Performance characteristics of the geotechnical cold rooms

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Performance characteristics of the geotechnical cold rooms Baker, T. H. W.; Frederking, R. M. W.; Hoffman, D. R.

PERFORMANCE CHARACTERISTICS OF THE GEOTECHNICAL COLD ROOMS

T.H.W. Baker R.M.W. Frederking

D.R. Hoffman

Two low temperature Iaboratories have been operating in t h e

Division of Building Resea~ch, National Research Council of Canada,

since July 1954. e initial design of these cold rooms w a s first

described by G o l d ( 3 . One cold room was used until 1970 by t h e

Division" Building Services Section f o r experiments related to cold weather building research but i s now being used by the

Geotechnical Section for frozen ground studies (Permafrost cold

room). The other cold room is used by the Geotechnical S e c t i o n for ice research (Ice cold room). Construction and layout of the two rooms and a b r i e f description of the associated r e f r i g e r a t i o n

and c o n t r o l equipment are included in this paper.

In the 2 1 years since t h e i r construction these cold rooms

have performed extremely well. Some modifications and changes

have recently been made, however, and new control systems were

installed following which a perf~mance study was made of the a i r

temperatures maintained during steady state and transient

conditions. This study permitted f i n e adjustments to be made for

optimum control of temperature stability and response. The

resulls of t h i s study are also included in this paper.

This information is necessary to evaluate experiments

performed in the cold rooms. The temperature distribution and

stability in the cold room a f f e c t s t h e temperature h i s t o r y of

samples of ice and frozen soil which are moved from zone to zone

during storage, machining and t e s t i n g , and also a f f e c t the time required for specimens to reach thermal equilibrium before testing. PLANS AND CONSTRUCTION DETAIU

Details of the frame construction and wall section are

presented in Figures 1 and 2. Both cold rooms are identical.

The only permanent service connections brought into the cold

rooms were t h o s e required for lighting, refrigeration, and o t h e r

permanent machinery. Three service panels each consisting of

twelve 2-in. (50-cm) diameter capped ports arranged in three rows

connections were brought i n t a the rooms through these ports.

Recording equipment used in the experiments is located outside the

cold rooms.

The layouts of the c o l d rooms, at the time of this study, are

given in Figures 3 and 4. The equipment used for machining and t e s t i n g can be moved to any location within the room thus

providing maximum flexibility to accomodate a variety o f t e s t i n g

procedures.

From Ffgures 3 and 4 it can be seen t h a t there was a greater

mass of equfpment in the Ice cold room than in the Permafrost cold room at t h e time of t h i s performance study.

DESCRIPTION OF REFRTGERATION P M

The cooling load of the two rooms is shared between three

similar r e f r i g e r a t i o n circuits (Figure 5). The compressor and

condenser are located in a separate machinery room, with only the

evaporator unit and expansion valve in t h e cold room. Each room

has t w o evaporator units (Figure 6). A main evaporator is

connected to one compressor, t h e auxillary evaporator is connected

to t h e t h i r d (auxillary) compressor which can be connected to the

auxillary evapararor in either of t h e two rooms. This t h i ~ d

(auxillaxy) refrigeration c i r c u i t serves several purposes, namely:

back-up in the event o f failure of one of the main circuits; extra

cooling capacity for rapid "'draw downtt or high heat loads; and, as

an alternate system during t h e defrosting o f t h e heat exchanger

coils i n the main evaporator unit.

The system was designed to have a low inertia temperature

control. This was accomplished through high voXwne a i r cixcnlatian

and electrical reheating for fine temperature control. The

compressors sun continuously under a constant load, cooling the air

to about -40°C. A f t e r being cooled, the air is warmed by

electrical reheat coils t o bring i t s temperature up to t h e s e t

p o i n t temperature. A fan draws a i r from the room through t h e

evaporator unit for conditioning and it is exhausted through a

plenum above the false c e i l i n g so as to provide relatively d r a f t free a i r circulation, A schematic o f the electrical s i d e of the

r e f r i g e r a t i o n c i r c u i t i s shown in F i g u r e 7 .

Temperature control is achieved by varying the power supplied

to t h e electrical reheat coils. Cold room temperatu~e is measured by a 100-ohm resistance temperature sensor placed in the air

intake at the bottom of the evaporator. This is the temperature

control sensox t h a t modifies an electrical signal f e d inta a

s o l i d - s t a t e c u r r e n t a d j u s t i n g proportional controller. The

t h e set point temperature are compared within the controller in order t o produce a milliamp level control signal. This signal is sent to a silicon controlled rectifier package in the current valve

which regulates the power supplied t o t h e r e h e a t coils. One h a l f

sf the maximum power output is supplied to the reheat coils when

the room temperature is equal to the set point temperature. If the measured room temperature i s greater than the set temperature then the power output t o t h e coils is l e s s ; if less than t h e set

temperature then the power output is greater.

TEMPERATURE PERFORMANCE

It is necessary to "tune*' the controller to the unique

characteristics of the cold room in o r d e r to optimize temperature

control. For t h i s reason a temperature performance study was

undertaken i n both cold rooms. The control u n i t i s shown i n Figure 8. Thirty copper-constantan themocouples were placed at

various locations within t h e rooms and temperature measurements

were made using a Kaye data logger having a resolution of

approximately 50. I 'F (20.06OC)

.

degrees Celsius and rounded o f f to the nearest tenth of a degree.

One thermocouple was placed in an ice bath to check the performance

of the data logger. Evaluation of the thermal behavioux of the

cold rooms included measurements of t h e temperature profiles moth

v e r t i c a l and horizontal) at different room t e q e r a t u r e settings,

temperature fluctuations under steady state and t r a n s i e n t

conditions, a ~ l d the change in room temperature during a defrost

cycle.

(a) Temperature Profiles

Typical profiles for t h e Permafrost and Ice cold roams are

shorn i n Figures 9 and TO, r e s p e c t i v e l y , f o r nominal control

settings ranging from -5 to -40°C. Thermocouples were spaced

every half metre. The horizontal temperature at midheight in

each room fluctuated only 2 0 . 2 ~ ~ from one position in t h e room t o another. The v e r t i c a l d i s t r i b u t i o n of temperatures i n the centre of t h e Ice cold room showed l i t t l e variation. In the Permafrost cold room a d i s t i n c t v e r t i c a l temperature gradient was found in t h e middle of t h e room with the difference in temperatures between the f loox and just above t h e false c e i l i n g being as much as 0 . 7 O C .

@) Constant Set Point

Temperature variations w i t h time were recorded Sn both rooms

for specific temperature s e t t i n g s . Figures I1 and 12 show the variations in temperature at t h e control sensor in the bottom o f

the evaporator and at the midpoint of the room when the Ice cold

shows t h e short-term fluctuation over a period of 70 minutes. It

was noted that opening the door for a few minutes raised the temperature at the midpoint of the room 0. S°C. Figure 12 shows the temperature fluctuation over a period of about 18 hours.

Fluctuations of about *0. I 0 C occur in both cold rooms a t any one

point. It should be noted that the temperature recorded a t t h e

control sensor was always a few tenths of a degree lower than t h a t

recorded a t the midpoint of the room and t h a t b o t h were below the

nominal set point. The temperature difference between the control

sensor and t h e s e t point were still within the specified limits of

the controller,

Step Change &-I Set Point

Changing the temperatures of the cold moms from one s e t

paint t o another was investigated. Figures 13 and 14 show t h e

temperatme change with time recorded at the control sensors and

midpoints of the two cold rooms when the setting was lowered from

-10 to -30'C over a period of 5 to 6 hours. Figure 15 shows the

temperature change in t h e Permafrost cold room when the nominal

set temperature was raised from -30 to -10'~ over a 4-hour period.

Figures 13 and 14 show t h a t the temperatures at the control

sensor and midpoint come to equilibrium in the Permafrost cold

room a f t e r approximately 3 hours and in the Ice cold room after

about 5 hours. Tfiis difference was probably due to the differences

in size and configuration of the two cold rooms and t h e bulk mass

of equipment in each of them. As shown in Figure 1 5 , when raising

the temperature in the Pemnafrost c o l d room, the temperature at

t h e controf sensor smoothly approached the set temperature a f t e r

3 hours. The midpoint of the room experienced an 'lovershooting"

of the set temperature by two degrees. F i n e tuning o f the control system can eliminate l ' o ~ e r s h o o t ~ ~ at any point in the room where

the control sensor is placed.

Defrost Cycle

Moisture in the air precipitates out, forming frost on t h e cooling coils o f the evaporator unit. For this reason, the

relative humidity in the cold rooms ranges from 35 t o 4 0 per cent.

The evaporator unit must be shut down about once a _{month for}

defrosting d u r h g which t i m e the temperature is controlled by the

auxiliary cooling system. Heating elements are used to remove the

frost build-up on the cooling coils. The e f f e c t of def~asting t h e

main unit in the Ice cold room on the measured temperature at the

control sensor of the auxiliary u n i t and midpoint of the room is shown 5n Figure 16.

As the de-frost heaters melt the frost the room temperature

in a i r temperature occurs. A t this paint the heaters are t u n e d off and within an hour the temperatures stabilize. When the main unit and fan are turned on a b l a s t of h o t air enters the room which raises the temperature considerably f o r a short time.

SUMMARY

The DRR Geotechnical cold rooms have the following

characteristics:

1. TemperatuTe can be controlled f r o m 0 to -40°C. A i r temperature fluctuates, at any one point,

3 . Temperature variation between different locations in

the rooms is f OC.

4. Relative humidity is between 35 and 40 per cent.

5, Three to 5 hours are necessary for the rooms to reach

equilibrfum when the s e t room temperature is changed

20 Celsius degrees.

6 . _{E r r a t i c temperatures occur during the monthly defrost}

cycle.

REFERENCE

1. Gold, L.W., New Snow and Ice Research Labo~atory i n Canada,

FIGURE 1

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