Psychrometric chart

Psychrometric chart

A Psychrometric chart graphically represents the thermodynamic properties of moist air। Standard psychrometric charts are bounded by the dry-bulb temperature line (abscissa) and the vapour pressure or humidity ratio (ordinate). The Left Hand Side of the psychrometric chart is bounded by the saturation line. Figure 27.2 shows the schematic of a psychrometric chart. Psychrometric charts are readily available for standard barometric pressure of 101.325 kPa at sea level and for normal temperatures (0-50oC). ASHRAE has also developed psychrometric charts for other temperatures and barometric pressures (for low temperatures: -40 to 10oC, high temperatures 10 to 120oC and very high temperatures 100 to 120oC)

Some of the contents of the download links

FREEZING SYSTEM

Introduction

Since properly frozen foods have a much longer storage life than cold stored foods and
retain better quality, the emphasis is on the development of frozen food storages and
techniques of quick freezing. Among the common products that are preserved by freezing
are fish, meat, vegetables, such as beans, sprouts, carrots, cauliflower, peas and spinach,
various tissues and organs, etc. Freezing requires the removal of heat, and the product
from which heat is removed falls in temperature lower than the freezing temperature.
The first stage involves sensible cooling. The temperature drops rapidly to the freezing
temperature below 0°C. The second stage involves the extraction of the latent heat of
fusion during which the temperature is constant. This stage is known as the period of
thermal arrest. When about 75 per cent of the water is converted into ice, the temperature
begins to fall again in the third stage in which the remaining water is frozen The unfrozen
water has a higher concentration of salts and other compounds, the effect of which
is to depress the freezing point. Even at temperatures as low as -30°C, a proportion of
water in the product tissues remains in the unfrozen state.
In order to produce a good product, the freezing must be accomplished quickly. Changes
take place during freezing as the salts tend to separate out. The product is altered from its
native state. Slow freezing is detrimental as it means a longer time being spent in the
zone of denaturation activity which is in the region of freezing.

Types of Freezers

There are three basic types of freezers
(i) Air blast freezer, in which cold air is blown over the product. U
(ii) Contact or plate freezer, in which the product is brought in direct contact with a
refrigerated surface.
(iii) Immersion or spray freezers, in which the product is immersed in or sprayed with a
liquid refrigerant.
Air blast freezers and contact plate freezers are the two most common types used in
industry. Contact plate freezers do not have the versatility of air blast freezers. The
former can only be used to freeze regularly shaped blocks. The latter can be used for a
variety of irregular shaped as well as, small sized products such as peas, shrimp, etc.,
which results in individual quick freezing (IQF) of each piece.
Immersion freezers use liquid freezants at atmospheric pressure, the corresponding
temperatures being -196°C with N2 and -30°C with R I2. Figure 1 shows the schematic
diagram of the principle of the operation of the air blast freezer. While the IQF system is
the commone use in the frozen fish production, the figure 2 shows the principle of the
IQF system. The belt IQF system shown in figure 3, the system is used for quick dry
freezing systems for the small sizes products.
Fans used in IQF (or general air blast freezers) developing a pressure of about 14 cm H2O
gauge. The air is in cross-flow to the product. Likewise, there may be three or more fans
along the length of the belt. To minimize dehydration, the temperature of the air is kept
close to that of the product. The temperature rise of air is thus about 2°C only.
Accordingly, the fan handles a large quantity of air. Also, the cooling coil is 12-16 rows

deep. As the frost formation is more in the front rows, their spacing must be larger so as
to maintain a certain free area with continuous operation without defrosting for at least 8
hours and without affecting the fan air flow. In well-designed freezers, the fan power is
of the order of 25 to 30 % of the refrigeration load.
Air-blast freezers with batch operation use trolleys or shelves for loading, the product
sometimes belt conveyor. Those with continuous operation make use of conveyors or
belts. The latter are used only if the product can be frozen quickly, the freezing time not
exceeding 30 minutes, otherwise the length of the freezer will become very large. To
reduce the length, sometimes double, triple or spiral belts are used. The speed of the belt
should be variable to increase its versatility to accommodate the varying freezing time
requirements of various products and their thicknesses

Some of the contents of the download links

Air Reversed Carnot Cycle

Reversed Carnot cycle is an ideal refrigeration cycle for constant temperature external
heat source and heat sinks. Figure 1 shows the schematic of a reversed Carnot
refrigeration system using air as the working fluid along with the cycle diagram on T-s
and P-v coordinates. As shown, the cycle consists of the following four processes:
Process 1-2: Reversible, adiabatic compression in a compressor
Process 2-3: Reversible, isothermal heat rejection in a compressor
Process 3-4: Reversible, adiabatic expansion in a turbine
Process 4-1: Reversible, isothermal heat absorption in a turbine
Figure 1 shows the flow diagram air reversed Carnot cycle, while figure 2 presents the
different process of the reversed Carnot cycle on the (T-S) diagram

Figure 1 Air reversed Carnot cycle

Figure 2 Carnot cycle (T-S) diagram.