Objectives Finish with heat exchangers (ch.11) Start with Air-distribution systems

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

For Air-Liquid Heat Exchanger we need Fin Efficiency Assume entire fin is at fin base temperature Maximum possible heat transfer Perfect fin Efficiency is ratio of actual heat transfer to perfect case Non-dimensional parameter tF,m

Fin Theory pL=L(hc,o /ky)0.5 k – conductivity of material hc,o – convection coefficient pL=L(hc,o /ky)0.5

Fin Efficiency Assume entire fin is at fin base temperature Maximum possible heat transfer Perfect fin Efficiency is ratio of actual heat transfer to perfect case Non-dimensional parameter

Heat exchanger performance (11.3) NTU – absolute sizing (# of transfer units) ε – relative sizing (effectiveness) Criteria NTU ε P RP cr

Air Distribution System Design Describe room distribution basics Select diffusers Supply and return duct sizing

Forced driven air flow Diffusers Grill (side wall) diffusers Linear diffusers Vertical Horizontal one side

Diffusers types Valve diffuser swirl diffusers ceiling diffuser wall or ceiling floor

Diffusers http://www.titus-hvac.com/techzone/ Perforated ceiling diffuser Jet nozzle diffuser Round conical ceiling diffuser Square conical ceiling diffuser Wall diffuser unit Swirl diffuser Floor diffuser Auditorium diffuser Linear slot diffuser DV diffuser External louvre Smoke damper http://www.titus-hvac.com/techzone/ http://www.halton.com/halton/cms.nsf/www/diffusers

Low mixing Diffusers Displacement ventilation

18.7

Diffuser Selection Procedure Select and locate diffusers, divide airflow amongst diffusers V = maximum volumetric flow rate (m3/s, ft3/min) Qtot = total design load (W, BTU/hr) Qsen = sensible design load (W, BTU/hr) ρ = air density (kg/m3, lbm/ft3) Δt = temperature difference between supply and return air (°C, °F) Δh = enthalpy difference between supply and return air (J/kg, BTU/lbm)