Heat Recovery Systems & CHP Vol. 12, No. 3, pp. 283-287, 1992 0890-4332/92 $5.00 + .00 Printed in Great Britain Pergamon Press Ltd

TECHNICAL NOTE

METHYLAMINE-SODIUM THIOCYANATE VAPOUR ABSORPTION REFRIGERATION

K. P. TYAGI Bharat Heavy Electrical Corporate Research and Development, Vikas Nagar Hyderabad-500593 India

(Received 5 September 1991)

Abstract--The performance characteristics of the methylamine-sodium thiocyanate vapour absorption refrigeration system have been analysed. The thermodynamic properties of the binary fluid have been expressed in polynomial equations. The thermal efficiency of this system has been compared with ammonia-water and ammonia-sodium thiocyanate absorption cycles. It has been observed that this mixture has a higher coefficient of performance.

N O M E N C L A T U R E

C~ specific heat of superheated vapor [kcal kg- ~ °C- ~] COP coefficient of performance m mass flow rate [kgh -t] P pressure [atmosphere] Q energy [kcal h- l] T temperature [°C] X weight concentration Subscripts A absorber C condenser E evaporator EX liquid-liquid heat exchanger G generator S saturated condition sup super saturated condition v vapour

1. I N T R O D U C T I O N

A considerable amount of work has been done in the search for promising binary mixtures for vapor absorption refrigeration (VAR) systems. Solubility data for these mixtures such as: ammonia with organic solvents, ammonia with salt mixtures, ammonia with organic solvents and salt mixtures, alcohol-salt mixtures, sulphur dioxide in polar organic solvents, and fluorocarbon with organic solvents, are available in the literature [1, 2]. Tyagi [3] has analysed the performance characteristics of some of the promising mixtures such as: effect of the generator temperature on coefficient of performance, mass flow rate, brake horse power/ton, in the low grade thermal energy temperature level of 70-120°C.

The thermal performance of an absorption refrigeration cycle depends upon the physical and thermodynamic properties of the refrigerant mixtures employed. Many promising combinations suitable for absorption systems are detailed in the literature [1-4]. Identification and evaluation of potential working fluids is one of the major prevailing problems. The most widely used binary mixtures are: ammonia-water and lithium bromide-water. But the fluids present some problems when used in residential air conditioning and air cooled systems. The lithium bromide-water system works at sub-atmospheric pressures, which are difficult to maintain. It is also difficult to use the system for air cooled units due to salt crystalisation under certain temperature levels. In the case of ammonia-water systems, the system pressures are quite high and a rectifier must be provided. In the search for better refrigerant-absorbent combination, Rush et al. [4] proposed a system using methylamine--sodium thiocyanate (MST) as the working fluid. This system is expected to be an improvement over the ammonia-water and ammonia-sodium thiocyanate systems. The vapour

283

284 K. P, TYAGI

pressure, temperature, concentration and other thermodynamic properties such as liquid and vapour enthalpy of refrigerant, saturated liquid enthalpy of methylamine-sodium thiocyanate solutions measured by the above authors were represented in polynomial form and system performance has been predicted. The performance of this mixture has been compared with ammonia-sodium thiocyanate and ammonia-water binary solutions.

2. V A P O U R A B S O R P T I O N C Y C L E

2.1. Vapour absorption refrigeration system The absorption cycle shown in Fig. 1 consists of a generator, condenser, evaporator, absorber,

liquid pump, throttling valves and heat exchanger. The pump delivers rich solution from the absorber to the generator through a liquid-liquid heat exchanger where the input of heat drives off some vapour of the refrigerant. The remaining weak solution passes back to the absorber through a throttling valve. The refrigerant vapour flows to the condenser where it liquified, and then drops in pressure through an expansion valve to the evaporator, where evaporation absorbs heat at low temperature thus producing the cooling effect. The refrigerant vapour then mixes with the weak solution in the absorber and thus enriches the solution.

2.2. Thermodynamic analysis 2.2.1. Assumptions. In order to analyse the thermodynamic performance of the absorption

refrigeration cycle, the following assumptions are made:

(1) the temperature of the generator, condenser, evaporator and absorber are constant and uniform throughout the components;

(2) the weak solution leaving the generator at state 4 is in equilibrium at the temperature and pressure in the generator;

(3) the strong solution leaving the absorber at state 1 is in equilibrium at the temperature and pressure in the absorber;

(4) heat loss to the surroundings is negligible; (5) pressure drop due to friction, etc., in the system is negligible; (6) pump work is zero.

I Generator Qg

Refrigerant B

Qex 3.<

L~

6

Qa

,r[ 5

Control. valve

=/ I0

Fig. 1.

7 i

I Condenser ~L

"x Qc

8

) Expansion valve

9

J Evaporator I Qe ~, \

Methylamine-sodium thiocyanate vapour absorption refrigeration 285

2.2.2. Analysis. The mass, material, and energy equations within the assumptions lead to the following:

~m =0 (1)

m X = 0 (2)

~ m h = 0 (3)

mlo = m, (X, - X6)/(Xlo - X6) (4)

QA = ml0ht0 + m6h6 - ml ht (5)

QEx = m3h3 - m2h2 (6)

QG = m7h7 + m4h4 - m3h3 (7)

Oc = m7 h7 - ms hs (8)

QE = mlohlo -- m9H9 (9)

COP = QE/Qc (10)

Three-state equations for thermodynamic properties:

(1) Vapour pressure of pure refrigerant (methylamine):

e = f(T)

In P = A + Ao/T.

(2) Pressure-concentration-temperature equation P - X - T for methylamine sodium thio- cyanate mixture:

P = f(X, T) 4 4

l n P = ~ Bo.X"+ ~ BI .X ~. n = 0 n = 0

1.0~

0.9-

0.8-

0.7

0.6

o 0.5

0.4

0.3

0.2

0 .1

( ~ TA=30=C, Tc=30°C

(~) TA=35°C, TC=35°C ( ~ TA=40oc, Tc=40=C

• Evaporator temperature=5°C

I I i I I I I I 60 70 80 90 100 110 120 130 140

Generator temp. =C

Fig. 2.

286 K . P . TYAGI

Q NH3"H20

Q NH3-Na SCN

Q CH3NH2-Na SCN

Q. o U

0 . 9 -

0.8-

0.7-

0.6-

0.5

0.4

0.3

0.2

0.1

0

• Evaporator temp. 5"C • Condenser temp. 35"C • Absorber temp. 35"C

I 60

i I I 70 80 90 100 100 110

Generator temp. *C Fig. 3.

(3) Enthalpy--concentration-temperature equation H - X - T

n = f(X, T)

H = Co+ Ct T + C2T 2

4

Co= Z Co.X" n=O

4

C 1 = £ CInXn n=O

4

c~= E c~.x". n=O

I 120

(4) Enthalpy of saturated (liquid) refrigerant:

IlL = Do + DI T + D2 T 2.

(5) Enthalpy of saturated refrigerant vapour:

Hv = Fo + FI T + F2 T2.

(6) Enthalpy of super saturated refrigerant vapour:

Hsv = Hv + Cp~(Ts.p - T,)

3. CONCLUSIONS

(1) The coefficient of performance of the methylamine--sodium thiocyanate VAR system has been plotted against generator temperature in Fig. 2 at the absorber and condenser temperatures of 30, 35 and 40°C and evaporator temperature of 5°C.

Methylamine-sodium thiocyanate vapour absorption refrigeration 287

(2) The coefficient of performance of above system has been compared with the coefficient of performance of ammonia-sodium thiocyanate and ammonia-water VAR systems.

R E F E R E N C E S

1. J. P. Roberson, C. Y. Lee, R. G. Squires and L. F. Albright, Vapour pressure of ammonia and methylamine in solutions for absorption refrigeration systems. ASHRAE Trans. 72, 198 (1966).

2. J. E. Aker, R. G. Squires and L. F. Albright, An evaluation o f alcohol-salt mixtures as absorption refrigeration solutions. ASHRAE Trans. 71, 14 (1965).

3. K. P. Tyagi, Compar ison of binary mixtures for absorption refrigeration systems. Heat Recovery Systems 3, 421 (1983). 4. W. F. Rush, R. A. Macriss and S. A. Well, A new fluid for absorption refrigeration. 4th Congress International du

chauffage Et De La Climatisation, May (1967).

A P P E N D I X . C O N S T A N T S F O R T H E R M O D Y N A M I C E Q U A T I O N S

Ao = 0.1134186E + 02 A~ = -0 .3006263E + 04

Boo = 0.1395709E + 02 Bo~ = -0 .4382246E + 02 Boo = 0.193516E + 03 Boo = -0 .3211592E + 03 Bo4 = 0.1854885E + 03 Bio = -0 .5015803E + 04 Bu = 0.1057704E + 05 B~2 = -0 .3852595E + 05 Bl3 = 0.6328807E + 05 Bi4 = -0 .3560337E + 05 Coo = -0 .1589127E + 02 Col = -0 .4570190E + 01 Co2 = -0 .2336584E + 03 Co3 = 0.3832696E + 03 Co4 = -0 .1518155E + 03

C,o = 0.2986066E + 00 Cll = -0 .3550777E + 00 C]2 = 0.2857421E + 01 C~5 = -0 .3653084E + 01 C]4 = 0.1657480E + 01 C2o = -0 .793224E - 03 C2, - 0.4104522E - 02 C22 - -0 .7175156E - 02 C23 - 0.121386E - 02 (?24 = 0.4996810E - 02 Do = -0 .1833748E + 02 DI -~ 0.7250143E + 02 D2 --- 0.1295105E - 02 Fo --- 0.1758190E + 03 FI -~ 0.4883584E + 00 F 2 -~ - 0.2264305E - 02