Heat Transfer Equations For “thin walled” tubes, A i = A o

Fouling Layers of dirt, particles, biological growth, etc. effect resistance to heat transfer We cannot predict fouling factors well Allow for fouling factors when sizing heat transfer equipment Historical information from similar applications Little fouling in water side, more on product

Log Mean Temperature Difference For Round, Thin-Walled Tubes

Log Mean Temperature Difference Parallel FlowCounter Flow Length Temperature T1T1 TT T2T2 Length Temperature T1T1 TT T2T2

Heat Losses Total Heat Loss = Convection + Radiation Preventing heat loss, insulation Air – low thermal conductivity Air, good Water – relatively high thermal conductivity Water, bad Vessels/pipes above ambient temperature – open pore structure to allow water vapor out Vessels/pipes below ambient temperature - closed pore structure to avoid condensation

Heat Transfer – Continued Hot wort at 70  C is transferred from one tank to another through a 2.2 cm id, 2.5 cm od stainless steel pipe (k = 120 W/m.K). The pipework is 150 m long and the wort has specific heat capacity of 4.0 kJ/kg.K and density of 975 kg/m 3. The heat transfer coefficients on the inside and outside of the pipe are 4000 W/m 2 K and 75 W/m 2 K and the temperature of the surroundings is 10  C. Assume that the pipe’s wall is “thin.” Calculate the rate of heat loss from the pipe by convection and the exit temperature at the end of the pipe. The velocity in the pipe is 1.0 m/s.

Heat Transfer – Continued Previous Problem continued… How would adding a 1 cm thick layer of insulation (k = 0.05 W/m.K) to the outer surface of the pipe effect the exit temperature of the wort. Our pipe has an external emissivity of 0.7. Calculate the heat loss by radiation (without insulation) and compare it to the heat loss by convection.

Refrigeration Terms Cooling Load, Cooling Capacity – Q in Compressor Load – W in Condenser Load – Q out Tons of Refrigeration – Rate of Heat Input Refrigerant – The Fluid Vapor-Compression Refrigeration Heat Pump – Same Cycle, Use Q out

Refrigeration Efficiency = desired output / required input Desired output = Heat removal from refrigerated space (Q in ) Required input = Work input to compressor Conservation of Energy: Q in + W in = Q out COP can be > 1.0 = Cooling Capacity

Refrigeration Applying Conservation of Energy…

Refrigeration Used when no other method of cooling is available Very expensive (40-60% of a brewery’s utility bill) Removal of heat from low T source to high T sink

Primary Refrigerants Ammonia (R-717), R-12, R-134a Saturation temp < Desired application temp 2 to 8  C Maturation tanks 0 to 1  C Beer Chillers -15 to -20  C CO 2 liquefaction Typically confined to small region of brewery Secondary Refrigerants Water with alcohol or salt solutions Methanol/glycol, potassium carbonate, NaCl Lower freezing temperature of water Low-toxicity (heat exchange with product) Pumped long distances across brewery

Theory and the Cycle Condenser Evaporator Compressor Q out Q in W in

Refrigeration 1-2: Constant entropy compression (s 1 = s 2 ) 2-3: Constant pressure heat rejection (3 = sat liq.) 3-4: Constant enthalpy throttling 4-1: Constant pressure heat addition (1 = sat vap.)

Coefficient of Performance Describes how well a refrigeration plant is running Heat removed divided by energy input COP increase with temperature difference between source and sink

Refrigeration Example An ideal vapor-compression refrigeration cycle using ammonia operates between the pressures of 2 and 14 bar. The system cools a secondary refrigerant at a rate of 25 kW. Determine: (a) The evaporator and condenser temperatures (b) The mass flow rate of refrigerant. (c) The COP of the system. (d) The power consumed by the compressor, in kW

Typical Manufacturers Performance Curves

Chemical structure of refrigerants

Refrigerant R12, CF 2 Cl 2

Dry Air Fin Condensers Fluid in condenser does not contact cooling fluid High electricity costs for fans

Wet Evaporative Condensers Fluid in condenser does not contact cooling fluid Water sprayed onto tubes to evaporate and cool

Cooling Tower Condensers A secondary fluid (water) sprayed Air passes across water droplets, cools Forced or induced draft, counter or cross Cool water to heat exchange condenser

Condenser Selection Considerations Ambient temperature (Air-fin?) Ambient humidity (evaporation?) Space, accessibility, maintenance Electricity costs (air-fin) Chemical costs (evaporative, tower) Legionellosis or L. pneumophila Major source cooling towers and evaporative coolers Name from 1976 meeting of American Legion – killed 36 people Kill by heating to 60 o C or chlorine

Evaporators and Expansion Devices Direct expansion with thermostat valve Regulates flow of liquid being throttled into evaporator Diaphragm to balance pressure between liquid in condenser and sum of evaporator and spring pressure

Evaporators and Expansion Devices Flooded with level control Level of liquid in reservoir (typically shell and tube heat exchanger) controlled with variable throttle valve.