and tap water (optional) flow rates before first use. In case of trouble (high wort exit temp): Speed up tap water Slow down wort Slow down cold liquor
CIP: the critical points Use a pump Use a noncaustic cleaner @ 140-160 F Backflush most of the time Steps: 1. Tap water rinse 2. Hot backflush 40 minutes alkali cleaner 3. Tap water rinse 4. (opt.) acid rinse – citric preferred Sanitizing: Star San or hot liquor, just before use
Q: I sanitize my heat exchanger by circulating hot wort through it for 10 minutes after the boil ends. Is this as good as hot water? A: It will certainly sanitize the heat exchanger, but it is not advisable for two reasons. First, you will foul the heat exchanger with hop particles and trub. This is a bigger issue with plate heat exchangers because the passages are so narrow. The second reason is that hot wort should be pumped only after it has cleared: In other words, after whirlpool, when the trub pile has formed on the kettle bottom. The impeller's shearing action will chop trub particles, which have grown bigger during the boil, into smaller pieces, which tend to stay in suspen- sion and can easily pass through the heat exchanger into the fermenter during knockout. Remember hot trub has a different chemical composition from cold trub. While professional brewers disagree about the importance of removing cold break material there is no such argument about hot break. Bottom line: Recirculating hot wort with a pump is undesirable. Brewpub kettle- whirlpools are a compromise. Best practice, which is almost universal in larger breweries with a separate whirlpool vessel, is to locate it below the kettle and drop the wort in by gravity. Home brewers can whirlpool in the kettle because all it takes is 15 or 20 seconds stirring with a paddle. Then after settling, pump the clean wort out.