R&T 2001 - Cold Storage Warehouse Dock Study - Jekel

8/18/2019 R&T 2001 - Cold Storage Warehouse Dock Study - Jekel

1/12

Cold Storage Warehouse DockParametric 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 dehumidifierEffects of infiltration and defrost

Conclusions


2/12

Objective

Develop a modeling tool for freezer anddock loads associated with dockoperation

Investigate effect of infiltration, defrostand dock setpoint on total refrigerationsystem energy use

Investigate effect of adding a desiccantdehumidifier to the dock

Refrigerated dock function

The most obvious function is to facilitate thestaging and transfer of stored goods

What about from the refrigerationperspective? “Protects” the freezer from infiltration by allowing

for removal of humidity at a higher temperaturelevel

“Protects” the employees from dangerous snowand ice at the freezer/dock door Assisted by a well designed and operated freezer door


3/12

Simulation of the Loads

Weather NOAA surface observations

of temperature and wetbulb

Dock People

Equipment (fans, forks, lights)

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

0 5 10 15 20 25

70

75

80

85

90

95

100

Hour Number

T e m p e r a t u r e ,

[ o F ]

Dry-bulbDry-bulb

Wet-bulbWet-bulb

ASHRAE 0.4% Design

ASHRAE 0.4% Design Mean Coincident Wetbulb Temperature

Drybulb Temperature

Infiltration

Ambient air that enters the conditionedspace

Uncontrolled

Unconditioned

Importance in refrigerated spaces

Largest source of humidity (i.e. frost)

Not as much of a problem now in WI


4/12

Infiltration through anunprotected door

The amount of air flow through a doorway asa function of only temperature difference isimpressive

Truck bay door 10’ wide x 9’ high

Dock 35F/86%, Ambient 91F/73F1 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 thatis protected from exchange

In other words, multiply the unprotected doorwayexchange by (1 – η) to determine the estimatedprotected doorway air exchange


5/12

Freezer doorway effectiveness

82-94%Strip curtain

78-93%Fast sliding doors

65-78%Dual horizontal

air curtain

49-80% Vertical air curtain

Values taken from:

Downing, C.C., W.A. Meffert, 1993, “Effectiveness of cold-storage door infiltration protectivedevices,” ASHRAE Transactions, vol. 99, part 2, pp. 356-366.

Freezer loads

Plots of sensible and latent load on freezer asa function of door effectiveness and dockconditions

5 10 15 20 25 30 35 40 45 500

2

4

6

8

10

Dock Dewpoint [oF]

L a t e n t L o

a d

[ t o n s ]

m

w [

l b / h r ]

Warehouse: -20oF, 90% RH

10 0

60

40

20

0

80

η = 95%

η = 90%

η = 85%

η = 80%

20oF Dry Bulb

50oF DB

40oF DB

30oF DB

20 25 30 35 40 45 500

5

10

15

20

Dock Temperature [oF]

S e n s i b l e L

o a d

[ t o n s ]

Warehouse: -20oF, 90% RH

η = 95%

η = 85%

η = 80%

η = 90%


6/12

How do desiccants dehumidify?

Adsorption of water vapor from humid air onthe 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 thesurface of the desiccant (gives off heat similar tocondensation)

Humid air is dehumidified and heated

Schematic and PsychrometricRepresentation of Desiccant

2,300 scfm

256 F / 0.022 lb/lb120 F / 0.038 lb/lb

9,000 scfm

42 F / 0.005 lb/lb74 F / 0.0008 lb/lb

R

PS

50 100 150 200 2500.00

0.01

0.02

0.03

0.04

T [°F]

H

u m

i d i t y R a t i o

Pressure = 14.7 [psia]

0.5

0.2

0.05


7/12

Desiccant System Schematic

35 F

> 40 FDesiccant

wheel

operational

Exhaust to

ambient

Conditioned

Supply Air

Intake air

from Dock

Intake

from

ambient

F r e e z e r

Dock Details

Located in Minneapolis, MN

Dock setpoint of 35F

Attached to a –20F warehouse

8,000 ft2 of dock perfreezer door (η = 85%)

900 ft2 of dock per truckbay door (open 2 minutesper hour)


8/12

Design Day Energy Use

Mechanical-refrigeration only Freezer load: 4.7 ton-hr/ft2 of door/SHR = 0.85

Peak dock load 200 ft2 /ton

0.33 kWh/ft2 per design day

$0.016/ft2 per design day1

With desiccant (flow rate 0.56 cfm/ft2 ofdock) Freezer load: 4 ton-hr/ft2 of door/SHR = 0.98

Peak dock load 175 ft2 /ton

0.35 kWh/ft2 and 0.007 therms/ft2 per design day

$0.019/ft2 per design day11 Assume $0.05/kWh and $0.25/therm

Mechanical-only LoadBreakdown

Credit

29%

Lights

7%

Forks

5%

Defrost

8%

Net load

72%

Ambient infiltration

38%

Occupants

0% Transmission

10%

Evaporator fans3%

Dock setpoint 35F

Gross Load 140 ft2/ton


9/12

Desiccant Load Breakdown

Credit

26%

Lights

7%

Forks

5%

Evaporator fans

4%

Defrost6%

Net load

74%

Transmission

10%Occupants

0%

Ambient infiltration

37%

Desiccant

5%

Dock setpoint 35F

Gross Load 130 ft2/ton

Effect of Dock Setpoint

0

0.005

0.01

0.015

0.02

0.025

0.03

0.035

25 30 35 40 45 50

Dock Dry-bulb Setpoint (F)

C o

s t p e r D e s i g n

D a y ( $ / s q f t )

Mechanical only/Hot Gas

Mechanical only/Electric

Desiccant, 0.56 c fm/sqft

Desiccant, 0.38 c fm/sqft

No Defrost

Required

Note: “Hot Gas” and “Electric” refer to the method of applying dock/freezer door heatto avoid frost, etc.


10/12


11/12

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/ft2 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


12/12

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 freezerfrost.

Desiccant Investigate sizing of desiccant system.

Investigate siting of desiccant system inlets and outlets.

Investigate alternative control of desiccant system.

Documents

R&T 2001 - Cold Storage Warehouse Dock Study - Jekel