HVAC Design: Field Trip Wednesday, October 22nd 9 am Location: St HVAC Design: Field Trip Wednesday, October 22nd 9 am Location: St. Edward's University Hosts: - Hannah James - Wesley Stidham, PE Principal Shah Smith and Associates, Inc

HW3 Cooling Cycles Problems: 1) Book: 3.1 (page 69), 2) Book: 3.5 ((page 70), 3) Same like 3.5 for R22 with no intercooler 4) Book 3.9 (pages 70-71) 5) Book 4.3 (page 110) Heat exchanger problems: 6) Book Problem 11.1 (page 326), 7) Problem 11.7 (page 327) Due March 23th

Objectives Learn about heat exchangers (ch.11)

Heat exchangers Air-liquid Tube heat exchanger Air-air Plate heat exchanger

Some Heat Exchanger Facts All of the energy that leaves the hot fluid enters the cold fluid If a heat exchanger surface is not below the dew point of the air, you will not get any dehumidification Water takes time to drain off of the coil Heat exchanger effectivness varies greatly

Heat Exchanger Effectivness (ε) C=mcp Mass flow rate Specific capacity of fluid THin TCout THout TCin Location B Location A

Example: What is the saving with the residential heat recovery system? Outdoor Air 32ºF 72ºF 72ºF Combustion products 52ºF Exhaust Furnace Fresh Air Gas For ε=0.5 and if mass flow rate for outdoor and exhaust air are the same 50% of heating energy for ventilation is recovered! For ε=1 → free ventilation! (or maybe not)

Coil Extended Surfaces Compact Heat Exchangers Fins added to refrigerant tubes Important parameters for heat exchange?

Overall Heat Transfer Q = U0A0Δtm Mean temperature difference Transfer Coefficient Mean temperature difference

Heat Exchangers Parallel flow Counterflow Crossflow Ref: Incropera & Dewitt (2002)

Heat Exchanger Analysis - Δtm

Heat Exchanger Analysis - Δtm Counterflow For parallel flow is the same or

Counterflow Heat Exchangers Important parameters:

What about crossflow heat exchangers? Δtm= F·Δtm,cf Correction factor Δt for counterflow Derivation of F is in the book: ………

Example: Calculate Δtm for the residential heat recovery system if : mcp,hot= 0.8· mc p,cold th,i=72 ºF, tc,i=32 ºF For ε = 0.5 → th,o=52 ºF, th,i=48 ºF → R=1.25, P=0.4 → F=0.89 Δtm,cf=(20-16)/ln(20/16)=17.9 ºF, Δtm=17.9 ·0.89=15.9 ºF

Overall Heat Transfer Q = U0A0Δtm Need to find this

Overall Heat Transfer Q = U0A0Δtm Need to find this

Resistance model Q = U0A0Δtm From eq. 1, 2, and 3: We can often neglect conduction through pipe walls Sometime more important to add fouling coefficients R Internal R cond-Pipe R External

Example The air to air heat exchanger in the heat recovery system from previous example has flow rate of fresh air of 200 cfm. With given: Calculate the needed area of heat exchanger A0=? Solution: Q = mcp,cold Δtcold = mcp,hot Δthot = U0A0Δtm From heat exchanger side: Q = U0A0Δtm → A0 = Q/ U0Δtm U0 = 1/(RInternal+RCond+RFin+RExternal) = (1/10+0.002+0+1/10) = 4.95 Btu/hsfF Δtm = 16.5 F From air side: Q = mcp,cold Δtcold = = 200cfm·60min/h·0.075lb/cf·0.24Btu/lbF·16 = 3456 Btu/h Then: A0 = 3456 / (4.95·16.5) = 42 sf