The frost formation on cold surface is a natural phenomenon when the surface temperature is less than dew point temperature of the moisture content present in the air. The frost is the result of two mechanism. The first mechanism is the formation of ice particle which are already present in the free air and they accumulate on the surface when they come to contact surface of evaporator coil [1 Malhammar 1988 , mango et al 2005 ].The atmospheric air is in the superheated state then only it can hold the water vapor.
when this air come to the cold air contact, due to sudden cooling moisture content of air transform into tiny ice crystals. These crystals are the fog and they are attracted toward the cold surface. These can build up very rapidly because they have an affinity of cold surface this type of frost formation take place where the moisture content is very high. Example: Evaporator coil of refrigerator. The second mechanism for frost growth is diffusion of water vapour [ Sanders 1974 ].
The moisture content of the air is divided into two part, first part of moisture is condense on the surface and increases the frost thickness and second part of moisture content diffuses inside the already present frost and increases the frost density.the speed of this type of frost formation is very slow but of high density. This type of frost must be removed periodically. This type of frost formation is possible where the temperature is low and moisture content is also low.
The frost formation problem has received varying degrees of attention over the last 70 Years.Many authors have published papers relating to the frost growth, heat transfer and frost properties. For the heat exchanges are complex geometries and their shape and size varies with application so that literature is not much, because large number of the variable are affecting the frost growth much, most of the studies are experimental and attempt to make a correlation with frost growth and different variable.Stoecker (1957), Hosoda et al. (1967), and Gatchilov et al. (1979) have experimented on fin tube heat exchanger and report that heat transfer increases during the initial stages and decreases with frost growth.The initial increase in the frost growth is due to higher velocity and higher surface roughness , These both increases the heat transfer coefficient on the air side , but this is only for short time as the first starts building up the heat transfer coefficient decreases because of thermal resistance of the frost.the frost thickness increases, resistance to airflow also increases because of which heat transfer coefficient decreases.Neiderer( 1976) studied the effect of frost accumulation on the capacities of different air coolers and and different fin arrangement. They concluded that with the frost formation heat transfer coefficient first increases then decreases with the frost growth and confirmed the stoecker findings( 1957). With wider fin spacing arrangement capacity of air cooler was affected less than closer fin spacing. From The result it was stated that only 15-25 % of heat is only used to melt the ice . rest of all the energy is used to heat up the compartment which indirectly increases the load.Fisk et al 1985 studied the cross flow and counter flow heat exchanger performance. This study shows that the decrease in efficiency is 7-15 % per hour with the frost buildup on the evaporator surface. A more rapid decrease in temperature efficiency was associated with lower cold air stream temperatures, higher warm air stream humidities and shorter periods o f freezing.Rite et al. ( 1991) carried out an small experiment on the domestic refrigerator during frosting and tried to quantify the effect of relative humidity, inlet air velocity, inlet air temperature. The air rate was made constant and reported that initially frost layer growth speed high due to higher heat transfer coefficient. The increase is due to decrease in contact resistance between fin and tube gap filled with frost , increase and air side heat transfer coefficient due to higher surface roughness and due to increase in higher surface area. They change the relative humidity and air inlet temp which showed that , increase in relative humidity will increase the frost growth and higher pressure drop, increase in temp also increase the frost thickness.This was carried out at constant speed so that does not represent all the conditions.Lee et al. (1996) experimentally studied the effect of fin spacing, air temp , humidity and velocity on the frost growth and thermal performance of finned tube heat transfer under frosting condition. It was found that the heat transfer rate decreases initially during the test then it become maximum and again decreases before the maximum heat transfer.Change in heat transfer is due to fin spacing from 5mm to 10mm. The staggered finned tube heat exchanger gives 17 % higher heat transfer than the an in-line finned tube heat transfer. Frost layer with less density and large thermal resistance increase with increase in relative humidity. If inlet air velocity increases, thermal resistance decreases , frost thickness increases and frost density also increases. Increase in inlet temperature , reduces the frost thickness but on the other hand increase in frost density and decreases in thermal resistance. During the experiment relative humidity was kept constant.Yen et al. (2003) investigated the effect frost formation on the finned tube heat exchanger with single tube and multiple tube row using various fin pitches.the effect of inlet temp, relative humidity, relative inlet speed inlet refrigerant temperature were examined. The speed of the air was changed using centrifugal fan speed and the test was run for 2 hours. For every test heat transfer was constant initially ( 30 min) and decreased continually until the end of the period. It was found that the frost formation is greater for lower air flow rate( opposite to lee). It was found the at the relative humidity increase the pressure drop also increases, also for the constant relative humidity of 70 % the increase in inlet temperature from 2.5 C to 5C increases the frost thickness and thus increases in pressure drop. However when the temp is increased from 5 °C to 7.5 °C leads to slight reduction in frost thickness. If fin pitch increases the effect of frost on the heat exchanger decreases.Despite of some difference in result, almost all the studies reaches to the same result regardie the relative humidity, inlet temp ,fin spacing effect on the performance of the finned tube under frosting condition. The results are as follows,1) High inlet air relative humidity leads to higher fost formation and thus a greater pressure drop across the coil2) If fin pitches increases, effect of frost build up on the performance of heat exchanger decreases.