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Operational characteristics of a new steam system and its potential for energy saving.

Operational Characteristics of a New Steam System and its Potential for Energy Saving

The market for large steam boilers and steam services has been greatly reduced over the last 20 years or so in the UK. The reasons for this are associated with the inefficiencies in distribution of steam from large boilers, water return, use of steam traps, flash vessels, etc.

Many processes in a factory/building, in fact, require steam of differing qualities and the use of steam has been systematically reduced as users have turned to alternative techniques with the benefit of eliminating the greater proportion of complex systems and energy losses.

Small steam generators have been widely available both in a format equivalent to a shell boiler as well as those based on coil designs. These units although being nominally efficient at their present conditions have little flexibility and often produce a mixture of water and saturated steam which has to be extensively treated before it can be used.

Such treatment can lead to considerable loss in steam quality as well as leading to considerable increases in energy usage. All such units are nominally set up to give dry saturated steam (ie, x=1) under low back pressure conditions. In fact, many processes do not really require dry saturated steam, ie, steam cleaning, steam tunnels, curing of plastics, food processes, etc. What is really needed is good quality steam of varying dryness fractions so that the user can choose the type of steam most appropriate to his process.

Large conventional steam boilers do not allow the user this choice as experience shows that attempts to produce quality wet steam, say at dryness fraction x=0.5, merely results in the production of slugs of steam interspersed with hot water.

Steam quality (below the superheat range) is defined in terms of dryness fraction x for a given pressure, dry saturated steam has dryness fraction 1, very wet steam, just about to condense back to liquid, has dryness fraction O. A whole range of steams of varying dryness fractions (sometimes termed quality) can be considered in the context of different applications. The important point is that the energy (or strictly enthalpy) content of that type of steam is primarily dependant on the dryness fraction x and only weakly dependant on the pressure as shown in

Fig. 1. As is well known the energy content of steam rises linearly with dryness fraction x.

It is here pertinent to examine the energy input necessary to produce steam at various dryness fractions. Comparing the figures for steam dryness fraction 0.5 and 1 shows that energy saving of around 40 percent can be made at all pressure levels; even better savings can be made if a lower dryness fraction is used.

The criteria is that the good quality steam at low dryness fraction can be made; it will be clearly demonstrated that this is possible using the new system described herein.

This paper thus describes the operation of a new type of steam system which displays the ability to produce good quality steams ranging from superheat to dry saturated and then to dryness fractions ranging from 1 down to 0.1. This is achieved by means of a special design of heating coil/heat exchanger/burner combination combined with a unique water injection system that carefully regulates the quantity of water passed into the coil. Emissions of NOx are also minimised, owing to the high rates of heat transfer achieved close to the flame front in the heating coil/heat exchanger.

The Steam System

The steam system produced by Cubit Ltd of Warwick incorporates several unique features. Mains water is fed to a water softener, then to a holding tank, and to a high pressure displacement pump. This pump feeds a specially designed block and, depending on the type of steam required, feeds a carefully regulated quantity of water to the heating coil/heat exchanger/burner system.

The performance of the system depends on the accuracy of this monitoring system which has been developed over the last 10 years and has proved to be extremely reliable.

An integrated burner and multiple pass heating coil/heat exchanger system, ensures not only excellent heat transfer and thus thermal efficiency (typically 80 percent for all steam conditions) but also, due to the technique of water injection and configuration of the coil, excellent steam quality is attained throughout its performance range. Steam from the coil is fed to a small storage vessel adequate for most applications; experience shows that returns from the storage vessel are extremely small even for very wet steam (ie, .42).

Significant water return usually only occurs at start up and for the first 2-3 minutes of the operation.

It is pertinent to examine the quality of the steam produced by one of these units when steam is directly discharged from the storage vessel via a nozzle.

The characteristic of dry saturated steam (x = 0.97) is that it is virtually invisible close to the nozzle; the jet only becomes clearly visible some 10-11 diameters downstream. This is because here are no water droplets in the steam leaving the nozzle; visible water droplets form later as the steam jet cools downstream. As the steam quality is reduced the visibility of the steam jet close to the nozzle steadily increases. It is not until x=0.42 that the visible jet of steam is seen to be attached to the nozzle. Thus it is only with wetter steam, ie x.42, that visible water droplets are present in the steam leaving the nozzle, (ie, water droplets are in the size range greater than 5-100 microns). Clearly at higher dryness fractions, fine water droplets are present but to be invisible must be smaller than 1-2 microns in diameter and thus have negligible propensity to wet surfaces as they will be swept along by the gas flow (ie, just superheated steam).

A steam jet of 'nominally' dry saturated condition (x = 0.97) produced by a conventional boiler is of inferior quality because of the presence of large water droplets (ie, diameter greater than 5 - 10 microns) and propensity to wet surfaces. Clearly to use this type of steam, extensive water trapping, etc, is needed, increasing the complexity whilst reducing the efficiency of the system. Moreover experience shows that providing good steam quality can be readily produced at low dryness fractions, wet steam can be used for many processes etc, ie, steam tunnels, curing, steam cleaning, some heating processes, with considerable benefit to energy utilisation as indicated by Fig. 1.

Fig. 2. Output of Steam System as a function of Steam Quality and level of Superheat.

It is not only in the wet steam region where the type of unit shows considerable advantage. As shown, superheated steam up to 350[degrees] C can be readily produced by the same unit enabling applications such as oven or vat purging/sterilisation to be undertaken. This steam can also be considered as a readily available low cost purge gas which, when superheated, will not wet surfaces. The operational characteristics of this system are shown in Fig. 2. Burner temperature output is monitored constantly, whilst the water flow through the heating coil/heat exchanger system is carefully regulated through up to 8 settings, giving the curve shown. For the unit tested, hot water through to very wet steam, dry saturated and superheated steam could be produced at flow rates ranging from more than 500kg per hr of hot water to 80-90kg per hr of superheated steam at up to 350[degrees] C. Cold start to superheat steam production is typically 4-5 minutes; dry saturated steam 2-3 minutes, wet steam 1-2 minutes. A Hi-Lo burner system enables a standby mode of operation to be used such that full steam head can usually be achieved in 1-2 minutes.

The unique ability of this new system to economically and rapidly produce different qualities of steam enables many potential applications to be re-examined. Steam is normally passed around a heating system and eventually condensed and returned to the boiler. Unfortunately a slow build up of dissolved solids occurs so that periodic blowdown of the system is necessary to keep the solids within acceptable limits.

Fig. 3. Schematic Layout of Integrated Steam Heating/Hot Water System to Avoid Steam Blowdown.

Often expensive monitoring equipment is used to minimise regular blowdown but, either way, considerable expense is involved. The unique abilities described enable a system, as shown in Fig. 3 to be considered. Here a control system is used to switch the system to periodically supply hot water for domestic needs, ie, baths/showers/washing hands/dishwashers, etc. Thus purging the steam system of dissolved solids is readily achieved whilst improving overall system efficiency. Similarly the system can intermittently supply different processes with different qualities of steam.

Fig. 4. Use of the New Steam System for Power Generation/Process/System/Heating.

Multiples of these units may also be considered in the context of power generation using superheated steam. This arises due to the availability of reasonably low cost steam turbines. A typical proposal is shown in Fig. 4. Five to six steam units are cloned together to produce a package capable of producing 350-420kg per hr of superheated steam at 20 bar and 350[degrees] C. This is normally fed to a steam turbine to produce 100kw of electrical power whilst the exhaust steam, of nominally 0.8 to 0.9 dryness, is probably fed to another process (say process heating), where it can be efficiently used; the condensate is returned to the boiler. The economics of such systems start to look particularly attractive when it is realised that fuels such as heavy fuel oil can be used.

Preliminary measurements also indicate that emission levels of NOx and CO are extremely low due to the careful integration of the burner/coil/heat exchanger surfaces.

Conclusions

This paper has described the characteristics of a novel steam generation system which allows the production of up to 8 differing qualities of steam and hot water ranging from hot water to very wet, wet, dry saturated and superheated steam. for quite wet steam (ie, x.5) the quality of the steam is excellent, comparable to that directly produced by many boiler systems nominally operating dry saturated (ie, x = 1). Hence considerable energy savings are available in many applications where dry saturated steam is not necessarily required. The flexibility of the system enables wasteful steam blowdown in closed circuit to be circumvented by producing hot water (for which there is normally a need in most buildings). The efficiency of the system is better than 80 percent for all modes of operation, whether operated on a light fuel oil, natural gas or LPG.

PHOTO : One of the new Cubit units
COPYRIGHT 1989 Food Trade Press Ltd.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Author:Syred, N.
Publication:Food Trade Review
Date:Sep 1, 1989
Words:1797
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