In a typical dairy plant, saturated steam is utilised in various process applications as a primary source of energy for heat transfer. Most of these applications require heating up to a predefined temperature that is less than 90°C.
Key areas of focus to achieve optimum energy consumption in processes
Selection of correct steam pressure
It is imperative to utilise steam at the lowest possible pressure in order to leverage the higher latent
heat available at lower pressures. As a rule of thumb, 25°C ~30°C is added to the desired process temperature to arrive at the required steam temperature. The steam pressure corresponding to the
required steam temperature is fed as input steam to the process.
Precise temperature control
Control of steam temperature for the process could be by traditional manual methods or automated
with on/off or PID based control. In on/off type control, temperature overshoot is commonly
observed which results in excess steam consumption and could adversely affect product quality. A
PID type temperature control system is the preferred option to achieve precise temperature control.
Selection of correct process trap
A correctly selected and sized process trap impacts energy consumption, process efficiency,
productivity, product quality and safety. The efficient removal of condensate from any process
equipment requires positive differential pressure across the steam trap throughout the complete
batch time of the process. When this condition of positive differential pressure is not met,
conventional process traps are unable to discharge the condensate and the system undergoes a
phenomenon known as ‘stall’. A steam operated pumping trap ensures efficient removal of
condensate during stall conditions.
Milk Pasteurizer
Pasteurisation of milk is a heating process to kill pathogens / bacteria and reduce the microbial load
of raw milk. This process is carried out at predefined time-temperature combination. The optimum
combination is defined region-wise, based on the laboratory experiments carried out on raw milk
from a particular region.
An alkaline phosphide test is the conducted on the pasteurised milk. The phosphate reacts with
enzymes in the milk to confirm the pasteurisation efficiency of the process. Active enzymes produce
ortho-nitrophenol, giving the milk a dark yellow colour. This indicates inefficient pasteurisation.
Inactive enzymes result in para-nitrophenol, which gives a light yellow colour, showing efficient
pasteurisation.
The main reasons for heat treatment of milk are
Warranting safety of the consumer
A fairly moderate heat treatment kills all microbial organisms, including pathogens like Mycobacterium tuberculosis. It also eliminates potentially pathogenic bacteria that may accidentally enter the milk.
Increasing the keeping quality
It helps retain freshness by killing spoilage agents and their spores, if present. Also, by inactivation of enzymes.
The Pasteurisation Process
Pasteurisation is primarily carried out using plate type heat-exchangers. Here, hot Water is utilised
to heat the incoming raw milk to the desired temperature. The incoming raw milk (at temperature
4degC) initially gains heat from the outgoing pasteurised milk. Such a milk-to-milk heat exchange
ensures reduction in the steam-utility demand of the pasteuriser PHE.
Regeneration Efficiency
Regeneration efficiency of a pasteuriser/ PHE is the amount of heat gained by the raw milk from the
outgoing pasteurised milk, in order to ensure reduction in the heating and chilling load in the
pasteuriser.
Regeneration Efficiency=
(Temp of the Milk after Regeneration Stage)- (Temp of the Raw Milk) /
(Temp of the Pasteurised Milk )- (Temp of the Raw Milk)
Efficient condensate evacuation plays a critical role in ensuring optimum steam consumption in the
milk pasteurizer.
Understanding Stall
‘Stall’ is the inability to evacuate condensate effectively from the heat exchanging equipment. The
expectation that the condensate should be evacuated from the heat exchanger as soon as it is
formed is adversely impacted when the pressure in the heat exchanger is equal to, or less than, the
total backpressure acting on the outlet of the steam trap installed on the heat exchanger.
Stall occurs when differential pressure across the steam trap becomes negative. During stall
condition, the steam operated pump trap uses external motive steam to create a positive pressure
for evacuation of condensate. When differential pressure across the trap is positive, it evacuates condensate by functioning as a float trap using the two orifice mechanism that is capable of handling condensate, both at peak loads and very low running loads.
In most heating applications, the process media has to be heated from the initial lower temperature
(most often ambient temperature) to its final set temperature. Here, the secondary fluid inlet
temperature rises as a result of the falling heat load.
As the control valve reduces the steam pressure to meet a falling heat load, the lack of differential
pressure across the steam trap causes condensate to waterlog in the steam space. This is illustrated in Fig No. 02 below.
Problems Caused by Stall
Water logging of condensate in the process equipment
Water logging reduces the effective area available for heat exchange hence the overall heat transfer
coefficient will reduce. This results in:
• Poor heat transfer
• Increased steam loads
• Fouling/ corroding of heat exchanger
Fouling in the PHE
Gasket Failure
Stall conditions lead to negative pressure inside the heat exchanger, causing waterlogging of
condensate that could lead to failure of gaskets. Gasket failure results in unsafe operations and
maintenance hassles, besides adversely impacting equipment uptime.
Hunting of Temperature Control Valve
When process temperature is not properly sensed, the temperature control valve operates
frequently. This leads to ineffective and unstable temperature control for the process.
Inadequate Product Quality
Lowered heat output and unstable temperatures across the heat exchange surface can impact
product quality, and most often plants suffer with lower yield.
The Forbes Marshall Solution
Conventional steam traps cannot remove condensate unless the trap bypass valves are kept open.
The opening of trap bypass valves result in higher steam consumption. A properly sized Steam Operated Pump Trap (SOPT) ensures complete condensate removal during stall conditions without
opening of bypass valves. This results in enhanced productivity due to consistent pasteurisation
temperature and steam savings of 5-7% at equipment level.
Condensate load of the process equipment defines the trapping capacity and computation of stall
load based on the operating parameters defines the required pumping capacity. Another important
factor to consider during the selection of SOPT is the available installation head, which is a site
specific parameter. A range of SOPT variants are available to give a reliable solution for stall in
varying condensate flow rates, stall loads and installation heads.
Principle of Operation: