AIR-CONDITIONING FOR FOOD PLANTS

JAMES HOLT Department of Mechanical Engineering, Massachusetts Ins t i tu te

of Technology, Cambridge, Massaahusetts

(Received for publication, November 12, 1937)

During these days of mass production and keen competition it is essential that the manufacturer of a food product, when he has once satisfied the taste of the public, be able to produce the article always the same not only in appearance but in taste and quality o r he will soon lose his fickle customers. The manufacturer is also interested in keeping his costs of handling at a minimum, with a minimum of waste owing to spoilage.

While every food product is more or less a special problem, there are many problems in common that can be solved or improved by the use of conditioned air. Air-conditioning finds its greatest appli- cation in plants where foodstuffs are in contact with air during the manufacturing process. I n the various food industries air-condition- ing is applied for one or more of several reasons:

1. To control the temperature of the product while in storage or during the varions stages of manufacture to prevent spoilage or loss of desirahle qualities, such as appearance. taste, plasticity, etc.

2. Since most foodstuffs are hygroscopic and hence take in or give OR moisture until in equilibrium with the surrounding air, it is im- portant to control the moisture content in order either t o maintain as nearly as possible the original moisture to prevent loss of weight, or to dry the product during the various stages of manufacture, as required.

3. To clean the air coming in contact with the product. 4. To produce a comfortable and healthful atmosphere in the fac-

tory for the benefit of employees. 5. In the retail trade to produce a comfortable atmosphere f o r the

benefit of customers. The heat and moisture content of the foodstuff, o r the enclosure

where it is processed. is seldom constant but in a continual state of flux. Where air-conditioning can be applied, it is the function of the air to remove or supply heat and moisture at the proper rate to main- tain temperature and moisture content a t an optimum.

By far the greatest number of problems involve the removal of heat and maintenance or removal of moisture, hence subsequent re- marks will be confined to this type of problem.

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128 JAMES HOLT

The rate at which sensible heat and moisture, with its associated latent heat, are removed is dependent upon the temperature, humid- ity, and motion of the air coming in contact with the product. Since the air must rise in temperature to pick up sensible heat and rise in moisture content to pick up moisture and latent heat, the permissible rise in temperature or change in moisture content, i.e., absolute hu- midity, whichever is the controlling factor, then determines the vol- ume of air which must be supplied to an enclosure. This volume, together with the volume of the space to be conditioned and the method for distributing air in the space, determines the air motion which will prevail. Careful design of the distribution system is essen- tial to control the air motion over the product a t the proper rate.

An air-conditioning system is only as good as its distribution sys- tem, and obtaining good distribution is one of the most difficult prob- lems of the engineer. His aim is t o insure that the temperature, hu- midity, and air motion in every part of the enclosure shall be the same within the permissible limits of variation.

Food technologists and engineers have, through their researches, determined the optimum temperatures, humidities, and air motions for storing and processing many foodstuffs. Unfortunately, the op- timum conditions are difficult and sometimes impractical of accom- plishment from an engineering point of view, and a compromise must be made in commercial practice. With meat in cold storage, for example, the loss of weight a t O0C.(32"F.) is practically three times as great at 85 per cent relative humidity as at 95 per cent relative humidity and approximately doubled at 10 feet per minute air move- ment as compared with still air. Loss of weight must be checked, not only for economic reasons, but on account of loss of color, flavor, and tenderness at the surface. On the other hand, for high humidities. above 90 o r 95 per cent spoilage owing to bacterial action is apt to predominate because of moisture precipitating on the meat when fluc- tuations of temperature in the room bring the meat below the dew- point temperature of the air coming in contact with it. Below 90 per cent relative humidity and fairly constant temperature the spoil- age is predominantly due to mold. For relative humidities below 85 per cent serious loss of weight occurs. As a compromise, commer- cial practice generally accepts 85 to 90 per cent as the optimum humidity range.

The possible limits of variation in temperature and humidity have an important bearing on the design and control of an air-conditioning system which can best be shown by one or two illustrations.

First, let us assume a room to be cooled and dehumidified where no fresh air is to be supplied but the same air is t o be recirculatecl.

AIR-COh-DITIOKING FOR FOOD E'LAS-TS 129

A (Fig. 1) represents a cooling surface usually cooled by a refriger- ant at a temperature T, and with a surface temperature in contact with the air ( 2 ) (Figs. 1 and 2 ) below the dew-point temperature of the air in the room. B (Fig. 1) represents a fan t o draw air over the cooling surface and create a positive air circulation in the room; (1) represents the condition of the air as it returns t o the condition- ing unit. The slope of the line 1-2 is determined by the ratio of sensible heat to moisture pick-up of the air in passing through the room.

Part of the air at condition (1) comes in actual contact with the cold coil surface and is cooled and dehumidified t o condition ( 2 ) ;

FIG. 1 D. 0. Temp

FIG. 2

the remaining part does not come in contact and by-passes the sur- face at its original condition, with the result that on leaving the fan at condition ( 3 ) it is a mixture of air at conditions (1) and (2). The air on leaving the fan at condition ( 3 ) , owing to aspirating action, causes a diffusion of air a t condition (3) with some room air a t con- dition (1) t o produce a mixture at condition (4 ) before coming in contact with the products in the room. I n the occupied portion of the room there will therefore be a variation in temperature A T (4) to (1) and a variation in absolute humidity AH (4) to (1). Proper engi- neering design involves the proper choice and control of the tempera- tures and amounts of air circulated, so that condition (1) A T and A H and air motions will be within the desired limits.

In most air-conditioning problems i t is necessary to take in some fresh air from outside for two purposes: (1) to supply fresh air for ventilation and (2 ) to create a positive pressure within the enclosure in order to cause a flow of air outward through cracks around win- dows and doors, thereby preventing as much as possible infiltration of unconditioned dust-laden air to affect conditions in the conditioned space. This affects the mechanism of cooling and dehumidification

130 J A M E S HOLT

in that it brings in the problem of cleaning, cooling, and dehumidify- ing this fresh air.

A and B (Fig. 3 ) represent cooling surface and fan as before. C represents air filters for cleaning both the outside and return air

J

4-

\ )L

L D B . Temp.

FIG. 3 FIG. 4

taken into the cooling unit. Condition (1) (Fig. 4) represents the conditioa desired in the room, condition ( 5 ) the condition of the fresh air, and condition (6) the resultant mixture of fresh air and return air from the room. The line (6) t o ( 2 ) represents the cooling of that portion of the air a t condition (6) actually coming in contact with the surface at temperature ( 2 ) . The slope of the line (6) t o ( 2 )

I

B. 43. Temp. FIG. 5

is determined by the temperature and humidity a t (6) and the tem- perature ( 2 ) where the humidity must be 100 per cent. Point (3) is the resultant mixture of air a t ( 2 ) which has come in contact with the cooling surface and air a t condition (6) which by-passed the surface. As air leaves the fan and duct system a t condition ( 3 ) , diffusion with room air a t Condition (1) produces condition (4), which is the condition of the air coming in contact with the products in the room.

AIR-COSDITIONING FOR FOOD PLANTS 131

so that in the occupied portion there will be a rise in temperature A T and an increase in absolute humidity AH. The slope of the line (I) to ( 3 ) is determined by the ratio of sensible heat to moisture pick-up in the room.

It frequently happens that the slopes of the lines ( 6 ) to ( 2 ) and (1) to ( 3 ) , dependent upon different factors, are such that they will not intersect at condition ( 3 ) . This is usually the case when the con- dition in the room must be maintained below approximately 50 per cent relative humidity. Within limits, this can be overcome by placing a heater between the cooler and fan, i.e., between A and B (Fig. 3 ) . The result is illustrated (Fig. 5 ) where the path (7) to ( 3 ) represents sensible heat added by the heater necessary to bring point ( 3 ) on the line through (1) and (4 ) , the slope of which was determined by the ratio of sensible heat to moisture pick-up in the room.