Cold Storage Warehouse Dock Parametric Study

Transcription

1 Cold Storage Warehouse Dock Parametric Study Todd B. Jekel, Ph.D. Industrial Refrigeration Consortium Topics Objective Modeling loads Infiltration Desiccant operation Effects of dock temperature setpoint and addition of desiccant dehumidifier Effects of infiltration and defrost Conclusions 1

2 Objective Develop a modeling tool for freezer and dock loads associated with dock operation Investigate effect of infiltration, defrost and dock setpoint on total refrigeration system energy use Investigate effect of adding a desiccant dehumidifier to the dock Refrigerated dock function The most obvious function is to facilitate the staging and transfer of stored goods What about from the refrigeration perspective? Protects the freezer from infiltration by allowing for removal of humidity at a higher temperature level Protects the employees from dangerous snow and ice at the freezer/dock door Assisted by a well designed and operated freezer door 2

3 Simulation of the Loads Weather NOAA surface observations of temperature and wetbulb Dock People Equipment (fans, forks, lights) ASHRAE 0.4% Design Mean Coincident Wetbulb Temperature Infiltration (ambient load and freezer credit) Transmission Defrost Freezer (incremental) Load from dock plus door heat (to avoid snow/frost) Defrost associated with latent load from dock Temperature, [ o F] Dry-bulb Wet-bulb ASHRAE 0.4% Design Drybulb Temperature Hour Number Infiltration Ambient air that enters the conditioned space Uncontrolled Unconditioned Importance in refrigerated spaces Largest source of humidity (i.e. frost) Not as much of a problem now in WI 3

4 Infiltration through an unprotected door The amount of air flow through a doorway as a function of only temperature difference is impressive Truck bay door 10 wide x 9 high Dock 35F/86%, Ambient 91F/73F 1 wetbulb Results in: 2 equivalent of 80 ft/min velocity through the door 36 tons sensible, 32 tons latent In other words, 1.1 ton-hrs per minute of open door 1 ASHRAE 0.4% Design conditions for Minneapolis 2 No influence of pressure difference from wind, etc. Freezer infiltration 10 wide x 14 high Usually have a protective device Strip curtain, air curtain, or both Doorway effectiveness, η Fraction of unprotected doorway air exchange that is protected from exchange In other words, multiply the unprotected doorway exchange by (1 η) to determine the estimated protected doorway air exchange 4

5 Freezer doorway effectiveness Strip curtain Vertical air curtain Dual horizontal air curtain Fast sliding doors 82-94% 49-80% 65-78% 78-93% Values taken from: Downing, C.C., W.A. Meffert, 1993, Effectiveness of cold-storage door infiltration protective devices, ASHRAE Transactions, vol. 99, part 2, pp Freezer loads Plots of sensible and latent load on freezer as a function of door effectiveness and dock conditions Sensible Load [tons] W arehouse: -20 o F, 90% RH η = 80% η = 85% η = 90% Dock Temperature [ o F] η = 95% Latent Load [tons] W arehouse: -20 o F, 90% RH 20 o F Dry Bulb 30 o F DB 40 o F DB 50 o F DB Dock Dewpoint [ o F] η = 80% η = 85% η = 90% η = 95% m w [lb/hr] 5

6 How do desiccants dehumidify? Adsorption of water vapor from humid air on the surface of the desiccant (solid type) A heat-activated water pump Use heat source to regenerate (drive-off moisture) the desiccant Adsorbs water vapor from humid air on the surface of the desiccant (gives off heat similar to condensation) Humid air is dehumidified and heated Schematic and Psychrometric Representation of Desiccant 2,300 scfm 256 F / lb/lb R 74 F / lb/lb S 120 F / lb/lb 9,000 scfm 42 F / lb/lb P Humidity Ratio Pressure = 14.7 [psia] T [ F] 6

7 Desiccant System Schematic Intake from ambient Exhaust to ambient Conditioned Supply Air Desiccant wheel operational Intake air from Dock 35 F > 40 F Freezer Dock Details Located in Minneapolis, MN Dock setpoint of 35F Attached to a 20F warehouse 8,000 ft 2 of dock per freezer door (η = 85%) 900 ft 2 of dock per truck bay door (open 2 minutes per hour) 7

8 Design Day Energy Use Mechanical-refrigeration only Freezer load: 4.7 ton-hr/ft 2 of door/shr = 0.85 Peak dock load 200 ft 2 /ton 0.33 kwh/ft 2 per design day $0.016/ft 2 per design day 1 With desiccant (flow rate 0.56 cfm/ft 2 of dock) Freezer load: 4 ton-hr/ft 2 of door/shr = 0.98 Peak dock load 175 ft 2 /ton 0.35 kwh/ft 2 and therms/ft 2 per design day $0.019/ft 2 per design day 1 1 Assume $0.05/kWh and $0.25/therm Mechanical-only Load Breakdown Dock setpoint 35F Gross Load 140 ft 2 /ton Credit 29% Net load 72% Ambient infiltration 38% Lights 7% Forks 5% Evaporator fans 3% Occupants Defrost 0% 8% Transmission 10% 8

9 Desiccant Load Breakdown Dock setpoint 35F Gross Load 130 ft 2 /ton Credit 26% Net load 74% Desiccant 5% Occupants 0% Ambient infiltration 37% Defrost 6% Lights 7% Forks 5% Evaporator fans 4% Transmission 10% Effect of Dock Setpoint Cost per Design Day ($/sqft) Mechanical only/hot Gas Mechanical only/electric Desiccant, 0.56 cfm/sqft Desiccant, 0.38 cfm/sqft No Defrost Required Dock Dry-bulb Setpoint (F) Note: Hot Gas and Electric refer to the method of applying dock/freezer door heat to avoid frost, etc. 9

10 Effects of Ambient Infiltration 50% decrease in ambient infiltration Mechanical Refrigeration-only Nearly 50% reduction in dock load Only 10% reduction in freezer load 25% reduction in design day energy use and cost With 0.56 cfm/ft 2 desiccant Approximately 40% reduction in dock load Negligible change in freezer load 17% reduction in design day energy use and cost Note: all parametric reductions are for 35F dock setpoint. Effects of Freezer Infiltration 67% decrease in freezer infiltration Mechanical Refrigeration-only 40% increase in dock load 60% reduction in freezer load 25% reduction in design day energy use and cost With 0.56 cfm/ft 2 desiccant 40% increase in dock load 60% reduction in freezer load 16% reduction in design day energy use and cost 10

11 Effects of Defrost Load Double the energy associated with defrost Mechanical Refrigeration-only 16% increase in dock load Negligible effect on freezer load 9% increase in design day energy use and cost With 0.56 cfm/ft 2 desiccant 10% increase in dock load Negligible effect on freezer load 6% reduction in design day energy use and cost Conclusions Infiltration Ambient Large effect on dock load Dock/freezer Large effect on both dock and freezer load Mechanical refrigeration-only 33-35F dock setpoint is near optimum Desiccant opportunities Benefits from higher setpoint in the dock Proper sizing important 11

12 Future work General Optimum dock temperature determination as a function of ambient conditions. Freezer door Dock/freezer air exchange conditions that result in no frost condition in the freezer. Methods for control of door heat addition to prevent freezer frost. Desiccant Investigate sizing of desiccant system. Investigate siting of desiccant system inlets and outlets. Investigate alternative control of desiccant system. 12