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 Ventilation Amount

1200 9000 Cumulative Hours Bin Hours 1000 7500

800 6000

600 4500

Bin Hours perYear 400 3000 Cumulative Hours per Year per Hours Cumulative

200 1500

0 0 0 More 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 55000 60000 65000 70000 75000 80000 85000 90000 95000 100000 Required Ceiling Ventilation, CFM

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)

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