Interstitial Spaces – Evaluation & Problem Solving Todd Jekel, Ph.D., P.E. Industrial Refrigeration Consortium 2006 IRC R&T Forum Madison, WI 1 What are we talking about? z Enclosed spaces adjacent to a cold space z Due to design (e.g. drop ceiling) or facility expansion Drop Ceiling -20°F -20°F Interstitial Space Interstitial 40°F Doorways 2 Why are we talking about it? z If the space is unconditioned or unventilated, infiltration will occur and likely result in z Condensation resulting in mold, bacterial growth, and possibly corrosion or z Frost resulting in potential structural issues 3 What Influences Condensing? z Condensing/frosting will occur if the surface temperature is less than the dewpoint temperature z The dewpoint is a function of the weather z The surface temperature will depend on the z temperature of adjacent cold space, and z amount of insulation 4 How to maintain non- condensing conditions z Simple… z Keep the surface temperature above the dewpoint z Methods z Add heat to keep the surface temperature above the dewpoint, z Dehumidify the air within the space to keep the dewpoint below the surface temperature, or z All of the above 5 Methods of Conditioning z Passive Heat/Dehumidification z Ventilation z Active Heat/Dehumidification z Heat z Dehumidification z Desiccant (also adds heat) z Cooling/Re-heat 6 Advantages/Disadvantages z Ventilation z A: Low capital & operational costs z D: Cannot always keep non-condensing (weather dependent) z All others z A: Given adequate capacity, can always keep non- condensing z D: Higher capital & operational costs 7 Ventilation z Most common method z Heat available with ventilation is a function of the ambient conditions z Temperature difference between the ambient dry- bulb and the space temperature that keeps the surface temperature above the ambient dewpoint z Some conditions result in the inability to keep non-condensing conditions (i.e. 100% relative humidity) 8 Industry Guidance z ASHRAE 2002 Refrigeration Handbook z Recommends 6 air changes per hour z Does not consider effect of insulation amount, ambient conditions, or connected refrigerated space temperatures 9 Factors Influencing Required Ventilation A Decrease in the… Resultant Change in Ventilation Rate Requirement Refrigerated space set point temperature ↑ Insulation R-value ↑ Surface area adjacent to cold space ↓ Dimension independent of surface area (e.g. height of interstitial space above ceiling or - width of interstitial space between walls) Temperature difference between the ambient dry-bulb and dewpoint temperature ↑ 10 Minimum Ventilation Amount z Performing an energy balance on the fully-mixed interstitial space & one-dimensional heat transfer through the wall area results in the following: ⎡⎤ A ⎢⎥TTT+Δ − CFM =⋅⎢⎥ADP, R 1.1⋅ R 1 ins ⎢⎥TT−−Δ−⋅ T T +Δ− TT ⎢⎥AADP,,() ADP R ⎣⎦hR⋅ ins Where A cooled surface area [ft2] CFM ventilation volume flow rate [cfm] ΔT allowable approach temperature of interstitial space to the surface temperature [°F] h convective heat transfer coefficient on the cooled wall surface [Btu/hr- ft2-°F] 2 Rins R-value of cooled wall insulation [°F-ft -hr/Btu] TA ambient dry-bulb temperature [°F] TA,DP ambient dewpoint temerature [°F] TR refrigerated space temperature [°F] 11 Example z Location: Northeast US z Refrigerated Space: -10°F z Wall area: 100,000 ft2 z Interstitial space volume of 300,000 ft3 z Wall insulation R-value: 20 °F-ft2-hr/Btu z Assume convective heat transfer coefficient in interstitial space of 5 Btu/hr-ft2-°F z high natural, low forced convection coefficient range z lower is conservative z Assume all credits from heat transfer between ambient and interstitial space are 0 (worst case scenario) 12 Comparison z ASHRAE Guidance z 6 ACH → 30,000 cfm z Using construction specifics and ΔT = 0°F (onset of condensing) & weather of z 0.4% Cooling Design Day z 89°F dry-bulb/66°F dewpoint z 15,100 cfm z 0.4% Evaporation Design Day z 85°F dry-bulb/71°F dewpoint z 27,300 cfm z 0.4% Dehumidification Design Day z 82°F dry-bulb/74°F dewpoint z 52,100 cfm 13 Minimum VentilationAmount Bin Hours per Year 1000 1200 200 400 600 800 0 0 5000 10000 15000 20000 25000 Required Ceiling Ventilation,CFM 30000 35000 40000 45000 50000 Bin Hours Hours Cumulative 55000 60000 65000 70000 75000 80000 85000 90000 95000 100000 More 0 1500 3000 4500 6000 7500 9000 Cumulative Hours per Year 14 Design Recommendations z Consider specifics of application z Especially for small volume per cooled wall area spaces Warm Mold 1 foot Frost Condensation Cold 15 Other Recommendations z Fan & Intake Placement z Should promote good air distribution within space z Poor placement or difficult space dimensions may require local distribution fans & increased amounts of ventilation Fan Fan Fan Poor Good 16 Other Recommendations z Provide adequate intake area to keep pressure difference low ensures z lowest cost fan operation z minimum exfiltration from the refrigerated space z Understand that ventilation alone may not be able to keep non-condensing conditions z Apply heat/dehumidification to problem areas if needed 17 Other Recommendations z Periodically Monitor to Insure z Fan operation z Intakes are clean & not obstructed z Non-condensing conditions within the space z Focus on stagnant areas (corners & areas farthest from intakes) 18 19.