Improvement of a Scraper Heat Exchanger for Pre-Heating Plant-Based Raw Materials Before Concentration

Abstract

When heating liquid and paste-type products, enterprises in the food industry commonly use scraper heat exchangers, which, given their high heat treatment intensity, make it possible to retain the original properties of the treated raw materials. Most heat exchangers demonstrate an unstable stabilizing effect: the vapor pressure ‒ the temperature that leads to damage to raw materials, under conditions of significant energy- and metal capacity. It is possible to eliminate these drawbacks by using a temperature-stable flexible film resistive electric heater of the radiating type as a heater in an improved scraper heat exchanger. We have proposed applying a hinged blade with a cutting edge (with a reflective heating surface) as a stirring element of the heat exchanger to obtain the uniform distribution of a product layer thickness at the working surface and to additionally heat by the blade's reflective surface. The heat exchanger can be supplemented with a cooling shell with ring channels to pass the refrigerant, which is placed on the outer surface of the non-thermally insulated flexible electric heater. Such a solution provides the possibility of cooling to −15 °C and it simultaneously serves the additional air thermal insulation in the absence of the carrier in it.

We have determined the uniformity of heat flow distribution over the heating surface of the model design of the improved unit (60.3...60.5 °C) and at the reflective surface of the hinged blade with a cutting edge (60.0...60.3 °C). The total thickness of a liquid layer has been established depending on the shaft rotation frequency of the proposed hinged blade with a cutting edge: at 50 min–1 – 1–2.65 mm, at 350 min–1 –1.5 mm, compared with a standard hinge blade (a layer thickness is from 5.0 mm to 1.5 mm), in terms of product consumption W=50 l/h. The improved scraper heat exchanger is characterized by a 1.48-time decrease in the specific energy consumption (170.4 kJ/kg), used to heat a product volume unit, compared to the heater with a steam shell – 252.6 kJ/kg. The research result is the confirmed efficiency of using the improved scraper heat exchanger, as well as its proposed structural scheme.

Keywords: heating; scraper heat exchanger; cutting blade; heat removal; flexible film resistive electric heater of the radiating type