Tecumseh Products Co.

Box Load Program v1.5

© Copyright 2011-2013

The Tecumseh Box Load Program allows users to determine the heat load of a refrigerated facility and select

the proper condensing units and evaporator coils to maintain design temperature.

The program allows equipment selection using one of three methods: (1) having the user perform a detailed heat

load analysis; (2) having the program perform a quick load estimation in which the user need only enter box

dimensions and design temperature; and (3) having the user simply enter the heat load of the refrigerated

facility without the program performing any heat load analysis.

The detailed heat load analysis requires the following heat loads be determined:

• Transmission Load

• Infiltration Load

• Product Load

• Miscellaneous Loads

A discussion of these heat loads follows the Quick Tutorial, along with the methodology the program uses to

perform its quick load estimation

Quick Tutorial

From the main page, specify the application, condensing unit type, desired voltage, refrigerant, and fan motor

type for the evaporator coil if an evaporator coil selection is desired. If you know the heat load, simply enter it

and click the right arrow:

The program will provide fields for you to identify the job. Entering information on this page is optional.

The program will then allow you to adjust the design room and ambient temperatures.

Clicking the right arrow once again brings the program to its equipment selection page.

The program will list condensing units and evaporators that are suitable for this application. The program will

highlight its preferred selection. You may change the quantities for either the condensing units or evaporators

desired. Clicking the right arrow brings the program to its summary page.

The program generates a report in pdf format which can be viewed, printed, or saved from the web.

You may also view, print, or save the selected condensing unit data sheet by click on the “Condensing Unit”

button.

The same is true for the selected evaporator by clicking on the “Evaporator” button

The report and datasheets may be emailed by clicking the “E-mail Report” button.

Enter the e-mail address in standard format. Multiple e-mail addresses may be entered by separating with a

semi-colon, e.g., john.doe@tecumseh.com;jane.doe@tecumseh.com.

Clicking the right arrow on the summary page simply returns you to the main menu.

If you do not know the heat load, the program will assist you in determining this value. From the main page,

specify the application, condensing unit type, desired voltage, refrigerant, and fan motor type for the evaporator

coil if an evaporator coil selection is desired, and leave the capacity field blank.

Click the right arrow. The program will ask if you want to do a quick or detailed load calculation.

If you elect to do a quick load calculation, you will only need to enter the refrigerated box dimensions. The

program will allow you to adjust the design room temperature, and specify whether the load will be “average”

or “heavy”.

Clicking the right arrow will bring the program to its equipment selection page and noted previously.

If you elect to do a detailed load calculation, the program will bring up its heat load calculation page.

If an outdoor unit is desired, this page will allow you to determine design ambient conditions by location. In

addition, this page provides four calculators: transmission load, infiltration load, product load, and

miscellaneous load.

The transmission load calculator begins by having the user select the shape of the refrigerated box:

The refrigerated box shapes page is then followed by the transmission load calculator:

The infiltration load calculator:

The product load calculator:

The miscellaneous loads calculator:

The heat load calculation page will collect all load calculations. Once finished with this page, clicking on the

right arrow brings you to the load adjustments page.

Please refer to the documentation below regarding how to properly enter data on these pages. Clicking on the

right arrow on the heat load calculation page brings you to the summary page described above.

Transmission Load

Sensible heat gain through the walls, floor, and ceiling is referred to as transmission load, and it may be

calculated as follows:

x

tkAq

∆

∆=

where:

q = heat gain, Btu/h (W)

k = thermal conductivity, Btu-in/h-ft2-°F (W/m-K)

A = area, ft2 (m2)

∆t = temperature difference, °F (°C)

∆x = wall thickness, in (m)

The program defaults wall temperature to design ambient dry bulb temperature, and it will add 20°F to the

ceiling temperature to allow for sun effect. These default temperatures may be changed in the event the

refrigerated room is not located outdoors, or if the default values are not accurate.

The program assumes no latent heat gain from moisture transmission through the walls

Infiltration Load

Heat gain from infiltration air is referred to as infiltration load. The program estimates infiltration load caused

by the differences in air density between the refrigerated space and outside of this space. It does not consider

infiltration load due to direct air flow through doorways.

The program can analyze infiltration loads for up to 3 different door types. Default infiltration air temperatures

for the door will be the design ambient dry and wet bulb temperatures unless the wall temperatures have been

modified by the user. If the wall temperatures are modified by the user, the program will default to the

minimum wall temperature entered, and determine a wet bulb temperature corresponding to a 50 percent RH.

Both the dry bulb and wet bulb temperatures of the infiltration air may be modified by the user.

The program calculates infiltration load using the Gosney and Olama air exchange equation1:

( ) ( ) ( )EDDFgHhhCAq ftmrirri −−−= 11 ρρρ

where:

q = heat gain, Btu/h (kW)

C = conversion factor, 795.6 (0.221)

A = area, ft2 (m2)

hi = enthalpy of infiltration air, Btu/lb (kJ/kg)

hr = enthalpy of refrigerated air, Btu/lb (kJ/kg)

ρi = density of infiltration air, lb/ft3 (kg/m3)

ρr = density of refrigerated air, lb/ft3 (kg/m3)

g = gravitational constant, 32.174 ft/s2 (9.81 m/s2)

H = door height, ft, (m)

Fm = density factor, see below

Dt = doorway open-time factor, see below

Df = doorway flow factor

E = effectiveness of doorway protective device

( )

5.1

311

2

−=

ir

mFρρ

( )d

op

t

PD

θ

θθ

3600

60+=

where:

P = number of doorway passages

θp = door open-close time, seconds per passage

θo = time door simply stands open, min

θd = daily (or other) time period, h

Doorway flow factor is the ratio of actual air exchange to fully established flow. Fully establish flow occurs in

the case of a doorway standing open to a large room or to the outdoors, and where cold outflow is not impeded

by obstructions such as stacked pallets. Under these conditions, doorway flow factor is 1.0.

The program uses ASHRAE recommended default values of 0.8 for flow factor when temperature differentials

less than 20°F, and 1.1 for higher differentials.

Effectiveness device is a value between 0 and 1 which refers to the effectiveness of an open-doorway

infiltration protective device such as an air curtain or push-through doors.

The program defaults to an effectiveness device of zero which indicates a wide open door with no protective

devices. Air lock vestibules with strip or push-through doors typically have effectiveness in the 0.85 to 0.95

range. Properly installed air curtains can have effectiveness up to 0.8. The following table may be used as a

guide for determining an appropriate effectiveness value for an open-doorway infiltration protective device2:

Protective Device Effectiveness Value

Air curtain, vertical 0.42 to 0.80

Air lock vestibule 0.85 to 0.95

Fast operating door 0.79 to 0.85

Plastic strip curtain, single 0.82 to 0.93

Plastic strip curtain, dual 0.92 to 0.98

Product Load

Product load is the heat that must be removed to bring product to storage temperature.

Product load may be calculated using the following equations:

( )21 ttmcq −=

where:

q = heat gain, Btu/h (kJ)

m = mass of product, lb (kg)

c = specific heat of product, Btu/lb-°F (kJ/kg-K)

t1 = initial product temperature, °F (°C)

t2 = final product temperature, °F (°C)

The specific heat of a product will depend upon if it is above or below its freezing temperature. In addition, the

latent heat to freeze the product must be accounted for when t1 is above freezing temperature and t2 is below

freezing temperature. In this case, the following equation is used:

( ) ( )[ ]2211 ttchttcmq fiff −++−=

where:

c1 = specific heat of product above freezing, Btu/lb-°F (kJ/kg-K)

c2 = specific heat of product below freezing, Btu/lb-°F (kJ/kg-K)

hif = latent heat to freeze the product, Btu/lb (kJ/kg)

t1 = initial product temperature above freezing, °F (°C)

t2 = final product temperature below freezing, °F (°C)

tf = freezing temperature of product, °F (°C)

An additional product load that must be considered is the heat of respiration generated by products (mainly

fruits and vegetables) in storage. This respiration heat varies by product and its temperature; the colder the

product, the less heat of respiration. The program applies an average respiration rate for products having a heat

of respiration load.

Miscellaneous Loads

The program allows the user to determine heat loads for lighting, people working in the refrigerated facility, lift

trucks being used in the facility, fan motor horsepower, and the number of glass doors.

Lighting in watts is converted to Btu/h as follows:

WhBtu 412.3=

The heat load from one person working in the facility is determined as follows:

tq 5.111295 −=

where:

q = heat gain, Btu/h

t = box temperature, °F

The program assumes a lift truck to have a 5000 Btu/h heat load while in operation. Note that this load can vary

substantially with the many available types and sizes of lift trucks.

Fan motor horsepower is converted to Btu/h as follows:

HPhBtu 2544=

The additional load created by a glass door is simply estimated as follows:

800=q Btu/h per glass door for box temperatures above 0°F

1300=q Btu/h per glass door for box temperatures 0°F and below

The miscellaneous load input screen proves an additional entry for any other heat load present in the facility.

Load Adjustments

The program allows the user to adjust heat load calculations in the following three areas:

Run time (h/day)

Run time is the amount of time per day the refrigeration equipment must run to maintain temperature. A 24

h/day run time would indicate the equipment must run all the time to maintain temperature. A range of 16 to 20

h/day is typically used. The program defaults to 18 h/day.

Safety Factor

The calculated load is often increased by a safety factor to account for possible discrepancies between design

criteria and actual operation. The program uses a default value of 10 percent. This factor is applied to all four

heat load segments.

Load Diversity Factor

When determining the heat load for a refrigerated facility, the peak value for each of the heat load segments is

normally computed. A load diversity factor is applied to this total heat load to account for the fact it is rare to

have all four heat load segments peak at the same time. A range of 0.70 to 0.85 is normally used for the

diversity factor. The program uses a default value of 0.85.

Quick Load Estimation

In the event the user doesn’t have good heat load data to work with, the program offers a quick load estimation

based only on box dimensions and design temperature. This estimation is based on the following assumptions:

Transmission Load

Surface temperatures are as follows: walls 95°F (35°C); ceiling 115°F (46.1°C); floor 55°F (12.8°C). Insulation

equivalent to 4 inch polyurethane board, k = 0.17 Btu-in/h-ft2-°F (0.025 W/m-K), for all surfaces.

Infiltration Load

The program uses an air change methodology3 to estimate infiltration load and applies it to the Gosney and

Olama air exchange equation listed in the Infiltration Load section.

Product Load

Load due to one pound of product per cubic foot of box volume being reduced in temperature by 10°F over a 24

hour period. Specific heat of the product is assumed to be 0.9 Btu/lb-°F (3.8 kJ/kg-K) above freezing and 0.45

Btu/lb-°F (1.9 kJ/kg-K) below freezing.

Miscellaneous Loads

Load due to lighting is one watt per square foot of floor space. Load due to personnel in the box is one person

per 25,000 ft3 (708 m3) of box volume. Load due to electric motors is one motor horsepower per 12,500 ft3

(354 m3) of box volume.

Heavy Usage

After these loads are calculated, the program will assume an 18 h/day run time and apply a 10 percent safety

factor.

1 ASHRAE Refrigeration Handbook 2010, p24.4

2 Downing, C. C., and Meffert, W. A., “Refrigerated Storage Door Air Infiltration Utilizing Infiltration

Reduction Devices”, ASHRAE 645-RP, August 1992.

3 ASHRAE Fundamentals Handbook 1977, p27.3 Table 5